Recently I replaced a transmission in a customer's '94 Seville STS with a low-mileage, used unit. There were some pieces of hard parts in the pan on this one that warranted an overhaul. Considering all the facts, we agreed this was the best route. Several days later the car came back with a rough running complaint that the customer swore was never there before we did the transmission work (bet you never had that happen). Well, open-minded as we shop owners are, I agreed that it was most likely coincidental, but said we'd be happy to do an initial diagnostic inspection at no charge to see if we had left something loose or worse yet ... damaged a component during the transmission transplant (doctors do that for free, don't they?)

Well, an hour into the free diagnostic inspection it was determined that the manifold air pressure (MAP) sensor had quit and I couldn't for the life of me see how we could have been responsible, seeing as how this one is buried safely inside the engine under the intake cover (Northstar, you know). The customer agreed and paid for the repair.

A new MAP sensor and all was well for another few days until another rather unsettling call from the same customer was received. This time the car was on the way back to the shop with a start and stall condition that rendered it undriveable. The customer, now understandably upset, was on the verge of a thinking pattern that required some phone finesse to help him understand that we don't have ways to make cars break down at certain intervals after they get serviced. After a lighthearted exchange of words, we were back on the hunt for this new gremlin. This thing starts and stalls nearly exactly like the pass lock system problem I just worked on - except for the fact that it's not equipped with that system. With no codes in the computer, a quick throttle body cleaning verifies that the problem is fuel-related and not an ignition system problem.

Figure 2.

Next step was to check for fuel supply, which I did through the easy access fuel pump fuse right under the hood, using my lab scope and current probe (see Figure 1).

The fuel pump continued to run at enough current to sustain fuel flow beyond the two-second point, indicating the presence of crank and reference signals (Figure 2).

Figure 3.

The next shot is of the injector current during the event (Figure 3). The fuel injector fuses, also in the same underhood fuse panel, are easy to access.

The first wide (nearly 40 mS) off the screen pulse you see in Figure 3 is the initial spritz of fuel the powertrain control module (PCM) gives all of the cylinders during cranking. The next quarter of a second the engine gets no fuel. Then the PCM starts firing the injectors individually based on the firing order. About midway down the screen in Figure 3, you can see the pulses tighten up as the engine starts for a moment, then widen back out as it dies. I want to see the actual pulse width when it's dying to see if the PCM is responding to the manifold pressure change by widening the injector pulse. To do this I need to set the scope to trigger at about three-fourths of a second into the event. First, I reset the time per division to 20 mS I, then I set the trigger to +18 divisions (Figure 4).

Figure 4.

The beginning of the waveform in Figure 4 shows the injector pulses getting close together as the engine starts, and then wider as it immediately dies. Notice that the width of the injector pulses never changes.

This system uses a MAP sensor to monitor air flow in the engine, so I went to the MAP sensor parameter and noticed a steady reading of 16. This number should be around 98-102 kpa at key on, engine off, at our elevation. I was a bit amazed that I wasn't getting a code for the map sensor circuit. I began checking the circuit voltages at the harness connector outside the manifold and was surprised to see that the harness had an area under the split loom that was gummy and had the insulation coming off the wires, with many of them bare (Figure 5). It made me think I was looking at a Contour harness for a minute. This explained a lot since this harness does rest atop the transmission and probably was disturbed during the transmission job.

This is also the injector harness and requires pulling the intake to replace. I just happened to have a good used harness to fit this model.

Figure 6.

I was now questioning the validity of the MAP sensor repair we just made and since it's right there under the intake cover, I put the original sensor back in to see how it reads now. The reading was now back to normal and I decided to credit the MAP sensor on this repair and eat some of the labor as a goodwill gesture. This is a good customer and more understanding than I expected.

I went ahead and got another shot of the injector's first pulses after the repair to see the difference in the pulse width (Figure 6).

This documentation went a long way in helping to explain things and allowed the customer to see how we came to our diagnostic conclusion.

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is johntjeff22@gmail.com and his Web site is www.currentprobe.com.

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