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Old 09-06-2010, 09:31 PM
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Join Date: Jul 2010
Location: Hamilton Ontario Canada
Posts: 394
tach swap question

Are tach's known to be non interchangeable between diesels and gassers or is this one a dud? 1986 190D 2.5NA diesel and 1988 190E 2.3 gasser

I swapped out my tach today with one out of my parts car. Donor was a 2.3 gasser. When started, the tach jumps to full rev, rev it up and it jumps to zero. I know the range is different, but only by 1000rpm. I assumed it would work. Looked the same, plugged right in. The tach signal is (or was) good, tach has worked in the past. I replaced the tach because I broke it while working on my cluster.

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Old 09-06-2010, 09:43 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,800
Two things to consider, the number of pulses and the voltage (or current?) of the signal. The Diesel tach counts the number of flywheel revs through an inductive pulse counter. IIRC there's only one lug on the ring gear. The gasser tach counts the number of cylinder firings from the coil and divides by the half the number of cylinders. Unless there is some intermediate device to take these differences into account, you won't get a proper display.

87 300D
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Old 09-06-2010, 09:52 PM
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Join Date: Jul 2010
Location: Hamilton Ontario Canada
Posts: 394
live and learn....

At least the clock works, so does the ambient temp!!!

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