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Old 05-04-2011, 04:13 PM
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Join Date: Feb 2011
Location: Redding, California
Posts: 229
Electrical Gremlins

I've been chasing several electrical problems that are giving me fits.

The two principal problems, that may be related are, inoperative tachometer and improper functioning coolant temperature gauge. The car is an 85 190D, the 601 engine. I'm far from a whiz on electrics (or anything else). The problem started a couple of months ago when the gauge started acting up, functioned intermittently and one morning it was trying to work, buzzed a but and gave up the ghost. I've replaced the gauge, the circuit board a couple or three times after resoldering all the connections which looked good, but not with much success. I initially replaced the temp sensor, later replaced ALL of them (3), replaced all the fuses, then the fuse box, cleaned all connections though the weren't corroded, and during all this the tachometer quit.

In researching the problem here I've read most of the posts relating to some of these problems (about 500 of them,) and about all I've succeeded in doing is getting the coolant temp gauge to very slightly come up off of shut down temperatures. It will work slightly yet show the temp at 60 or slightly under. The engine is operating at the right temperatures apparently. The thermostat is new. I've just ordered a new OVP which is about the last of the relays I haven't replaced. I pulled the combo relay and resoldered it, and a jy replacement, with no effect. I also haven't replaced the EGR and I'm not sure how to test it.

I replaced the Klima relay with one out of a wreck, with no effect. Today I cleaned the tachometer sensor and after cleaning all the contacts on the relays and sockets in the compartment behind the battery, I checked the output for the rpm sensor. Everything I've read here says it should put out from 2.8V to 4V. This one puts out 1.8mV at idle and goes up to about 2.6mV increasing the r's. Somewhere I read that any output meant the sensor and cable was OK. Is it?

I traced the contacts to the Klima and everything is fine there, however, for kicks I tested continuity at the posts relating to the tach. On the Klima which was in there originally, there is complete continuity between the post marked 31 and the one marked GM+. On the replacement there is resistance between those two posts. I have no clue what this means except that one is possibly OK and the other is not. Can anyone direct me on which is OK, and what other tests I can make? Those puppies are expensive and I hate to buy a new one just to test.

Any help or suggestions, including gentle leading, will be appreciated.

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Old 05-06-2011, 04:12 PM
Registered User
Join Date: Feb 2011
Location: Redding, California
Posts: 229
I thought surely one of the brilliant Mercedes minds around here would have at least a comment.

I've replaced the OVP with a new one and there is absolutely no change. Also switch relays (again) in the electrical center and cleaned the last of the contacts in the wiring in the engine compartment. No change.

Still would like info on the tach problem. The sensor seems to be working but I'm uncertain if the output from the sensor is enough as it seems below the numbers I read in some of the old posts.


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