A simple concern and simple circuit leads to an unexpected and unusual cause.
I have trained and placed quite a few technicians in the past six years, and up to this point, I've managed to place all my graduates in dealerships or independent shops. I like to see my guys do well. They call me sometimes for advice, sometimes to tell me about a particularly interesting job they did or just to say hello.
I've had a lot of good students, but some of them really stand out. Jimmy Batchelor is a natural, a great example of a young technician who has the mettle to become a Top Gun tech in just about any shop. He's in his early 20s and is doing quality service work in a Ford dealership. He's a great investigative troubleshooter and does his work quickly, efficiently and with great interest and energy.
He recently called me about a situation he ran into on a 1997 Town Car with the overdrive light illuminated. It was an interesting story that I'll let him tell you in (mostly) his own words.
No Overdrive Town Car
I just recently became the lead transmission technician at a Ford dealer in Enterprise, Ala., after the previous transmission tech was promoted to shop foreman. After a couple of successful transmission overhauls, I finally got the first (and inevitable) transmission ticket that slapped me around. It was a work order that said the transmission wouldn't shift into overdrive (O/D) on a 1997 Lincoln Town Car.
I started the Town Car and found that the O/D light on the instrument cluster was illuminated. I tried to turn it off by pressing the button on the end of the shift lever Transmission Control Switch (TCS), but the light stayed on. The TCS is merely a momentary trigger that lets the Powertrain Control Module (PCM) know whether the driver wants to let the transmission shift through all four gears or just three gears. It has no direct connection to the transmission; it simply sends a 12-volt signal to PCM pin 29. The PCM illuminates the O/D OFF light on the PRNDL stick or the cluster and doesn't let the transmission shift into O/D.
I test-drove the car and found, as I expected, that it wouldn't shift into overdrive.
On Ford vehicles with the TCS in the end of the shift lever, it's fairly common for the TCS harness (which passes through the hollow gear selector lever to a small connector above the steering column) to chafe somewhere near the shifter and short out, because it moves around a lot due to the driver's operation of the PRNDL stick.
While this condition normally blows the fuse, it also can cause the O/D light to illuminate when the O/D OFF light is located in the gear selector handle right by the TCS. The O/D OFF light on the Lincoln, however, was in the cluster. With a possible short circuit in mind, the first thing I checked was the fuse feeding the TCS. It was OK, but that didn't mean the wires hadn't suffered the chafing failure. Intermittents abound in this type of situation, but I couldn't duplicate the concern with any movement of the PRNDL stick.
Next, I removed the steering column cover and checked the TCS wire harness for physical indications of chafing, but there was no evidence of it.
I decided to see if the switch was working by connecting my ohmmeter to C2005 (the TCS connector by the steering column) since it was easily accessible. The switch checked fine with the ohmmeter (continuity with the button depressed).
My next step was to check for shorts to ground or between circuits in the switch harness, but I found no problem there.
Disconnecting the PCM, I checked the circuit from the switch to the PCM. I checked for shorts to ground and power, shorts between the wires and open circuits, but I found nothing out of the ordinary.
I noticed as I was doing my circuit checks that the speed control deactivation switch on the brake master cylinder was leaking. Ford has a recall in progress (Field Service Action 05S28) to replace thousands of these switches due to a fire hazard presented by brake fluid in the connector, but this car wasn't part of that recall, according to the Ford Web site. Mustangs and Taurus models generally have a brake pedal travel switch for speed control deactivation instead of a hydraulic pressure switch.
I have developed a habit of checking speed control deactivation switches because these pressure switches are prone to leak brake fluid regardless of whether or not the vehicle is covered under the recall. More about that later.
There was only one other connector between the TCS and the PCM, but since the circuit checked good all the way to the PCM, I decided not to check that connector.
I like to double-check everything else, so I reconnected everything, started the Lincoln and initially the TCS worked fine. But after a few moments, it started doing the same thing again, i.e., the switch would stop working and the O/D OFF light would remain illuminated on its own.
A Different Approach
I decided to disconnect the battery and reset the Keep Alive Memory (KAM) to get rid of any trash data the PCM might have stored. After reconnecting the battery, I connected a laptop to the Data Link and pulled up the PID for the TCS, working the switch physically while watching the display on the screen. The PID reflected that the TCS was working fine.
So far, no good.
The problem seemed to evaporate for a few moments every time I fiddled with something.
Well, the problem returned in short order, but with a twist. This time when the button stopped working, the O/D OFF light was not on, and pressing the switch button had no effect; for some reason, the PCM was no longer seeing the switch.
I test-drove the vehicle again to see if it would go into overdrive. It shifted into overdrive just fine. So, let's take a look at what we know so far.
- I found that the Lincoln would not go into overdrive with the indicator light on (TCS inoperative) and that it would go into overdrive with the O/D OFF light off and the TCS still inoperative.
- Furthermore, the datastream PID matched whatever the O/D OFF light was doing even with the TCS inoperative. Obviously, something besides the TCS was sending a voltage signal to the PCM on pin 29 and the PCM was interpreting that as a command not to shift into overdrive. It simply had to be a short circuit in some part of the loop.
In my mind, that eliminated any transmission concern that might have kept it out of overdrive. The problem had to be related to the circuit I was checking, as simple as that circuit is, and there obviously was something I had missed.
Since I already had checked the TCS circuit for continuity and shorts to power, ground or each other, and because there was no blown fuse, a short to ground seemed not to be an issue.
As I continued my diagnosis, I noticed that the warmer the vehicle got after I reset the KAM, the quicker the TCS would stop working again. Could this be a faulty PCM? It seemed plausible. I installed a new PCM and did a test-drive.
So far, so good. Patting myself on the back, I saw this as my third successful transmission job (even though it was no overhaul). Then the O/D OFF light came on again, bright and angry. That's when the Lincoln reached up and slapped my face for the first time and let me know I had made my first nasty misdiagnosis as a transmission tech.
Going Back to the Drawing Board
I don't know how any other auto tech feels, but misdiagnosing something is one of the worst feelings I can have at work. It's worse than backing a truck into the handrail in front of the show room with all the salespeople and customers watching, but I would know nothing about that.
I pulled the Town Car back in and rechecked everything. As I studied the schematic, I realized that the only thing I didn't check was connector C102, which is a 16-pin in-line connector between the PCM and the TCS. Even though I hadn't checked that connector, I didn't believe the problem could be in that area, because I had checked the circuit that passed through it so many times. But after two years of doing mostly drivability work, I reminded myself that electrical circuits can be tricky, particularly if the signal passing through the circuit simply carries a voltage signal instead of a load.
If everything else has been eliminated, whatever remains has to be the answer, so I went looking for the C102 connector.
Finding the connector was annoying; neither the DVD shop manual nor the Ford PTS Web site had a location illustration (that page is blank), even though a check box was there to link to the non-existent illustration. The description of the connector's location put it behind the left wheel well.
As it was, I located the C102 connector on the left hand side of the firewall not far from the PCM. What I found when I disconnected it was interesting: it was full of a liquid that I thought was brown water.
"That sure is some dirty water in that connector," I thought. I dumped a little of the liquid on my hand and smelled it to find that I had a handful of brake fluid. So how did it happen?
When performing Ford's deactivation switch recall, I have encountered vehicles where so much brake fluid has traveled up through the speed control servo wiring harness to the servo-amp that brake fluid actually comes out of the servo-amp connector. I have never seen the brake fluid travel as far and take as many turns as it did on this Town Car to get to this one 16-pin connector over in the left corner of the bulkhead.
An insulated wire makes a nice little pipe, and those weather-tight sealed connectors along the way are a dandy conveyor of pressure and liquid. And old brake fluid tends to be fairly conductive, particularly with the moisture and minerals it picks up as it does its job.
I sprayed the connector with brake parts cleaner and then blew it out with shop air before reconnecting it. I then replaced the speed control deactivation switch and connector with the recall parts package.
I test-drove the car for an hour to make sure the problem was gone. Aside from that PCM debacle, my third successful transmission repair was in the bag.
Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at firstname.lastname@example.org.