Recently bought an 82 300SD, tach & cruise are both inop. Both appear to be on the same fuse, (fuse appeared to be ok but changed it anyway). Any suggestions on common problem spots?
Haven't picked up a shop manual yet so am flying blind in many respects. With only 144K and good maintenance history engine is tight and runs like a swiss watch, but mileage is not great, only shifts through 3 gears (smoothly I might add). Is the W4A 040 transmission a 4 speed?
Any thoughts would be most welcome,

Jim

Do a search for tach amp - it's usually an easy fix. CC is probably the CC amp - located under the driver's side knee panel. About $150 for a rebuilt one, 10 minute swap. The transmission is a 4-speed. Sometimes these cars start off in 2nd. Manually shift to first before starting out then quickly shift to D and count the shifts as you accelerate. When your tach is working you should see about 2600-2700 RPM at 60.

Mileage is all over the place depending on your driving. Some folks get 22-23 in town. My long commute is at 60 most of the way and I routinely get 29-30 (no AC).

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

I should have mentioned that I replaced the tach amp to no avail.

Have done some searching and alot of reading and it appears that the tach sensor also feeds to the ac; the ac works well, which should eliminate the sensor from the list of potential problem source.

Are there any elements common to the tach and cc other than that they are on the same circuit? Could be that problems are entirely unrelated but....

From what I have read on this forum and have experienced myself that the cruise control amp is many times the problem...I did what this forum suggested and took mine out of the car and resoldered every connection on the circuit board and presto back in the car and it again functioned as new. Although it was very tedious and I had the right kind of soldering equiptment it was a very satisfying experience. If you can't do this yourself you might inlist the aid of a local electronics repair agency to do it for you....It wouldn't be that expensive.

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html