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Discussion Starter #1
Hi all,

I've got a 2008 fusion I4 with 142k miles and the FNR5 transmission with a suspected bad TCM. Below is the whole story of what happened but bottom line is: If I do in fact have a bad TCM what's the best route to fix it? Find a used one on ebay then get it programmed or use a service such as moduleexperts to repair it?

A couple of weeks ago, my idler pulley bolt broke and I read a number of threads about problems with those. I replaced the idler pulley and bolt along with the tensioner. 2 days later, driving home from work my transmission shifts hard and yellow wrench light comes on. I make it home and pull P0778 code: Pressure Control Solenoid B Electrical.

I drop it off at a local shop that does work when I can't or don't want to. They don't do transmission work other than basic maintenance such as fluid change which I had them do about 40K miles ago. They did what they could and so we called the transmission shop they work with. Guy there says it's probably a rebuild or remanufacturer to fix it. The cost of that would likely be more than half the value of the car so I drove it back home and decided I would see what I could find.

I found a diagram of the solenoids and dropped the transmission pan. I didn't see any wiring or other immediate problems so I go back inside and look closer at the service manual (should have done this first). Turns out Pressure Control solenoid B (PCB) is on the secondary valve body. After some searching I figured out secondary valve body == 5th gear valve body which is under the 5th gear valve body cover. I guess they shoved a 5th gear on the 4 speed transmission this is based on?

Anyway, I didn't have time to pull that cover until this past Saturday. Halfway through removing the cover, I decided not to do a repeat of last weekend, and go inside and look through the diagnostic steps for this P0778 code. First 2 steps required tools to do checks with the computer (run some test and try to cycle the solenoid), so I skipped to step 3 where testing for shorts to power, shorts to ground, solenoid impedance, etc start.

Step 3 has you check for voltage on connector C1535 to see if there is a short to power. After a couple of hours searching for the connector and finally getting the wiring diagrams to show up, I found the connector, unplugged it, turn ignition on, and sure enough, 12 volts on the pin feeding PCB as well as the 5th gear shift solenoid. At this point, it's either a short to power or bad TCM. While I had the connector off, I ohmed out the solenoid using the connector on the transmission housing (really glad I didn't pull that cover at this point) and got 3.5 Ohms which is in spec of the 2 to 7 ohms in the manual. 5th gear solenoid read 13 ohms but I didn't look up the spec for it.

Step 4 is to unplug the TCM and check for voltage again. With cables unplugged and ignition on, I check both ends and no voltage. I also checked if there was short between the PCB and 5th gear shift solenoid lines but there was none.

Based on this info, the service manual says to replace the TCM. I suspect there is a bad transistor in the solenoid drive circuit in the TCM or either the MCU pin is bad and the transistor is floating off/on (depending on the circuit). My mother in law has the same car but a year older so I'm going to do the same test with hers to make sure there is no voltage on that same connector.

Any thoughts or suggestions? I would just buy one from ebay but I'm pretty sure it will need to be programmed which could cost more than getting my TCM repaired unless maybe I can program it with ForScan.

Thanks!
 

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Hi all,

I've got a 2008 fusion I4 with 142k miles and the FNR5 transmission with a suspected bad TCM. Below is the whole story of what happened but bottom line is: If I do in fact have a bad TCM what's the best route to fix it? Find a used one on ebay then get it programmed or use a service such as moduleexperts to repair it?

A couple of weeks ago, my idler pulley bolt broke and I read a number of threads about problems with those. I replaced the idler pulley and bolt along with the tensioner. 2 days later, driving home from work my transmission shifts hard and yellow wrench light comes on. I make it home and pull P0778 code: Pressure Control Solenoid B Electrical.

I drop it off at a local shop that does work when I can't or don't want to. They don't do transmission work other than basic maintenance such as fluid change which I had them do about 40K miles ago. They did what they could and so we called the transmission shop they work with. Guy there says it's probably a rebuild or remanufacturer to fix it. The cost of that would likely be more than half the value of the car so I drove it back home and decided I would see what I could find.

I found a diagram of the solenoids and dropped the transmission pan. I didn't see any wiring or other immediate problems so I go back inside and look closer at the service manual (should have done this first). Turns out Pressure Control solenoid B (PCB) is on the secondary valve body. After some searching I figured out secondary valve body == 5th gear valve body which is under the 5th gear valve body cover. I guess they shoved a 5th gear on the 4 speed transmission this is based on?

Anyway, I didn't have time to pull that cover until this past Saturday. Halfway through removing the cover, I decided not to do a repeat of last weekend, and go inside and look through the diagnostic steps for this P0778 code. First 2 steps required tools to do checks with the computer (run some test and try to cycle the solenoid), so I skipped to step 3 where testing for shorts to power, shorts to ground, solenoid impedance, etc start.

Step 3 has you check for voltage on connector C1535 to see if there is a short to power. After a couple of hours searching for the connector and finally getting the wiring diagrams to show up, I found the connector, unplugged it, turn ignition on, and sure enough, 12 volts on the pin feeding PCB as well as the 5th gear shift solenoid. At this point, it's either a short to power or bad TCM. While I had the connector off, I ohmed out the solenoid using the connector on the transmission housing (really glad I didn't pull that cover at this point) and got 3.5 Ohms which is in spec of the 2 to 7 ohms in the manual. 5th gear solenoid read 13 ohms but I didn't look up the spec for it.

Step 4 is to unplug the TCM and check for voltage again. With cables unplugged and ignition on, I check both ends and no voltage. I also checked if there was short between the PCB and 5th gear shift solenoid lines but there was none.

Based on this info, the service manual says to replace the TCM. I suspect there is a bad transistor in the solenoid drive circuit in the TCM or either the MCU pin is bad and the transistor is floating off/on (depending on the circuit). My mother in law has the same car but a year older so I'm going to do the same test with hers to make sure there is no voltage on that same connector.

Any thoughts or suggestions? I would just buy one from ebay but I'm pretty sure it will need to be programmed which could cost more than getting my TCM repaired unless maybe I can program it with ForScan.

Thanks!
With the connector plugged, did you measure the voltage across the two solenoid wires? The voltage should vary in order to proportionally position the solenoid to control the line pressure.
Even if the electrical readings seem fine, the issue may be that the solenoid is not moving due to a blockage or other mechanical cause.
 

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Discussion Starter #3
I didn't pull the cover on the valve body so I didn't have access to the solenoid terminals to measure voltage across the solenoid (although I might be able to probe on the back side of the connector). The PCB solenoid is a PWM solenoid though, so you are right the effective voltage measured across those terminals should change based on the duty cycle.

However, I measured a solid 12V with the connector off and the ignition on. According to the service manual, there should be no voltage there as the vehicle shouldn't be driving either PCB solenoid or the shift solenoid F (5th gear) with the car off and ignition on. Also, according to the service manual, P0778 means "PCB circuit failed to provide voltage drop across solenoid. Circuit open or shorted or TCM driver failure during on-board diagnostic".

If the TCM is stuck at 12 V output (TCM driver failure) that is effectively 100% duty cycle, so you wouldn't see a voltage drop. I will double check the voltage on the same connector with my mother-in-law's car since wrong information is worse than no information.
 

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A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.
In British English, the term transmission refers to the whole drivetrain, including clutch, gearbox, prop shaft (for rear-wheel drive), differential, and final drive shafts. In American English, however, the term refers more specifically to the gearbox alone, and detailed usage differs.
The most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a relatively high rotational speed, which is inappropriate for starting, stopping, and slower travel. The transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are also used on pedal bicycles, fixed machines, and where different rotational speeds and torques are adapted.
 

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I didn't pull the cover on the valve body so I didn't have access to the solenoid terminals to measure voltage across the solenoid (although I might be able to probe on the back side of the connector). The PCB solenoid is a PWM solenoid though, so you are right the effective voltage measured across those terminals should change based on the duty cycle.

However, I measured a solid 12V with the connector off and the ignition on. According to the service manual, there should be no voltage there as the vehicle shouldn't be driving either PCB solenoid or the shift solenoid F (5th gear) with the car off and ignition on. Also, according to the service manual, P0778 means "PCB circuit failed to provide voltage drop across solenoid. Circuit open or shorted or TCM driver failure during on-board diagnostic".

If the TCM is stuck at 12 V output (TCM driver failure) that is effectively 100% duty cycle, so you wouldn't see a voltage drop. I will double check the voltage on the same connector with my mother-in-law's car since wrong information is worse than no information.
Measure the voltage on the backside of the connector directly across the two solenoid pins. The measurement referenced to ground is meaningless as the PWM occurs on the ground side of the solenoid. Seeing 12V on the supply side is just leakage through the high side FET.
 

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Discussion Starter #6
Measure the voltage on the backside of the connector directly across the two solenoid pins. The measurement referenced to ground is meaningless as the PWM occurs on the ground side of the solenoid. Seeing 12V on the supply side is just leakage through the high side FET.
I don't think this is a typical solenoid driver circuit where you switch between one solenoid terminal and ground and the other terminal is connected to voltage. All the wiring diagrams in the service show the internal wiring harness connecting one side of the solenoid to ground and the other side of the solenoid to the TCM so the TCM is switching voltage not ground to the solenoid. My guess is they are using an NPN and PNP together on the drive circuit and the NPN failed so the PNP is stuck on. The driver circuit is more complicated but it would be a lot cheaper than running two wires to every solenoid.

Besides, the attached image shows the steps where I should not be seeing 12V output between the TCM and ground with the ignition on and the TCM disconnected from the transmission/solenoid. Unless the procedure is wrong, there is a failure in the TCM driver.

I'm going to contact the module experts place and see what they say. Otherwise, I'll just buy a used TCM and get a dealer to program it if I can't do it with FORScan.
 

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I don't think this is a typical solenoid driver circuit where you switch between one solenoid terminal and ground and the other terminal is connected to voltage. All the wiring diagrams in the service show the internal wiring harness connecting one side of the solenoid to ground and the other side of the solenoid to the TCM so the TCM is switching voltage not ground to the solenoid. My guess is they are using an NPN and PNP together on the drive circuit and the NPN failed so the PNP is stuck on. The driver circuit is more complicated but it would be a lot cheaper than running two wires to every solenoid.

Besides, the attached image shows the steps where I should not be seeing 12V output between the TCM and ground with the ignition on and the TCM disconnected from the transmission/solenoid. Unless the procedure is wrong, there is a failure in the TCM driver.

I'm going to contact the module experts place and see what they say. Otherwise, I'll just buy a used TCM and get a dealer to program it if I can't do it with FORScan.

It might be worth nothing, Pressure Control Solenoid A (which is called Linear PCA) does have + and - wires run to it. All other solenoids including Pressure Control Solenoid B have a single wire driving the solenoid and is grounded inside the transmission (or to a common ground run inside the transmission).
 

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Discussion Starter #8
I don't think this is a typical solenoid driver circuit where you switch between one solenoid terminal and ground and the other terminal is connected to voltage. All the wiring diagrams in the service show the internal wiring harness connecting one side of the solenoid to ground and the other side of the solenoid to the TCM so the TCM is switching voltage not ground to the solenoid. My guess is they are using an NPN and PNP together on the drive circuit and the NPN failed so the PNP is stuck on. The driver circuit is more complicated but it would be a lot cheaper than running two wires to every solenoid.

Besides, the attached image shows the steps where I should not be seeing 12V output between the TCM and ground with the ignition on and the TCM disconnected from the transmission/solenoid. Unless the procedure is wrong, there is a failure in the TCM driver.

I'm going to contact the module experts place and see what they say. Otherwise, I'll just buy a used TCM and get a dealer to program it if I can't do it with FORScan.
I meant n-channel and p-channel instead of NPN and PNP since the TCM is almost certainly using FETs and not BJTs. If it's a single FET driving the solenoid, I'm not sure why you would see 12V unless the FET failed on which is unlikely but not impossible. I'm willing to bet the circuit is more complicated than a single FET due to diagnostics and such though.
 

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It might be worth nothing, Pressure Control Solenoid A (which is called Linear PCA) does have + and - wires run to it. All other solenoids including Pressure Control Solenoid B have a single wire driving the solenoid and is grounded inside the transmission (or to a common ground run inside the transmission).
Did you perform the diagnostic tests on Pressure Control Solenoid A?
 

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Any updates on this? I've been fighting with p0778 in my fnr5 for several months. (09) . I've pulled the secondary valve body cover twice. I replaced the internal harness and pressure control solenoid B, no luck. getting P0778 still..OP, This sounds exactly like my problem id love to know if the TCM could fix this..I'm desperate! Ive at least acquired an autel code reader and multi meter through my struggle. if there's anything else you guys could suggest I do or check let me know, Thanks!
 

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Welp, got the new TCM in and went through the relearn procedure, within 5 minutes after I started driving it, I got p0778 (pressure control solenoid B electrical) again. The code seems to happen more when im downshifting from 5th to 4th than when its going from 4th to 5th... either way its still screwed up. Im open to more suggestions on how to troubleshoot this. So far ive replaced the internal harness that connects to shift solenoid F (5th gear) and pressure control solenoid B inside the secondary valve body housing.. Ive replaced pressure control solenoid B itself and now ive replaced the TCM. I don't think it would make a difference but im running redline d4 atf...
Thanks.
 

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Welp, got the new TCM in and went through the relearn procedure, within 5 minutes after I started driving it, I got p0778 (pressure control solenoid B electrical) again. The code seems to happen more when im downshifting from 5th to 4th than when its going from 4th to 5th... either way its still screwed up. Im open to more suggestions on how to troubleshoot this. So far ive replaced the internal harness that connects to shift solenoid F (5th gear) and pressure control solenoid B inside the secondary valve body housing.. Ive replaced pressure control solenoid B itself and now ive replaced the TCM. I don't think it would make a difference but im running redline d4 atf...
Thanks.
Hi ovalnut, sorry for the late reply, I'm just now getting my fusion back up and running. Glad to know I'm not the only one working on this problem.

Last weekend I finally got the main transmission pan back on (what a fiasco that was) and the transmission filled up with fluid. I left the original TCM in and ran it for a quick trip to town and back with no codes. Yesterday I ran a longer trip (over 40 miles on the interstate) with no problems. I'm going to attempt to commute with it tomorrow for a longer test (~150 miles round trip).

I don't really know why it's working now although I do have a few theories but no guarantee it's fixed long term. Here's all what I've done since I originally got that code.


  • Drained transmission and refilled with Idemitsu Type M from amazon. Put a total of 6 quarts and am still a little low. Side note: when you refilled your transmission did the transmission pump make a horrible sound for 15 seconds or so? I assume it's because I had my pan off for so long I drained at least 6 or 7 quarts so it was dry until it pulled in the new fluid.
  • I plugged and unplugged several times the connector on the transmission housing that goes to that solenoid when doing the diagnostics.
  • Completely removed the original TCM and left it disconnected for several weeks.
I doubt the transmission fluid had anything to do with it although the shop that serviced it last may have put Mercon V in it. The fluid for this transmission is confusing as hell. From what I read, you need something that meets the Mazda M V spec and Mercon V doesn't so it won't last long (been about 20-30k since mine was last serviced). That shop I suspect is also responsible for cross threading some of the bolts on the pan which is why it took me a while to figure out the right replacement bolts and get them installed. I've read online where others have used the redline D4 so I think you are ok there.

Plugging and unplugging that connector might have fixed it if the connection there was not good. There was a good connection internally since I was able to ohm out that solenoid and it was in spec.

Leaving the TCM disconnected for a few weeks means it completely discharged so that could have reset something to make it work again. Since you replaced your TCM, I don't think that is your problem. Question: did you have to do anything to your ECU when you did the swap? I figured the TCM was keyed to the ECU so you would have to reprogram the ECU. Oh I see now you said it was programmed to your VIN when you bought it.

In your case, you might have a wiring problem from the TCM to the solenoid connector. Or it could be possible that your solenoid doesn't have a good ground.

Here's what I suggest you do:

  1. Ohm out the solenoid from the transmission housing connector pin to ground. I will have to check the manual again but it should be between 2 and 7 ohms. If that looks good, then your solenoid and internal wiring should be fine.
  2. Disconnect TCM and measure continuity from the solenoid drive pin on the TCM connector to the other end that goes to the transmission housing.
  3. Check for short to ground or short to voltage on that pin.
If you need any help with those tests let me know and I can try to walk you through it.
 

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Discussion Starter #14
Did you perform the diagnostic tests on Pressure Control Solenoid A?
No. The code is for pressure control solenoid B. PCA is on a completely different valve body so no need to test it.

I never did get around to checking my mother-in-law's fusion to see if that same pin is driving high with ignition and car off. Since mine appears to be working (for now) I will check it again after a few round trips with no problems. Although I'm worried that might break it again knowing my luck :surprise:
 

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Mine was working decent for a couple days and then yesterday it goes into a fit, wrench light/limp mode... p0778, every time I hit 5th gear or downshift from it. This is the same behavior it had before I started throwing parts at it unfortunately. I would think it has to be a short since its so intermittent. Im going to look over my harness again today maybe ill find something, It would be a lot easier to test if it was more consistent. Anyway, ill post back with any revelations. Hopefully someone who has dealt with this will stumble onto this thread.
 

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Mine was working decent for a couple days and then yesterday it goes into a fit, wrench light/limp mode... p0778, every time I hit 5th gear or downshift from it. This is the same behavior it had before I started throwing parts at it unfortunately. I would think it has to be a short since its so intermittent. Im going to look over my harness again today maybe ill find something, It would be a lot easier to test if it was more consistent. Anyway, ill post back with any revelations. Hopefully someone who has dealt with this will stumble onto this thread.

I posted a long reply to you yesterday with the latest on my troubleshooting and some things you should check but apparently it has to be approved. I guess the post was too long or something.

I just drove in to work over 75 miles (long commute on a new job until I move) without any problems. I was worried transmission fluid was a little low but checked it when I got here according to the manual and it's just below the top full mark after putting in the 6 quarts.

It definitely sounds like you have a wiring problem somewhere. I mention a few things to check in that post once it gets approved.
 

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Hey, Thanks for the in depth reply. Ill try those 3 troubleshooting tips this weekend and post back with the results.

Question though, doesn't the 2 solenoids in there share a common ground? Wouldn't shift solenoid (F i think-5th gear) also be throwing a code if i had a bad ground? Out of curiosity, I removed the connector on top of the housing, turned on the car, checked the codes and sure enough i got shift control solenoid f electrical and pressure control solenoid b electrical. That was before i decided to pull off the secondary pan and replace the internal harness. If there is still a short inside the pan still I have no idea how. I inspected the connections thoroughly when i installed the new harness I thought... ill test it

Another Question, how do I identify the pcb pin on the tcm side of the harness?

Like you I also removed that connector on top of the housing and plugged it back in several times, wiggled it, twisted it, etc but it didn't seem to help. I agree that it almost has to be a wiring problem based off its behavior, it could simply be the lone wire feeding PCB. At this point it almost seems like the wire is partially broke in the harness somewhere

Oddly enough my car is back to normal for the last three days, when i say its back to normal I mean no p0778 code. It does seem to shift a little hard going into 5th still..maybe I need to try the relearn again since I haven't been getting the code lately. idk.. This whole deal has me baffled, seems like we're the only two fusion owners on earth with this problem.

I see you already answered yourself about the TCM, but yea a guy on ebay programmed it to vin. I was surprised the one he sent had the oem sticker on it and all, I was impressed.
As far as the transmission pump whine, yes i had it but only briefly. I've just been doing drain and fills though..
 

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Discussion Starter #18
Yeah the solenoids share a ground so it is unlikely that is the problem. It definitely sounds like a short in the wiring harness somewhere. Very odd indeed that we are both having the same problem. Maybe there is some spot on the wiring harness that is rubbing on the firewall or is getting too hot from the exhaust?

You can ohm out that solenoid if you want (one probe on the pin on the transmission connector and one probe to chassis ground) but it likely is fine since it works on and off for you.

The best way to find the short in my opinion is:

1. Disconnect TCM and connector on transmission
2. Put one probe on wiring harness PCB pin from TCM side
3. Put other probe on wiring harness PCB pin on transmission side
4. Set multimeter to continuity test so it beeps as long as there is a connection
5. Flex harness and listen for beep to drop out

That should help to find the short if there is one. Easier said than done since you will need some long probes and way to keep them on the harness pins (or find a couple of helpers).

I will lookup the pin number on the TCM side and get back to you. An easy to find it is connect your probe to PCB on transmission side and probe TCM connector pins until you find it.
 

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That should help to find the short if there is one. Easier said than done since you will need some long probes and way to keep them on the harness pins (or find a couple of helpers).

I got a wife ha..maybe i can make some jumpers though, my probes are definitely too short. Tomm ill try it out
 

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Discussion Starter #20
Pin 15 is the PCB pin on the TCM side. If you send me your email on private message I can email you some pdfs with the connector layouts and some other info.
 
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