Re: Good Job! Thanks for passing this info along . (archive)

No real good ideas. There are 3 banks of resistors in the CCM and this was the only one in all three modules to measure out of spec. There was no sign of overheating or physical damage to the part either. The thing that got me thinking about this as a possible source of the problem is that when I was running the CCM with its cases off, I put my finger on the diodes that are in series with the precision resistors (series is important because it means the same current flows through both parts). The diodes were all warm. From my design days, I noted that as rather odd since there parts are small signal diodes, not ones designed for passing lots of power. Then I realized that the design was a current loop type circuit and it made more sense that the circuit had to operate with reasonable current in order to keep the impedence down (high impedence circuits are susceptable to noise, ESD etc. not good for cars). But, this also caused me to wonder about the precision resistors since they are not made to pass any real current at all. The resistors used a what are known as thin film resistors. essentially they are made using the chip making process which a) makes them cheap and b) guarantees a good match between resistance values. My guess is that when compromising a number of design requirements, the engineer(s) opted to use the thin film resistors because it would ensure the highest yield of functioning CCMs on the assembly line, yet they had to recognize that passing relatively high current through the thin film parts can cause them to fail - one of the failure modes is a change in the properties of the oxide the resistor is made of that - at least in my case - caused the resistance to go up. So, my best guess, considering the little I know about the CCM is that it was a heat induced failure that was not catastrophic (i.e. the part did not just burn up).

Kevin

Now, what do you suppose caused that resistor to get out of spec?

gale

Some may have seen a message posted earlier this week on "ESD Regelt" and my response about that message possibly indicating a fault in the check control module... well, as fate would have it, less than 24 hours after I posted that message, I got the same dreaded message! OK, time to take some of my own medicine. I pulled the check control module today and began to do some serious triage. For those who have cracked the cases on the CCM, you'll no doubt recognize a highly symetrical design. This is for a reason... there are about 30 separate, but identical test circuits in the CCM that check everything from your brake lights to the key in the ignition. Every single one of these test circuits is set up as what is known in the business as a "current loop". That means that the system sends a current through the closed loop of the and measures the voltage drop across a precision resistor. If the drop is out of range, you get an error. Not to difficult. Anyway, to debug my CCM, I took the cases off and put it back in the car and started the engine. Immediately I got the ESD Regelt error. Using my multi-meter, I started testing the voltage at the precision resistors. All were the same... accept for 1. Hmm. I pulled the CCM and put it on the bench. Again using the multi-meter, this time in ohms mode, I measured the precision resistors. Each bank of precision resistors has 4 2K ohm resistors and 1 1K ohm resistor... again, nice symmetry. Problem was, one of my 2K ohm resistors was 2.7K ohms... since I did not have any spare precision resistor networks laying around, I instead calculated that an 8.2K ohm resistor in parallel with the suspicious 2.7K ohm resistor would bring the value back down to the required 2.0K ohms. 30 seconds of soldering and the new resistor was in place. I left the case off the CCM and plugged it back into the car. Hey, no more ESD-Regelt! I checked the voltages and all were exactly the same - as they should be.

Anyway, thought I would post this since this is one of those gremlins that could be hard to track and cure... it's sort of the analog to the capacitor fix for the cluster. Of course, this problem could have been solved by purchasing a new or refurbished CCM... but then again, we're all trying to keep that cost of ownership down!

Kevin

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