I’ve always loved cars and knew from an early age that I wanted to be a mechanic when I grew up. Well, I haven’t grown up, and I’m not a mechanic; I’m a Professional Automotive Technician. I’ve been fortunate to have been able to see the advancement of automotive technology over many years. The evolution and integration of computer-controlled components and the speed and accuracy that comes with it has and will continue to change the challenges of diagnosing these systems.

How we go about diagnosing electrical and computer problems in transmissions depends on what rolls in the door, but the same basic strategies can be applied to just about anything with four wheels and a battery. That being said, you have to take into consideration your resources, capabilities, tooling and experience to know what you can make a profit on.

The first step is performing an initial evaluation on the vehicle. Does it start, run, and move? Everybody does this differently, but every evaluation should include a fluid-level check, a visual inspection and a test drive with a scan tool. I like to take movies of the scan data in case I need to go back and look at anything; volumetric efficiency comes to mind as an important one.

OK, so you got paid for your first half-hour for your professional opinion based on the limited information collected during that time. So, what else do we need to know? Won’t move, full of burnt fluid at 175K miles, needs a unit, very straightforward. Need to do pressure tests or electrical evaluations? That’s different. Got a 2008 BMW 550i that intermittently has the transmission go into limp mode and has low-voltage and communication codes in just about every module? Off to the dealer it goes – with the aftermarket wet-cell battery that wasn’t registered to the vehicle. Got a 2000 Dodge Ram 5.9 gasser with a code P0753 / 3-4 shift solenoid circuit problem? I’m all in. How about a 2006 Cobalt with intermittent TCM communication issues that comes from another very good shop that wants a second opinion? I’ll swing a bat at that because I’ve got plenty of resources to help me deal with issues like that.

So what are your resources? OE and aftermarket scan tools, service information from various providers, information from professional trade groups, and personal experience in the field to name a few. I have access to some OE scan tools and reprogramming, but BMW isn’t one of them. I also have your standard service information providers, and it seems a lot of information is very limited on European vehicles. That’s why the BMW left and the old Dodge truck and Cobalt stayed. Everyone has strong and weak points; take advantage of your strengths and try to learn more about your weaknesses.

Let’s talk about this fine 2000 Dodge Ram with the P0753 code / 3-4 shift solenoid circuit issue. I spotted some questionable repairs on my initial inspection (Figure 1).

I recommended two hours of diagnostic time. I’m pretty used to seeing bungled-up wiring, but it’s usually caused by rodent damage or bad installs; I wasn’t sure where this diagnosis would lead me. The duct tape on the TV cable was an interesting touch. Yes, duct tape.

The code was set for a specific circuit and I had a good idea where I would need to go with this. One of the first things I considered was that this was a circuit code as opposed to a performance code. If I have a circuit code, the controlling device has detected an electrical issue, such as a short or open, rather than a failure to respond to a command. Failing to respond to a command would usually result in a performance code. The module in control doesn’t see the expected results from the command but sees no issues electrically. The computer can be applying a clutch pack, VVT solenoid, fuel injector, or whatever, and it knows what changes it wants to see under certain conditions. This will help to lead you down the correct diagnostic path. If I have a circuit code, I’m busting out the electrical diagnostic gear. If I have a performance code, I may be checking pressures or other data on the movie I took while on my initial test drive.

Spoiler alert: It’s another Chrysler with a bad PCM. After seeing the multiple butt connectors on the injectors (yes, butt connectors), I was really thinking wiring damage was part of the problem but I went after what the PCM was seeing to set this code. The PCM is seeing the wrong voltage on the 3-4 shift solenoid circuit when it’s commanding the solenoid on or off. On this particular transmission electrical diagnosis is fairly easy, as there only a few electronically controlled devices: the pressure control solenoid, TCC solenoid and the 3-4 shift solenoid. Looking at a schematic in (Figure 2), we can see B+ supplied to the transmission from the transmission-control relay to the solenoid pack at pin 1. If I had an issue with the power supply to the solenoid pack, I likely would have seen codes for the other solenoid circuits or transmission relay stuck on or off. Ground for each circuit is provided by the PCM, so I can easily check total circuit resistance by removing the transmission control relay and measuring the resistance between pin 87 of the relay and the control pin at the PCM. I had 28 ohms. Spec for this circuit is 20 to 40 ohms, so I felt I was good there. Easily from there, I can check for a short to ground (which there wasn’t), or to power.

Now, with the transmission being in limp mode, it shuts off power to the transmission control relay, so you will have to supply power to that circuit by activating the relay with a scan tool, or by using a fused jumper wire or relay bypass and check voltage at the PCM control wire. With power supplied to the circuit, I had close to no voltage at the PCM control wire when the solenoid was commanded off, where I should have close to B+. Unplugging the connector to the PCM gave me a reading of battery voltage at the PCM connector, proving the PCM had an internal short on the 3-4 shift solenoid circuit. A used PCM and a TV cable (remember the duct tape?) and this one is out the door.

By the way, the story behind this particular vehicle is a comedy of errors, so to speak. A young kid bought this truck knowing it had a transmission problem, stuck in limp mode. He’d taken it to two different shops and was told it needed a solenoid, or some other misinformation that I don’t know. He purchased a new transmission, installed it and had the same problem as before. It took less than a half hour to reach the correct conclusion as to what was wrong with it, using proper diagnostic techniques.

What about this ’06 Cobalt with intermittent TCM communication issues? We’re going to fix this one by using a different approach. Resources and experience is going to be the key on this one.

We had a very good wholesale customer asking us to take a look at this Cobalt in which the transmission would go into limp mode intermittently (several times a week), and then be OK after the key was turned off and restarted. The shop had already replaced the TCM and flashed it to the latest calibration.

I’m a big fan of the IATN website, (International Automotive Technicians Network), and have used its database, waveform library, and forums to expand my knowledge and help me gather information about a lot of issues that I can use in my everyday routine. If you have Identifix, there’s a link to IATN on the home page. Some time ago, I had come across a thread in the Technical Discussion Forum about the use and issues that aftermarket devices such as insurance company tracking dongles, remote start systems, etc., can have on communications, drivability and transmission operation on GM vehicles. It gives a pretty detailed list of codes and symptoms that can be caused by such devices. If you want to view it, it’s in GM Tech Connect from Feb 11, 2013.

Anytime I get a vehicle in with communication issues relating to the transmission, one of the first things I look for are any added switches, LEDs, or anything that would indicate a non-OE device has been installed. Another good thing is to take a quick peek under the trim panel below the steering column. If you find something like this (Figure 3), you might want to remove it and see if your issue goes away. On this Cobalt, I found a wire tapped into the hi-speed CAN circuit that was intermittently causing a communication error. This picture isn’t from the Cobalt (clutch pedal?) but gives you an idea of what to look for. And while you’re looking, be especially wary of those dang Scotch-Locks that seem to be so popular. Once you cut into a wire, the damage is done and it can be very difficult to trace a wiring issue down after the fact.

Expanding your knowledge base and resources is often crucial to your success and sometimes doesn’t cost very little if anything at all. Talking with other shops, technicians, networking with other professionals through electronic media, and sharing with others is almost necessary today to keep at the top of the pack.