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Old 12-30-2021, 08:37 AM
jay_bob's Avatar
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Join Date: Oct 2011
Location: Columbia, SC
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I think your gas tach must have been bad. If you try to plug a gas tach into a diesel 124 the needle will be buried above the red line at idle. BTDT

This is because the gas and diesel engines get their signal from completely different means.

Gas uses the ignition coil trigger from the distributor, so 3 pulses per revolution (assuming a 6 cyl).

Diesel uses the ring gear via a Hall sensor with 144 pulses per rev.

The ring gear signal goes to the EDS module which does rely on the OVP working. The car will run without the OVP, but you'll have poor idle control, and also the ABS won't work, as the ABS computer also uses this protected voltage source.

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Old 12-30-2021, 09:21 AM
vstech's Avatar
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,549
Originally Posted by Corvo Gold View Post
I now have a working tachometer in my stick shift 1987 300TD. The other day I got a wild hair and swapped the tach with a spare that came from a diesel w124. I think the inop tach that I had in there was from a gas model.

Either the tach itself was broken or there is a difference between diesel and gas tachs.


Corvo Gold
1987 300TD, converted to stick shift
Where did you mention this was a gas conversion? How did a gas tach get in your car, and how did it work before the manual trans was there with a diesel motor?
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Old 12-30-2021, 10:13 PM
ykobayashi's Avatar
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Join Date: Jun 2005
Location: Irvine, CA
Posts: 1,157
Thanks for following up. Great you got it going. The diesel tach operates on a hall sensor. The gas probably works off the ignition coil. I bet the gas wants to see a higher voltage than the diesel tach.
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
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Old 07-27-2022, 09:25 PM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 38,224
anybody know of a source for a wiring diagram for my 1992 200
td? The AC quit working and we cannot find the wiring piece by piece.

[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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