As a specialist in hard-to-find driveability and electrical problems, I have encountered my fair share of intermittent idle surge complaints from indiscriminate sources. I've seen causes range from low fuel pressure to weak distributor magnets and everything in between. I remember having idle problems come from wrong resistance distributor brushes, noisy EST signals (due to four-wire harness position too close to distributor cap), worn camshaft lobes, worn throttle body shafts, bad ISC nose switches, worn distributor gears, loose grounds, purge canister problems, bad ECMs, high resistance plug wires, bad alternator diodes, you name it. But I had a 1981 Cadillac Deville recently that held my goat hostage for a while.

This car was a sharp old '81 coupe, with an 8,6,4, 368. It had been garage kept and well maintained. It had a new ISC, TPS and throttle body. The cap, wires and rotor were all new parts and the steady fuel pressure was within specs. The MAP reading was normal, no EGR flow at idle and no vacuum leaks. The timing was right on and it had very little dwell variation. All the testing I had done showed a perfectly good running engine and yet at times the idle would surge up and down radically for just an instant with seemingly no reason. Every scope signal I checked was normal and I was beginning to run low on ideas. I began to experience that frustrating feeling that occasionally accompanies one of these “challenging” jobs. This is the feeling that comes over you when you realize that you have allowed yourself to go too far to think about turning back, yet you're not sure how far it is to the end.

That “Challenging” Feeling

Kind of reminds me of the time when my brother, Eric, and I went after a rock he had found while collecting fossils on a hillside. He asked me to go with him to help load it. We went up in the old Jeep Cherokee that dad had made into a pickup complete with a hydraulic tilt bed. The hillside was cut by the railroad through the Ohio Shale layers, leaving the Huron member exposed. Eric digs into the rock layers with his rock hammer and chisel, finding what he calls “concretions,” which are spheroidal fine-grained aggregates with calcite crystals inside them. They appear as round rocks protruding from the flat shale layers and are heavy for their size. I have seen them up to two feet across. A concretion this size can weigh 1,000 pounds or more. This would be the one (unbeknownst to me) he needed help loading. He neglected to mention that it was on the other side of the tracks and not near a road crossing.

After a short debate, we decided to cross the tracks and get the rock, then cross back. I knew the thing wasn't light or Eric wouldn't have needed me. I've seen 300-pounders that he had managed to retrieve by himself. Crossing the tracks looks fairly easy until those big loose rocks banked up to the sides of the tracks roll out like big marbles and you sink rather quickly down to the frame, right when you get halfway across. Did I mention that the rock was in a bend in the tracks? We actually had only about 1,000 feet of visibility in either direction and with no train schedule, we had the added excitement of not knowing when or from which direction a train might come barreling down the tracks and round the curve to find us hung halfway across the second rail by just the head of a 3/8 cross-member bolt.

Trying to maintain calm while working at a flat-raters pace, and feeling a rather invigorating adrenaline surge, I used a floor jack I had brought (to possibly help load the rock) to free the frame and help the jeep to a place of safety on the other side of the tracks. There, Eric and I had a rather dry-mouthed discussion of how much lower the frame was going to be with the extra weight of the boulder. We decided to cross back over empty, then drag the rock across the tracks with a chain and load it with the engine crane, which I had brought also. “Your turn,” I said to Eric as I handed him the keys.

Back to the Caddy

I had all but given up on this Caddy, and was thinking about rechecking all the basics again. That's another way of saying, “I was going around in circles,” when I noticed that during one of its rare surges, the A/C clutch speed seemed to slow down just for an instant while the engine speed was increasing. I watched the clutch plate intensely for a few minutes and saw it happen again. Just idling along and for just an instant, the clutch plate slowed down and sped up again while the engine maintained constant speed this time.

The wheels began turning again.

Could it be that the A/C pressure was getting too high and increasing the load? Maybe the compressor itself was trying to lock up. Maybe the clutch coil was starting to short, weakening the magnetic field, then it could grab and let go instantaneously, making it difficult for the map sensor to properly relay the load to the ECM. Could it be that the low-pressure switch in the back of the compressor was not providing a good ground for the clutch coil? Maybe there was a flapper in the high side line or in the muffler. Most of these questions I figured I could answer with a simple waveform of the compressor clutch coil current with the current probe. I hooked the current probe to my scope and clamped it around the blue compressor clutch coil feed wire. Since I can't stand to see my scope's “channel two” sitting there spectating like a county supervisor, I put it to work monitoring the voltage signal on the same circuit.

Figure 1 shows the clutch engaging during a normal cycle with the corresponding voltage signal.

The engagement of the clutch looked normal enough, but it wasn't - as we are fond of saying in the intermittent business - “acting up” at the time of the picture.

With the scope set on “min/max,” monitoring both the voltage and current levels of the compressor clutch circuit, I left for a while to go do some “reading,” confident that the scope would be capable of relaying any abnormalities in either the voltage or current.

I was just finishing up an article by former Tech to Tech writer Mark Warren on the limitations of OBDII misfire detection when I thought I heard the engine surge for a moment. I hurried back out to see if I was right, only to find the engine still purring like a cat.

When I looked at the screen image, though, I was pleasantly surprised to see that the scope had recorded the image shown in Figure 2.

This was undoubtedly one of the noisiest looking voltage signals I had ever encountered. The current for the compressor clutch comes from the power module.

The voltage feeding the module and its ground were both clean signals (straight lines). I ordered a new module, but continued to collect signals, thinking this may be an article some day if I ever get it fixed.

The car “acted up” a couple of more times. The waveform in Figure 3 was captured during a surge while I was watching it.

I found a thermal sweet spot that occurred about 10 minutes into a hot soak that lasted for about 20 minutes in which the thing would act up about three or four times. (Nice article, by the way, Mark.)

Several times the surge occurred while I was monitoring the clutch engagement current and I didn't see the current let the clutch go. I still wasn't quite sure why the radical surge occurred when the clutch current was not dropping.

I decided to set the scope to catch the idle speed control motor current while the ECM was commanding it to increase the throttle angle to take up the compressor load. For this test I used one probe to monitor the A/C clutch current with a second current probe around the ISC circuit. The image in Figure 4 shows that the ISC current pulse kicks the throttle up 620mS before the clutch engages.

I was working the heat soak sweet spot for a few more waveforms when during a particularly nasty surge I was able to capture the image in Figure 5 while triggering from the ISC pulses and monitoring the clutch voltage signal.

It wasn't until this interaction that the light went on for me as to why the idle was so crazy-acting. The ECM is monitoring the voltage signal that feeds the clutch and gives the throttle a kick when it sees the A/C clutch circuit go high. Then, when the idle goes way too high because the clutch load didn't get there, the ECM backs the ISC down. Note that during the idle control confusion, the ECM is not turning on any light or setting DTCs. While I had this thing acting up I wanted to get a shot of the clutch current while the voltage was dropping and noisy. I reset the scope to trigger on the down slope of the voltage signal to capture the voltage dropping below 10 volts. I received the image in Figure 6 during a surge while the clutch stayed engaged.

The new power module solved the idle speed problem and smoothed out the voltage signal to the A/C clutch. I stacked the current and voltage waveforms from a good and bad engagement to show the time difference in the clutch engagement. These were taken during a period of just slightly unstable idle but the clutch engagement seemed fine to the naked eye.

The images in Figure 7 demonstrate just how slow the current reacts to rapid voltage changes and made a great picture to send the customer away with ... just in case he's the sort to talk to his friends about his car problems.

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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