This article is about how important it is to troubleshoot the electrical devices used to shift today’s transmissions correctly. Now, don’t stop reading just because I mentioned electricity: We’re not going to talk about Ohm’s Law or try to get you to use fancy meters or oscilloscopes. There are plenty of magazine articles and textbooks that already cover this subject very well.
What I notice is that a lot of guys who are new to this industry don’t start to learn about electricity until they run into a problem. I can’t blame them: “If it ain’t broke, don’t fix it.”
But, in reality, the best time to learn about electrical circuits is by testing circuits that are good. That way you know how they’re supposed to work. The following examples that I’ve come across may help guide your own troubleshooting… when electricity goes bad.
Let’s take a look at the basic, on-off electrical circuit. Remember: these types of circuits involve a load device such as a light bulb, a fan, or a transmission solenoid. And they typically include a power source, a switch, and a ground, which make the device work.
The textbook examples of a bad circuit include several scenarios, such as an open or a short circuit or a continuity problem. In the real world, it usually comes down to the device or on-off switch being bad; there’s no power or ground. And while you could spend a lot of time and effort tracking down the specific problem, very often, it’s not necessary.
Let’s take a look at a few examples and you’ll see what I mean:
A general repair shop next door had a customer with a 1996 E250 van equipped with a handicap lift and modified driver controls (Figure 1). I noticed that the tech had been working on this van for a few days. One day after work, he told me the story. The issue was that it wouldn’t pass the smog test because the check engine light was on.
He started in the right direction: He recorded and cleared the code. During a test drive, he noticed that it drove fine until the check engine light came on; then, it started shifting hard. He found the same VSS code in memory. He then verified that the battery, grounds, and fuses were all okay. He also found out from the customer that the dash and parking lights didn’t always work.
At this point he pulled the dash to check the circuits at the PSOM (Programmable Speedometer Odometer Module). Since the power and ground circuits checked out, he assumed the PSOM must be bad. He installed a used dash from a salvage yard. The owner picked up the van and returned the next day with the same problem.
The tech told me that the problem was erratic and seemed to be related to driving with the lights on. Sure enough, the wiring diagrams seemed to indicate a possible relationship between a common “joined in electrical lingo” voltage feed to the PSOM and the headlight switch.
So at this point he installed a new headlight switch and new turn signal switch, and was seriously considering ordering a new dash assembly. I couldn’t help but feel compelled to help him — not just for his sake but also because I wanted to see the handicapped owner get his van back.
The wiring diagrams showed a relationship between fuse numbers 1 and 5 in the dash panel fusebox. Number one provided the battery feed to the PSOM, brake switch, turn signal switch, and ABS module.
Fuse number 5 was an ignition feed to the transmission range sensor, turn signal switch, OD cancel switch, daytime running light module, and some other circuits. He decided that one of the switches was shorted, which caused a voltage drop before the PSOM.
“WOW,” I said, “that’s amazing.” “Yeah,” he replied, “it would be if I could find what’s doing it.” I offered to help him for a few minutes. He’d been showing me the wiring diagrams to support his theory. I noticed the headlight switch hanging from under the dash.
The first thing I did was check for battery voltage from fuse number 5 to the headlight switch. With the ignition off, there was no voltage. When the ignition switch was on, there was only 8 volts. I think this is what had him confused.
Clearly, a battery fuse circuit should always have voltage, whether the ignition is on or off. To be sure, I got similar results when I checked for voltage at the fuse side of the brake switch.
Since he said he checked the fuse, I was getting suspicious of an open between the fuse and the circuits it fed. So I pulled out the number 5 fuse. Lo and behold, there was a broken terminal on the feed side of the fuse (Figure 2). He’d probed the fuse test sockets on top of the fuse, and it tested good. I don’t know what he charged his customer for the new fuse, but I haven’t seen the van back again.
This next example involves a classic case of how a snap diagnosis can benefit a shop owner who was willing to diagnose the real problem with some basic troubleshooting skills. I was asked by the owner of a shop to take a look at a 2000 VW Jetta that occasionally set a VAG 00652 slip code.
The owner was told by the first shop that he needed a transmission because it was setting a slip code. The vehicle owner was skeptical: He wanted a second opinion from another transmission shop. The owner of the second shop told me the transmission worked perfectly until the computer system went into limp mode.
As soon as they cleared the slip code, the transmission would drive and shift great. These symptoms didn’t seem to indicate the transmission needed to be rebuilt. Even though the TCM hadn’t set any other codes, we did a wiggle test on the OSS (Output Speed Sensor).
All of a sudden an OSS code set. A new OSS fixed the problem, and it cost a lot less than a complete transmission rebuild. Remember, just because there’s no a trouble code doesn’t mean electricity can’t go bad.
Let’s take a short review of computer-related troubleshooting: For a computer to control a transmission, it needs sensors to monitor the physical conditions the vehicle is under at every moment. Unless a code is set for one of the sensors, a lot of techs seem to ignore the potential problems a bad sensor can create.
Remember, the speed sensors, temperature sensors, pressure sensors, engine load sensors, driver inputs (Tiptronics), and more supply vital information for the computer to control shift timing and shift feel.
This final example illustrates what sort of extreme situations you may come across. I was asked by a friend of a friend (always a bad sign) to fix a 1996 VW that seemed to be in limp mode. The complaint sounded like the transmission only drove forward in high gear and would back up okay in reverse.
An initial check revealed several engine codes. It was hard to believe the engine would even start and run. Someone had wired the fuel pump to run all the time; otherwise, the engine wouldn’t even start.
Since the engine was able to run, his concern was getting the transmission to shift. As the story unfolded and I spent several hours diagnosing, troubleshooting, and testing circuits, I concluded that this was going to take a lot more time, money, and parts to fix than it was probably worth.
Here’s why:
The previous owner had replaced the engine with a used one (Figure 3). My friend’s friend bought it this way and assumed some minor thing was causing the transmission not to shift right. The engine and transmission looked like they were all there and seemed to have the wiring connected to the usual sensors. In most typical situations with multiple codes, the logic is to first solve the circuits setting the lowest number code.
One of the lowest codes present is related to the ECM power circuits. This made sense because someone had forced the fuel pump relay to work all the time. It should have taken only a few minutes to verify the ECM fuel pump relay control circuits were good. At first, I wasn’t too concerned that none of the wiring diagrams matched the car’s wiring. “No big deal,” I thought, “it’s a Volkswagen.”
To my amazement, I couldn’t find any wiring diagrams from any model or year Volkswagen that matched this car! To make a long story short, we finally realized that the engine had come from a 1995 Volkswagen. A few of the sensors (TPS, IAC, Idle Switch, wiring, etc.) were different between a 1995 and a 1996 VW. Since the 18-pin engine harness plugged right into the main wiring harness near the distributor (Figure 4), both friends assumed it was going to work. It’s been over a year, and as far as I know, the owner still hasn’t decided what to do.
I hope these examples help illustrate that when it seems electricity has gone bad, fixing the problem may not require advanced diagnostic strategies. Sometimes you can make it right by using simple logic and knowing what it takes to make it good.
