What is the most common reason my tach will not work, and is there a procedure to diagnose it?

[This message has been edited by shoe (edited 06-13-2000).]

It's broken... :D :D :D. Seriously, usually a bad connection in the cluster, either a ground or the lead for the tach or the multi connector plug. Did it quit suddenly or die a slow death?? I'm not 100% sure, but I think the tach picks up its signal from the ECU. The tach itself is nothing more that a fancy voltmeter. You can check it with a digital low voltage multimeter by checking the resistance across the two leads for the tach. Should not be zero or infinite.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 06-13-2000).]

I've only had the car eight month's, it did not work when i bought it. It jumped and worked for about a minute but has been dead since. I put an ohm meter on the TDC sensor at the balancer and it showed a small amount of resistance so I assume it's ok.

You'll need to check the resistance across the tach head itself.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[QUOTE]Originally posted by shoe:
[B]What is the most common reason my tach will not work, and is there a procedure to diagnose it?

The tach on my '83 300D has quit on me several times over the years. Each time I did no testing, just went out and purchased a new cap for the unit where you plug in to diagnose the engine. That two minute fix has always restored my tach. 2 yrs ago the cap was $80, this yr I paid about $120.

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Robby Ackerman
Past President
Virginia Section MBCA

thank's for the info. I will check the tach. itself then get a new piece for under the hood