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Old 11-12-2012, 05:32 PM
Diesel Nut
Join Date: Sep 2012
Location: Santa Cruz, Ca
Posts: 24
603 tach & a couple of other questions

The tach on my 87 300tdt is intermittent. When it works, it runs better. Just where is the sensor/how do you get to it. I think it's somewhere on the left back of the engine.

The seal on the shaft of the aux. pump for the heater has decided to leak. Are repair parts available? The seal has some like ax 5 molded into it.

Finally, last night the alarm(I presume factory) went off for no apparent reason, quit, went off again. Now have the old burnt electrical smell from somewhere in the dash/front passenger area. Haven't been able to locate it better. Everything seems to work, but any ideas of where to look first?


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Old 11-12-2012, 05:53 PM
oldiesel's Avatar
Registered User
Join Date: May 2007
Location: florida
Posts: 677
Tach problem and rough idle is very likely caused by the over voltage protection relay that is located just behind the battery ,a silver relay with a red top and a 10amp fuse at the top.If it is intermittent it is more likely the relay failin than the fuse but it would not hurt to replace the fuse and hope for the best . Easy to do and cheap,however the relay is not a real hi dollar part either. Don
Red Green "This is only temporary,Unless it works!"

97 E300D 157000 miles
87 300TD ?141k? miles
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Old 11-12-2012, 07:12 PM
jay_bob's Avatar
Control Freak
Join Date: Oct 2011
Location: Columbia, SC
Posts: 3,639
Tach signal is derived from the EDS controller that lives behind the battery.

If you are running crappy with no tach then it clears up and you have good tach I would suspect the OVP relay as well. It's back there with the EDS controller.

First I would check the big plug on the EDS and be sure it is plugged in securely and not corroded.

Aux pump, try the yard first if you want to go cheap, new ones are $$. Bring a 12 V drill battery and a couple clip leads with you to test it before you pull it.

Alarm controller under passenger feet, pull back carpet and remove plate. Alarm controller is the black box on the floor. Leave the red plug alone, that is the maintenance plug for your airbag/seat belt pyro tensioners.

The OM 642/722.9 powered family
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)
2008 ML320 CDI (Older sonís DD)
2008 E320 Bluetec (Younger son's DD)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
gone but living on
1983 240D (body to greazzer, engine to t walgamuth)
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