I recently had another opportunity to play detective while trying to repair an engine overheating condition. The vehicle was a 1988 Ford Club wagon with a 460 engine. The complaint was that the engine temperature would go almost to the red zone when both blowers were running on high and the fuel gage would read improperly. Now you have probably already said to yourself as I did, "That doesn't make sense. If anything, the temperature should come down a little when the blowers are on if the problem were actually heat related." So I figured I had better check the temperature with the infrared gun to eliminate overheating as being the problem and get on with checking the electrical system.

This vehicle was a typical Midwestern winter vehicle with its share of corrosive chemical damage evident from a slight distance. After cleaning up several of the fender, body and engine grounds, I added an extra one to the dash circuit, then I road tested it to see if the grounds had helped. They didn't.

Now it was time for some voltage drop tests. I found slight drops on the groundside, but none significant enough to cause a fluctuation in the gages.

The customer had been slowly leaking more information about this problem with frequent phone calls as he was trying to get the van working properly so he could migrate to Florida. There, he would wait out the inclement weather conditions common to the Ohio valley until tax time. I was checking the voltage supply side of the system when he showed up with an instrument cluster and relayed the fact that the van had had this problem for quite some time and had been to the dealer where they replaced the whole instrument panel to the tune of more than $500. Apparently there were some similarities that were becoming evident to the customer between my test findings and those of the dealer technicians. He produced proof of the replaced components as if to say, "Don't even go there; that has already been tried."

Among the parts was a temperature sending unit and a thermostat.

I continued testing the power circuit and discovered that the voltage was dropping on the supply feed wire to the gages as the blower motor was turned to high. There was no problem on the ground circuit. Since I was losing supply voltage under the additional load of the blower, I figured that there must be a loss in the power supply that was common to both circuits. Using the wiring diagrams in Figures 1, 2 and 3, I found that the first common power supply connection for these two circuits was splice S102 where the blower motor fuselink and the ignition switch feed circuit were joined.

Figure 1 - Dash gages circuit
Figure 2 - Ignition feed to dash and heater
Figure 3 - Blower feed circuit
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I tested the voltage level and found no appreciable drop on either leg of the circuit. I probed the connection at the ignition switch and found the level remained acceptable there also. The voltage drop was found to be across the circuit from the ignition terminal out, to the gage feed wire in, at the instrument cluster plug. I took another look at the wiring diagram. It shows this circuit to be a loom wire with a built-in 8-9 ohm resistance, which measured within spec using the ohmmeter. I started wondering about the possibility that the resistor wire circuit could somehow be tied to the blower feed and the additional current across the loom wire was reducing the voltage feed for the gages. I decided that if this were so, the best way to prove my theory would be to measure the actual current decrease in the temperature sending circuit. (Can you tell I'm in this deeper than I expected when I gave the guy the estimate of an hour or two?)

Time for the big guns.

Armed with my current probe and my Fluke 99, Figure 1 shows the pattern I received when I clamped the probe around the wire to the temperature-sending unit. The pulse illustrated in Figure 4 is a result of the constant voltage regulator, which works similar to a hazard flasher. It uses a bimetallic strip and a set of contacts, continually opening and closing to send a constant string of current pulses to the temperature, fuel and oil pressure gages.

With the engine warm and the gage reading normal, I switched the blower on high and received an unexpected surprise in the pattern shown in Figure 5.

Now that is one trashy signal!

The noise evident in the low current cycle was actually the pitted regulator points making contact when they were supposed to be open. The slight vibration in the dash from the blower running was enough to let the contact occur. I replaced the constant voltage regulator with one from my local Ford dealer and solved the creeping temperature gage mystery.

Just for the satisfaction of seeing if the regulator was the problem with the original gages, I plugged in the original regulator to the new cluster and took a reading of the temp gage current, then the known bad one, then the new one, all with the blower on high. Figure 6 illustrates those findings. I attribute the fact that the noise lessened during these last tests to my not bolting the cluster securely in place after plugging in each regulator.

The customer was very pleased to have his gages finally reading properly. He made a statement to me that really sank in, making me again realize how helpful it is to be able to document my findings and relay them to customers in an intelligible manner. His comment was, "If I hadn't seen this with my own eyes, me and you would be goin' around and around." He then asked for a copy of the images, which I happily provided. I printed Figure 7 for him and sent him on his way to Florida.

If you find that sometimes logic seems to serve you best in hindsight by helping to rationalize what a problem was after throwing on a part that fixed it when the traditional logical diagnostic approaches have failed, and you have to pull out the stops and are reduced to throwing a "Hail Mary," I would suggest that a current probe will give you the real picture of what is going on in a circuit when a voltmeter seems to fall short in tracing down a tough electrical problem. It sure has helped a tremendous amount to bring sanity back to my sometimes-crazy diagnostic dilemmas.

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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