Intermittent Short Circuit Diagnostics
I first got the call to program a TIPM in this truck. When I arrived the TIPM had been installed and the technician said that it didn’t need programmed and that I shouldn’t have been called out. I explained that I was charging the service fee so I might as well hook up the Micropod II and take a look. While the vehicle would start and run the TIPM had codes in it for the left and right headlamp leveling system that would not clear. Once I showed them the codes and the fact that this vehicle was not equipped with that system they agreed to have me program the module which in this case is a configuration procedure.
Once completed I checked all DTCs. I cleared all of them, started the engine and noticed a P0882 TCM power input low. I cleared them again and it did not return. When I headed up to the office with a no DTCs sheet printed the service writer told me how they had been fighting this truck for a few weeks and were happy to get rid of it. I asked what the symptom had been and he said a fuse kept blowing in the TIPM. They had already replaced “components in the transmission” and a few other things. I mentioned the TCM code and the service writer showed me a workorder with that same code. I explained that it had cleared but there was a good chance that it may not be fixed.
A week later I got the call to come diagnose the truck. It sounded like an intermittent issue. I usually dodge these calls as much as possible. I explained to them that if it took me four hours to get it to act up enough to test it that I would have to bill them for the four hours. If I drove the truck for four hours and it didn’t act up I would still have to bill them for the four hours despite my company’s guarantee that if we can’t diagnose it there is no charge. They were too far into this truck and needed to get it diagnosed so on the schedule it went.
I started the diagnostics by verifying the code was present and then doing some research on the code. P0882 Describes a fault in the power supply from the TIPM to the PCM. There is a relay in the TIPM labeled transmission control relay. This relay is used to cut power to the transmission so that it will go into limp home mode. The PCM monitors the high (circuit T16) and low side (circuit T15) of the relay for faults. If a fault is found the relay is shut off and the code is set.
Looking at the wiring diagram listed for this code it shows “electronics” in the TIPM that close the relay. This tells me that the TIPM is in charge of the decision to close the relay even though T15 appears to be a control wire from the PCM to the TIPM labeled “transmission control”. Since we knew we were also blowing fuse J18 which is inline on the high side of the relay I can assume that there is a short in one of the components down the line.
Since the code was “stored” when I arrived. I decided to clear it and hook up a 20 amp circuit breaker and an ammeter inline in place of the fuse and drive the vehicle. The code did not reset right away and I saw 1-3 amps steady on the ammeter while driving. When the engine was stopped and then restarted the code reset however it did not seem to trip the circuit breaker and I did not catch anything on the ammeter. I drove back to the shop and decided to check the circuit for intermittent shorts.
The wiring diagram looks a little intimidating showing that circuit going to splice S136. Of course, there is no info listed in the aftermarket or OEM service info for S136. The nice thing about the OEM service info is that it will also list numbers for any additional diagrams in this case 8W-13-11. This diagram breaks down the splice a little better.
This truck has a gasoline engine and a 42RLE transmission. Looking at that diagram I was able to see that this circuit goes to the PCM, the transmission line pressure sensor, and the solenoid pressure switch. I decided to disconnect C100 and check the circuit for a short to ground on pin 25 at each side of the connector. The Powerprobe gave me an audible beep telling me there was ground on that circuit on the connector end opposite the relay which really only isolated the wiring back to the relay. I unplugged connector C4 at the PCM and found no change. I unplugged the line pressure sensor and the solenoid pressure sensor and still no change. Finally, with the Powerprobe still connected, I grabbed the harness where it runs behind the engine and on top of the transmission and was able to wiggle it and get the beep to stop.
Normally I would make the repair as part of the diag but because this short was in a very inaccessible place I showed the technician and service writer what I had found and explained that they can remove the transmission or the harness to repair the wiring or just replace the harness.
I got a call a few days later that they had removed the transmission and wanted me to do the wire repair. When I arrived the cross member for the transmission and transfer case had been removed so that the transmission was lowered and some sensors unplugged. I removed the rest of the connectors and pulled the harness up to repair it. Here you can see the damage to the harness where it had been rubbing on the top of the bellhousing. Once repaired and the harness reinstalled the code cleared and has not reset.
I shared this because I do a LOT of short finding. It seems like shorted wiring is thought to be this mysterious undiagnosable problem to some technicians. I suppose I felt the same way a while back but especially in this case it can be pretty simple. Using things like circuit breakers which you can find for older vehicles in a variety of amperages can help to keep from having to replace fuses and a Power Probe or a voltmeter with an audible tone can make intermittent shorts easy to find as well. The Power Probe’s uniquely irritating tone also helps streamline the diagnostic process by motivating the technician to stop the beeping as quickly as possible. This is also a great example of how to use clues like a fuse blowing in conjunction with a code description to isolate a fault to a specific area or circuit.
Service information is also key. Manufacturers are also starting to include better wiring harness routing diagrams that can help in determining where a circuit may run throughout the chassis.
One important thing to note is that many modern vehicles will disable a circuit when a short is determined even before a fuse will blow. This is very common on circuits like turn signals where aftermarket lighting or trailer harnesses may cause the module to shut down the circuit. In this case a code will set letting you know that a fault has been detected and the circuit disabled. It is important to check for these codes prior to performing any diagnostics rather than spending time trying to determine why there is no power on a circuit that you suspect should have power.