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Here's One For The Books!

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A buddy of mine just purchased a nice '90 Reatta that had a few issues.  The big issue was that he had no taillights or parking lights.  He found that fuse #10 was blown and would continue to blow if replaced. He brought the car over to my house as I suspected he had a bad headlight switch and I had a good used spare to try.  We replaced the switch and the fuse blew again.  We then pulled the taillight assembly and started checking the bulbs.  We found 3 that the base had separated from the glass bulb so we replaced them with new bulbs but still the fuse blew.  I then began to suspect the issue was in the headlamp circuit and not in the taillight circuit. He had replaced the relay and that didn't fix the issue.  I pulled both headlights and Lo and Behold! the fuse didn't blow.  Evidently the headlamp bulb had a dead short in it on the high beam.  It lit up fine on low beam but when you hit the high beam switch, they didn't go on high beam. I replaced it with a known good used bulb I carried as a spare and the problem was solved.


This was a really strange but true adventure as I played a mechanic in my garage today!


One issue we didn't get to has to do with his trip odometer.  He says that when he resets it, it goes to zero.  The next time he restarts the car, it goes back to the mileage it had before.  Any ideas on this one?

Edited by ol' yeller (see edit history)
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And then it increments when he drives it ? That is a new one. Know that it is stored in the EEProm along with the ODO but should zero when requested.

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I would suspect a BCM malfunction. The main odometer is stored in EEPROM but the trip odometer is stored in only BCM RAM, hence it's being zeroed when the battery is pulled. The cluster stores nothing, it only displays what is broadcast in the ALDL data stream sent by the BCM. Button presses (trip reset, E/M, etc.) are sent to the BCM from the IPC (bus is bi-directional) and the BCM is then to take the requested action in turn.

It appears to me that somehow the BCM RAM is holding the value of the trip odometer even after a reset is issued to zero it. What is odd is that it would display zero but then revert to the previous value at a subsequent ignition cycle. Causes me to wonder of the trip odometer data in RAM is copied to another memory location before being called for inclusion in the data stream sent to the IPC from the BCM.

Since the BCM is a "black box" inasmuch as never having been reverse engineered to my knowledge, this is all speculation on my part. Having some embedded system experience however, it seems a solid theory.

Pull the battery for a couple of minutes when the trip odometer has a non-zero value and then reconnect and see what it displays on the next start. Should be zero as the RAM should have been wiped, but if there is an internal malfunction I suppose some random garbage value could end up there.


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