Rain makes my tach jump!

[bthompson]

Okay, here's one for the troubleshooters. Driving to work today, I got caught in a sudden torrential downpour. (Put the hood up just in time!) My tach, which has never given me any trouble before, started jumping at idle. It would twitch erratically from 500 to 2000 rpm. It only happened at idle; once underway, the tach calmed down. On the way home from work, it was not raining, and the tach behaved normally again.

So, I assume this is a water issue, shorting something out. What would make the tach jump like that, and where would the water have to be dripping to do it? It's not a huge problem...but I'm puzzled.

 

[JPSmit]

If I had to guess Bill, it is where it attaches to the coil as that is the most exposed

 

[Morris]

Better to make your tach jump then make your frog[eye] jump.

Okay... that was sooo bad I have to groan at it myself. I hope I don't get banned for that.

 

[Sarastro]

Any change in the idle, such as missing, when this happens?

This really sounds to me like there is some arcing at the coil, like some electrical leakage or discharge between the HV and LV terminals. If there is a crack, or just some dirt, a little moisture could cause an occasional zap. If there's no water getting into the instrument itself, I can't imagine what else would do that.

 

[bthompson]

Nope, the idle stayed the same, as far as I could tell. Didn't detect any missing, but I might have just not noticed in the rain. All symptoms point to the coil...I'll have to give it the old once-over and see if there's any tracking or cracking.