Greetings from the pre 97 side!

I have a question about my recently acquired 1998 F150 4X4, 4.6, 4R70W. It already has about 275K km on it and runs reasonably well most of the time.

I am having an intermittent problem that sets code P1747. The definition I find for this code is a short circuit in the EPC solenoid circuit. I have read many threads on this and I have checked the common trouble spots like the transmission electrical connector plug. To date, I have not found any damage or wear.

So, please allow me to explain that I have a very strong electrical background and I do understand the EPC circuit function. I am not nearly as well versed in transmission function and OBDII diagnostics, but I have an ELM 327 Bluetooth OBDII device and Torque on my Android phone and tablet. I also have an OBDII reader on a cabled dongle. I have internet diagrams of the 98 F150 electrical system, but not the coveted EVTM.

So, the problem becomes how to diagnose the relatively simple EPC circuit. My overly analytical mind sees two legs of this circuit on either side of the solenoid itself and 3 possibilities of a short circuit. Either + or - power, or a short within the solenoid coil itself. Depending on where such a short may be in the circuit, a variety of symptoms may occur including a blown fuse, or no control over EPC pressure, or even an inoperative EPC. I find myself considering the much more likely possibility of an open circuit which would result in no control of EPC pressure and possibly other symptoms again depending on where such a fault may be. I lack the necessary map to actually
​​​find these circuits in the vehicle and I am not anxious to dissect the wiring harness.

So, the symptoms I have noted is that the vehicle had a report of this fault when I bought it, but I inspected the transmission wiring harness, cleared the codes and the fault did not reappear for a few weeks. There is an apparently unrelated issue with a leaking fuel injector causing the fuel rail to depressurize when hot with the engine off, resulting in hard starting, (long crank times). The latest occurrence of the P1747 code was immediately upon startup in warm conditions. The code set immediately and would reset after being cleared when the truck was placed in gear. Driveability was terrible and concurs with what I read on threads of the same subject. The transmission would actually select gears manually and this did seem to improve driveability somewhat. After a cool down period, the code was cleared and has yet to reappear with limited operation.

The fuse supplying power to the transmission solenoids has not blown and I interpret the ability to manually select gears as an indication that this power does make it to the transmission. I have not ruled out a faulty internal wiring harness in the transmission and due to the degree of difficulty, I will likely save such a test for last. The remaining possibility is a fault either open or short in the leg between the EPC solenoid and the ECM. As I understand it, the ECM provides a variable current sink to ground to operate the EPC and that would be measurable on the ECM side of the circuit as a variable voltage pulled to battery + through the EPC solenoid coil. Full 12 volts anywhere in this circuit along with 0 volts at the ECM would indicate a break in this circuit and that frankly is what it expect will likely be found, but I would like to know wire colors, connectors and locations within this circuit.

I hope you will excuse the lengthy description, but I hope to put all necessary info up front. I did find many threads on the subject, but no fixes. I also can consider the possibility of a failing ECM, but that too is a check I can save for last.

One final note on OBDII diagnostics, is there any reader or procedure that will monitor the EPC signal output from the ECM? My meter would be my choice, but this is a difficult point to access. Also wondering if there is any monitoring of EPC pressure. I'm sure I have heard of a test port, but I was hoping this could be done electronically without plumbing in a gauge.

 

The ECM monitors the EPC solenoid voltage and current signature loading across a sampling element to infer its operation.
From the connector into the transmission valve body is a wire or ribbon harness. It could be that harness or the EPC Solenoid.
You need to drop the Pan and either find a harness fault or replace the solenoid valve.
The code is very specific due to the system ability to see the nature of the fault.
Power is applied to all from fuse #23.
The signature is looked at on the ECM signal ground side in the ECM.
The self diagnostic resolution designed into the system can detect Short, Open and Intermittent operations and report as separate codes.
.
If a short in the primary power that blows fuse 23, nothing in the trans operates as well as other circuit that use that same power source.
The result of that would be many codes that seem unrelated until you would see they are fed from the same source.
Good luck.

 

Thank you bluegrass7.

I did find a generic list that includes other codes related to EPC, so it would appear that P1747 is specific to a short. So much for my open circuit theory. Since the fuse did not blow, it would be reasonable to conclude that the detected short is between the EPC solenoid and the ECM. The greater likelihood is that it would be a short to chassis and if I am correct, that would cause full current through the EPC solenoid with a corresponding effect on internal transmission line pressure. High or low, I do not know, but this pressure would now be unregulated and all the related driveability symptoms follow.

I still need to identify the physical path of this portion of the circuit. The transmission and ECM connectors are easy enough to identify, but what lies between? I did suspect there may be multiple harness connections in this path, but a failure there would more likely be an open and I already determined the transmission connector has not deteriorated. So the hunt leads me to the possibility of chafed wiring, which I have not yet detected. I did read one potential trouble spot is behind the right hand cylinder head, but I inspected this area and found no damage. Anywhere else to look?. If I can't find anything external# next step will be to open the transmission and replace the harness and EPC solenoid. That is a task I will farm out and I don't want to have to do it twice.

Thanks again for your help.

 

Thought I would update this.

Still chasing a very intermittent P1747 code. I did manage to map out the EPC control path from the transmission to the PCM. It is ckt 925, white/yellow stripe and the path is through C172, which is the gray square 40 pin connector which connects the transmission harness to the main PCM wiring harness under the hood. It is paired up with the black 40 pin connector that carries most of the engine PCM connectors and both these connectors are mounted on the bracket that also holds the megafuses. This is located on the upper right side of the firewall.

So, now the problem becomes how to diagnose what may be wrong in this circuit. The PCM is smart enough to figure out if the circuit is OK, open, or shorted. Presumably, a short would be a wire bared or crushed and completing a circuit to the vehicle chassis. P1747 indicates a short and P1746 would indicate an open. Logically, this short must be located on the PCM side of the EPC circuit as a short on the battery side would result in a blown fuse 23. Since this remains intact, I am directing my attention to the PCM side of the EPC circuit. This path contains the transmission connector, (C183 pin 6), the PCM connector,(C174 pin 81) and the main chassis connector,(C172 pin 37). The circuit number is 925 and the wire is an 18 GA colored white with a yellow stripe.

What remains unknown is the nature of the signal from the PCM applied to the EPC solenoid. This is a variable solenoid, so the signal is more than a simple closure to ground. I suspect it may be a PWM switched ground which may best be viewed with a scope. PWM means pulse width modulated for those unfamiliar with the term. I do know that a short to ground would effectively make this signal a 100% duty cycle ground and I do not know what effect that would have on the pressure set internally in the transmission by the EPC solenoid.

I am becoming increasingly convinced that the problem is internal within the transmission and will require replacement of the EPC solenoid and/or the internal wiring harness. I likely will change both items along with a transmission service and fluid change. In the meantîme, the search continues. The P1747 code really kills driveability and it seems like the engine also misfires, but I was able to confirm the PCM remains in closed loop mode when the P1747 condition appears.

 

I have dealt with this p1747 code before, on one vehicle it was the EPC solenoid itself and on another it was a faulty PCM.

I dont have any test for you to try but I thought I would pass along my experiences with this to you.

 

Is it possible to bypass the OE wire in the harness and run a test wire from point A to point B? Perhaps that could eliminate some connectors and that OE length of wire as the possible culprit. Or maybe I missed that you have already done that, your posts are pretty long and mostly above my paygrade and sometimes I get lost going to the refrigerator.

 

Thanks jk080, PCM is on the list of possible suspects. I have been on the lookout for compatible hardware. Most of my Ford experience has been with EECIV hardware and they can become pretty application specific. OBDII hardware supports flash programming, but such capability is beyond what tools are accessible to me, so I have been keeping my eye out for a similarly equipped truck in the scrapyard. Biggest obstacle so far has been uncertainty about the transfer case controls, or in my case, the lack of as I have a manual stick. All of the other 4X4 that I have found have electric shift transfer cases. If my PCM proves faulty, I may attempt repair. 20 year old electrolytic caps start to fail and this has been well documented in the older ECM's.

Sam I Am, I apologize for the lengthy technobabble. What I have in mind is documenting a complete record should someone search this in the future. Hopefully, I will eventually include a resolve.

That P1747 code is a bit of a diagnosis problem. That PCM is outsmarting me as well by declaring the fault to be a short. The question is where? I actually snipped the harness out of a scrap truck and dissected it to find the path I described in the last post. Becomes quite a bit less intimidating once you get your hands directly on it. Your suggestion of a bypass wire is quite logical, but I am reluctant to take such a step. Reason is a wire is dead simple and reliable. Most failures can be visually identified and I have inspected the entirety of the path this circuit takes in my truck. Rigging a bypass would require that I cut and splice into my existing harness which would introduce potential failures in the future. Not my brand of troubleshooting.

Knowing that the circuit passes through C172 gives me the opportunity to open up the circuit and test segments. The main harness segment from the PCM to C172 is quite short and well protected. Disconnecting C172 and C174 allows testing for faults between pins 37 C172 and pin 81 C174 with an ohm meter. Similarly, you can test between pin 37 C172 and pin 6 C183 at the transmission. A faulty harness between C172 and the transmission can be easily replaced. This section contains the common faults of the transmission connector,(C 183) and the area behind the right cylinder head that can get crushed.

 

Or you could repin the connector.
Sounds like I have a higher than average aptitude for splicing wires, I have enough confidence in my ability to make it better than it was to start with.
But I totally get where you are coming from. I would much prefer to leave an OE wiring harness intact. And then again, I would much more prefer to not have to chase sparks at all.

 

or you could drop the pan as Bluegrass said last week and look at the harness / solenoid.

 

Absolutely. As I mentioned, I'm saving that action for last.

I have owned this truck for about 2 months. I have zero history before that and the friend I bought the truck from only owned it briefly before deciding the problem was over his head.

What history I have been able to gather suggests this truck does need a little love. I have fixed numerous small issues, replaced tires, shocks and battery and the P1747 problem was a known, but intermittent issue. What evidence I have to date does point to an internal transmission issue and the job of replacing the solenoid and wiring harness is easy enough to accomplish when you drop the pan. At 275K with evidence of apparent neglect, it's a pretty safe bet the transmission could use a flush and fluid change, but when the truck works, the transmission works w ll. The fluid is red and does not smell burnt.

I have been associating the intermittent driveability problem with the P1747 code. I have since discovered if I reset the code, the driveability problem remains and does not set any additional codes. The truck drove fine for 3 weeks without setting any codes. The P1747 code only appears when the truck performs poorly, yet it may not be the trigger for the poor performance.

I should have described the symptoms of poor driveability earlier, now that I have some history, I can. The engine starts and idles fine from cold. I described a hot start problem earlier and I have diagnosed that to a fuel pressure drop when the engine is shut off. Likely a leaky injector. Fuel pressure remains slightly above 30 psi under all running and driving conditions, even during the power loss. When the engine power drops off, it does seem to pop or stutter, but not something I would describe as a miss. I am watching various measures with my Torque app and the only odd thing is the O2 sensor readings bounce a lot. Wiring has been checked and no problems found. The engine barely has the power to reach and hold highway speeds, 100kph here in Canada. When running normally, none of this is a problem.

I am splitting the driveability problem from the P1747 problem. I have some concern that the code may not be the cause, but rather the effect. I want to solve the driveability problem before I pursue the code further. I am suspecting a possible plugged exhaust, but more investigation is required

 

This is a current controlled circuit. The PCM controls the amperage to the solenoid. It is not a PWM.

When the control circuit is shorted to ground the EPC is at minimum pressure. If I remember correctly, and it's now been over two decades since I worked on engineering OBDII systems when this code is stored engine torque is severely limited. It won't get out of it's own way. This is done because at minimum EPC pressure there isn't much capacity in the clutches to prevent slipping.

I've done that when finding the actual problem in the harness is difficult.

As I said above it's been quite a while since I worked on OBDII things, but to me this makes sense. The driveability is a result of the code.

I think that's from torque limiting from the P1747 code. The PCM has the ability to shut off cylinders to limit torque. A non-firing cylinder dumps a bunch of O2 in the exhaust each time the exhaust valve opens.

I think that's a mistake. I believe the driveability problem is caused by the P1747 code. The FMEM (Failure Mode Effects Management) protocol will limit engine power when the error (short circuit) is first detected, but it has to be detected for a certain amount of time to set the code.

I think it is impossible to solve the driveability problem without first fixing the P1747. Fix the code and you most likely won't have a driveability problem.

 

Thanks for your input Mark. Wonderful knowledge as always!

Sam, I think your repinning suggestion would work. I still am quite convinced that the wire from the PCM to the transmission is OK and this very intermittent short is in the transmission.

I am ordering up a new transmission harness and EPC solenoid. Replacement will be the next step along with a trans flush , filter and new fluid.

I am getting a better understanding of the sequence of events within the OBDII diagnostics. Thanks again Mark for your explanation of torque limiting. Makes sense.

So, exploring the idea of the problem still being caused by the P1747 code, I had occasion today to experience the brief engine blips that coincide with the power loss. The check engine light was off and remained off. I checked for codes and there were no logged faults. I checked for pending codes and there was my friend, P1747. The pending codes do not light the check engine light, but the logged faults do. I do not know the exact sequence which moves a code from pending to verified, but the truck can drive quite a while between these driveability episodes, which is quite in line with the intermittent nature of the problem.

I'll post again after the solenoid and wiring harness have been changed.

 

Well, transmission servicing was not the answer.

I have been able to accumulate about 5000 km on this truck over the past few months. It will work flawlessly for weeks and suddenly, the driveability problem returns. Just as suddenly, it clears up.

So, the question of what to monitor and where returns. As Mark pointed out, the problem is clearly associated with the P1747 code and the driveability problem is due to the torque limiting from the FMEM system. He also supplied an important clue here in that the EPC circuit is a variable current control with maximum current associated with minimum pressure. I know it is circuit #925 and the white wire with the yellow stripe carries it through C172 pin 37 from the ECM connector C174 pin 81 to the transmission connector, C183 pin 6. I also know all the transmission solenoids receive + battery on a common wire at C183 and are switched to ground by the ECM via individual wires via the C183/C172/C174 path. So, I used Sam's suggestion and unpinned the circuit #925 wires at C172 pin 37 and extended both wires into the cab of the truck where I arranged 2 meters to monitor current and voltage simultaneously in circuit #925. I found in normal operation, the current would range between 250 and 950 ma. Voltage would vary from about 7.5 to 12.5 volts with respect to ground.

Jk080 correctly predicted this fault could lie in either the ECM or the EPC solenoid and wiring. Visual inspection had confirmed the external wiring and connectors were intact. With the meters in place, I finally was able to monitor the electrical connections to the EPC solenoid. Today, the fault occurred and I hit paydirt! Voltage at this point increased to 13.9 VDC and current dropped to 0. There was no short, rather the fault pointed to an open circuit towards the ECM. Of course, the infamous P1747 code could be read as a pending fault and all the poor driveability symptoms were present.

I had found an ECM in the junkyard from a 97 F150, also equipped with the 4.6/4R70W/4WD combination. The part number and catch code were F75F-12A650-ZD DJJ3 as opposed to the F85F-12A650-MB FOU1 that was in my 98 F150. Of course, they were not interchangeable. The engine started, ran poorly and numerous ignition coil related codes were set. Switching back to the original ECM restored proper engine operation. The hunt is on for the correct ECM.

What to do now?. Of course, I checked the ECM wiring for ground integrity. The ground connection is quite robust. 5 pins are used, spliced into a single wire which is grounded at the firewall near the megafuse and drivetrain connector assembly. I took the covers off the ECM and found no visual damage. I reverse engineered the EPC control circuit which consists of a single unmarked device in a TO220 transistor package clipped to the heatsink frame. I reflowed the solder connections on this device, reassembled and reinstalled the ECM. A short test drive was successful.

This has been a very intermittent problem and I am not quite ready to pronounce it healed. I will report on any other developments as they occur. Hopefully my next update will resolve this.

 

It was the ECM. Reflow of the solder connections did not help. Likely an output device failure.

Found the correct ECM about a week ago. Installed it and has not missed a beat since. Kept the meter in line with ckt #925 and no more intermittent current loss.

To sum up, this case was not a short, rather it was a loss of the current sink through the ECM. I do not know why the ECM trouble code protocol would report this as P1747 , but it is what it is. Best troubleshooting tool was the advice here and thanks to all. Best place to measure was ckt #925 as described in my previous post. Meter will be coming out today and repinning C172 back to the original configuration. No wires were harmed!

 

Thanks for the detailed information in this thread. I too am chasing a p1747 and now think my next step is to replace the ECM/PCM.