By Jeff Bach
Tracing intermittent problems with cars can be frustrating at times. Why anyone would choose to tackle these types of problems whether they are occasional shorts, infrequent battery drains, the “sometimes it won't turn over” complaint, or the “once in a while it dies at stops” problem is a mystery to some.
I can only speak for myself when I say a lot of it has to do with the satisfaction of problem solving. If you have an appetite for such things, then you have probably figured out where your gifts are. I can still hear my dad giving me advice on how to be happy. He'd say, “All you have to do is figure out what you don't like, and don't do it.”
Often I get to hear customer descriptions for intermittent problems that sound similar but may have totally different solutions. With these types of complaints, often the solution lies in the details. For this reason, it's better if the technician gets to hear the customer's description of the occurrence of the problem firsthand. Abbreviated descriptions on a repair order such as “car stalls at times,” or “engine misses once in a while,” or “sometimes car won't crank” can cause confusion, frustration and sometimes unnecessary time and money spent trying to figure out just what the customer is experiencing.
For instance, I recently had a customer in my shop with a '92 Pontiac Grand Prix that had an infrequent stalling complaint. By infrequent we're talking two stalling episodes in six months. He stated the second time the car stalled, his wife was driving it and now she won't drive it anymore until she's sure it's fixed. I didn't get to the schpeel about how difficult these kinds of problems were to track down when they're not acting up, or how I could drive it for weeks and it may not happen while I had the scope hooked to it and even if it did I may not catch it on the screen unless I got lucky enough to be scoping the right part when the problem occurred. I could tell by the way he was just taking the keys off the ring while he was talking that he wasn't planning on driving the car back home unfixed. He was describing the problem as occurring on occasions when the car came to a stop. As he was talking, I was thinking, “possibly fuel related, check basics, check pump current and injector waves, check throttle body for carbon.” He mentioned the car had only seemed to start stalling when he was coming back from seeing his daughter at college. He said he worked locally and never really got the car on the road much. The car always started right back up and ran fine but may or may not stay running at the next stop.
After assuring the customer I would do my best to track down his problem, I sent him on his way.
Now I know the odds of catching a problem as intermittent as this one on demand during a brief road test aren't that great, but from something the customer mentioned during his description I got the feeling his problem might possibly be related to a sticking TCC solenoid. I know from previous experiences with TCC-related stalling conditions that by monitoring the solenoid's current with a scope and probe, it is possible to spot a potentially sticking solenoid even though it may not be stalling at the time of the test-drive.
Figure 1 shows my first waveform when the TCC solenoid first engaged.
The waveform shows a slight dip where the movement of the solenoid occurs and it also seems a bit high on the current slope. I like to see a more pronounced dip and I like it about half to two-thirds of the way up the pattern. The pattern in Figure 2 shows an even less-defined dip where the ball in the solenoid should be moving.
Figure 3 shows a little better dip, but it has moved higher up the pattern and looks like it has a secondary dip, which I associate with the solenoid ball sticking then moving again.
When watching the TCC solenoid pattern on this road test, I was continually touching the brake and looking at the scope when the ECM energized the circuit while storing a waveform at each mile marker.
Seeing these slight changes in the waveform pattern made me even more suspicious that I was looking at a TCC solenoid on its way to sticking in the engaged position, but not enough to make the call. The location of the TCC solenoid on this transmission makes it difficult to replace and it's not a job I wanted to do on a guess.
Figure 4 shows the waveforms I captured during the road test at one-mile intervals.
Seeing them stacked in order made me even more suspicious of the solenoid, but still a little skeptical since the car was still idling fine at stops.
I let the car sit overnight to allow the transmission to cool down completely. This test works better from “stone cold” than from just a several-hour cool down. I then readjusted my scope to bring in the pertinent details of the solenoid's current pattern. I wanted to keep the current probe from shifting around under the column while I maneuvered the brake and gas with my size 11 square-toed work boots. Keep in mind, the hush panel was hanging by one finger-tight screw. So to minimize any current probe shifting, which can cause erronous readings, I fastened the probe to the wiring harness with a “Mark Warren special-
edition ball-type ponytail holder.”
Figure 5 shows a screen capture of the solenoid current min/max from this laboratory condition road test.
The shaded area in Figure 5 shows a substantial gradually shifting pattern.
The stored waveforms when stacked from left to right for comparison (Figure 6) now had me convinced that the solenoid was the culprit.
The pattern in Figure 6 shows the balls' movement dip start low and distinguished, then with each passing mile, gradually diminish and a secondary dip begins to appear.
By the time we finished installing the new solenoid it was Tuesday - stone cold again and time for another road test.
The min/max results are shown in Figure 7, with the subsequent left-to-right stack of waveforms shown in Figure 8.
I feel confident that this one is fixed even though it never did stall for us.
The customer asked me when he picked up the car: “Looks good on paper, but will it work in rehearsal?” I didn't get it, but I gladly gave him a copy of the waveforms when he asked for them.
I know it's just a car repair job, but somehow I get the feeling I know how Don Quixote must have felt.
