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Old 05-20-2002, 12:23 AM
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Join Date: Jul 2001
Location: Olathe, KS
Posts: 40
Lost Tach and A/C

The Tach and A/C compressor in my 1985 300SD have finally stopped working completely. For the past year the problem has been intermittent -- when the tach would stop working the A/C compressor would disengage. I searched this forum and found a thread from 1999 that discussed two possibilities. 1) a blown fuse on the overvoltage relay and 2) loose wire that connects to the positive lead of the battery to the EGR control circuit. I have checked both of these and they are OK. Tach and A/C are still dead.

Anyone have any ideas?


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Old 05-20-2002, 08:08 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
If the EGR computer IS powering up then the crank sensor(flywheel) must generate at least 5V ac current at idle for the system to operate properly.
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Old 05-20-2002, 07:19 PM
Registered User
Join Date: Jul 2001
Location: Olathe, KS
Posts: 40
I disconnected the engine speed sensor at the connector behind the first firewall and measured the voltage. With the engine at idle speed in read about 25mv. The reading did not change significantly when increasing the engine speed. Do I have a bum sensor?

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Old 05-30-2002, 11:03 PM
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Join Date: Jul 2001
Location: Olathe, KS
Posts: 40
Bingo!!! I finally got around to replacing the engine speed sensor. The tach and AC now work perfectly -- just in time for a hot Kansas summer. Thanks for your help.

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