Three import case studies

Jan. 1, 2020
Case study No. 1 is a 2001 Mazda Protegé that had a bunch of transmission P07XX Diagnostic Trouble Codes (DTCs) and a problem with a hard shift.

Case study No. 1 is a 2001 Mazda Protegé that had a bunch of transmission P07XX Diagnostic Trouble Codes (DTCs) and a problem with a hard shift. This vehicle uses a Powertrain Control Module (PCM) that controls both the engine and transmission functions. Using our most important tool (our head!) we know when there are so many DTCs, the first thing to check is the PCM.  

Not an every day occurrence, but not uncommon either. Failed seals can allow fluids to wick up wiring harnesses for a surprisingly long way.

With the covers hiding the PCM removed, we were surprised to see transmission fluid oozing from the wiring harness leading up to the control module. Our next step was unplugging the PCM connectors to see if there was any fluid there. We weren’t too surprised to see the connector pins coated with the red stuff and even found traces of fluid inside the module! Sensors and output devices, both on the engine and in the transmission, are very low current devices. It doesn’t take much to add enough resistance to make them act funny or even fail completely.

And fluids entering the wiring harness are also not unusual. A single failed seal at the point where the harness enters the transmission is all it takes on some models to allow the fluid to wick its way up the harness. Other common sources of fluid damage to wiring come from coolant leaks (in or outside of the cabin) and water leaks (from the evaporator drain or failed weather seals allowing rain water in).

The fix for this one? The Rav4 needed a new PCM and we cleaned up the wiring and repaired the source of the leak.

Case Study No.2 is a 2002 Nissan Maxima with electronic motor mounts. This vehicle came in to the shop with a Malfunction Indicator Lamp (MIL) illumination caused by a motor mount problem. The shop that was working on this vehicle used the “plug and play” diagnostic method, replacing the PCM only to get the same results…another blown PCM. Perhaps they should rename their method  “plug and pay”!

A professional remote starter installation – or maybe not.

Remember that the PCM is the most expensive fuse on the vehicle,  so don’t guess why it failed but test and know why it failed! The shop decided to install yet another new PCM thinking the used PCM they tried the first time was the problem but they yielded the same results. After smoking a few PCMs, they knew that they were in over their heads. They decided to call TST board member Rich Peterson, who is a mobile technician who just diagnoses and repairs electrical problems.

Rich told me that he had not seen this electronically controlled motor mount system before. Reading the description in AllData provided some information on the operation of the electronically controlled motor mounts, covering only the electrical side of the system. After studying the wiring diagrams, Rich learned that each mount had two separate coils, one coil to activate “hard” mode and one coil to activate “soft” mode, with a mount on each side of the engine. Battery power (B+) is fed to each coil and drivers in the PCM independently ground the coils. Since the problem was only on one side of the engine, it was easy to use the comparison test by measuring the resistance from the known good coil and comparing it against the bad one.

Using an ohmmeter to measure the static resistance of both, Rich found the set of coils on the good mount to be 75 Ohms (baseline) while the other set read 7 Ohms (too low). Looking at the test results revealed the obvious problem; low resistance in the shorted coils caused a high amperage draw that resulted in the fried driver in the PCM. The defective mount was replaced and a new PCM was installed, solving both the mount problem and the shop’s headache.

Can you believe what you see in this photo? I’m sure most of you have seen the same.

Case Study No. 3 is on a 2007 Infiniti G35 Sedan that came in with a complaint of the horn blowing at random times. The first thing I like to look for after performing some basic checks is the addition of any aftermarket devices installed on the vehicle. Since this vehicle came from a used car lot there was no telling what was done prior. In this case, the vehicle had a remote starter installed. This system is the most likely cause of the horn blowing complaint since all of the components involved with the horn circuit checked out. Without having information on how this system operates, and with the car lot’s permission, we disconnected it.

As we were taking a careful look under the dash determining on how best to proceed, we found wires that had stripped insulation with wires twisted to open copper strands. As any good technician would know this is not the recommended procedure for making electrical connections or splices! There was only one wire that we found that was properly crimped and heat shrunk. We disconnected the remote start system and repaired all the wires using heat shrink solder connectors. No more reports of a blowing horn.

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