I sometimes say Oc-tow-ber is the beginning of wrecker season.

Opportunities abound to find and sell work as the fair weather that has been permitting the borderline electrical components to live an extended life has begun to fade into the cold drizzly season we get just before the cold dry season.

Fuel pumps have been dropping like flies all month. We get a fairly decent temperature spread this time of year, which serves as both a blessing and a curse. The curse being that a lot of intermittent problems come in on the hook, but seem to start right up when the wrecker sets them down. This scenario is all too familiar, I'm sure, to most of us. The wheels start turning and thoughts come to mind such as, Probably one of those bad PCMs, or It must be the crank sensor. Perhaps the ICM is giving some trouble; it probably needs a coil pack also. Fortunately, many times these problems are caused by the fuel pump, which can't hide its trace from the current probe. Even if it's not acting up.

The trace in Figure 1 came from a car with a very frustrated owner. The car had been towed three times in two days. Once it gets running, the thing runs fine; you just don't know if it's going to start back once you turn it off. A ride on the wrecker always seems to bring it back to life temporarily. It had several parts hung on it in an attempt to find the problem by the owner's regular repair shop. We only get the special jobs from them.

I reached in and cranked it while the driver was unloading it and guess what? It fired up like you had just shut it off. I ordered an electrical system visual inspection (ESVI) and a current probe scope trace (CPST) of the fuel pump.

The result of the test was positive. The fuel pump was causing the problem. I obtained consent from the owner and found an exact match for the transplant.

The owner came to pick it up later that day and asked the inevitable question: How can you be so sure this will fix it? I get tickled, but I manage to keep a straight face.

You see these low spots here and here, I say pointing to the pattern in Figure 1, Those are caused by degeneration of the comutator bars on the armature of the fuel pump. If the armature happens to stop while one of those low spots is between the brushes, which looks to me like about a one in five chance, you get exercise. I pull out the diagram in Figure 2 and continue. This is the trace from your new fuel pump. As you can see, it's perfect. It has the proper speed and current level with only about 1 amp peak to peak, which will diminish as the brushes seat in.

Finding these kinds of problems is what I do for a living. I probably shouldn't enjoy it so much, but I still feel like the kid that found the prize egg at the Easter egg hunt when I find myself staring at one of these traces.

The next one (Figure 3) came from a '93 Blazer with a 4.3 CPI W motor.

While this one was towed in as a no start, the down spikes in this pattern are caused by the armature being worn clear through in spots. This was another no-brainer.

The new fuel pump in Figure 4 was drawing more than 10 amps, which is normal for this particular engine.

One of the blessings about these kinds of temperature-sensitive intermittent problems is that with all of the possibilities of things that can go wrong and cause them, the guessers and parts changers odds of getting them fixed reasonably are lessening. This makes customers think twice about the old logic of well, those parts had a lot of miles on them anyway so it's probably a good thing we replaced them. The word seems to be getting around that it's cheaper to pay one of those high-priced diagnostic shops to find and fix the problem right the first time.

This next one came to us as a referral from a local coffee shop. Auto repair seems to be a hot topic these days; almost everyone who drives a car that's out of warranty around here has had some sort of electrical intermittent problem recently.

This particular car ran great most of the time but when the temperature drops to the 40s at night (especially if the air is damp) the car won't start in the morning. Every time she had it towed, it started for the shop she took it to. By the time it got to us it was the same story. Started right up as soon as the wrecker driver set it down. Having two similar cars with the same symptoms only hours apart and not being totally immune to the blue car syndrome, I scoped the fuel pump first. (Figure 5)

While this reading at first glance does not seem to be enough current compared to the 5 and 10 amp readings of the previous two cars, keep in mind that this is a TBI system, which has a relatively low fuel pressure requirement.

Since I have a database of literally thousands of case studies to reference, I chose the waveform in Figure No. 6 - for its similar symptoms - to compare this one to. I decided the fuel pump was not the problem and to check the ignition system next since the car had no codes stored in the computer.

I hooked the current probe around the red wire feeding the distributor and got the signal in Figure 7. This is the inductive angle of the coil primary circuit during its charge cycle. I knew as soon as I saw this that the problem had been identified.

The small curve up at the beginning of the waveform is an indication that the primary circuit windings are shorted. I ordered and installed the new coil and took another shot of the primary circuit.

The inductive angle should go up at a linear rate, as shown in Figure 8.

I felt confident that I had repaired this intermittent problem without ever having it act up.

While having the current probe still connected around the primary feed wire, I changed the time base and trigger point to check the secondary system for any obviously bad wires. Figure 9 was the result. The down spike is a result of a very high-resistance plug wire, probably what caused the coil to short in the first place. After being informed of our findings, the customer wasn't the least bit reluctant to let us do a complete tuneup with secondary system service and all the filters.

It's amazing how much difference there is in a customer's attitude when you can call and say, We fixed your no-start problem, and it needs some further maintenance. Rather than when you try to sell work on a car like this after saying, Well, we couldn't get it to do it for us; it started up as soon as the wrecker let go of it but would you like us to go ahead and tune it up while it's here?

The current probe has opened a window into a heretofore-unseen world in electrical circuits. Intermittent problems that have been hidden from the technicians' view in the past are unveiled by this amazing new tool.

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 northstarguy@zoomtown.com

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