My 1985 300CD's battery died. After

several unsuccesful attempt's to recharge,

I had it load tested, and I ended up

replacing the battery. Ever since then,

the tachometer quit working and the a/c

compressor no longer comes on. Is this

just a coincidence, or are they related

somehow?

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Related! Your car has an overload protection relay that protects the EGR computer from excessive voltage. The relay has a 10A fuse that blows when the voltage is to high or has incorrect polarity. The relay is mounted next to the right A/C outlet vent beside the glovebox.

To add to DOC. You have to take out the inner panel of the glove box. The small rivets have to come out then just pull.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

The 1985 model also has an electronic EGR control unit.The tach signal runs through it.There is a wire at the positive battery terminal that supplies power to the EGR control box. Be sure that wire is tight...I've seen them loosened during a battery swap and not retightened.
Randy D.

You guys are awesome!!! It took all of

15 minutes to replace the blown fuse on

my relay and restore my tach and A/C.

The A/C is particularly satisfying as

the temperature has been 100 degrees for

the past several days. Thanks again.