Problem is the tach works great with air con off. When on, at idle, tach registers zero. Upon reving up in gear, starts reading again at about 2000 rpm and continues until 3500 in hi gear when it again begins to jump all over. Tach revs normally from 2000 to shift point thru the gears, just bounces when it hits 3500-3600 in hi gear. Any ideas

Regards

Run-em

OK your first sin is that you didn't state what brand/model/year of car you have. I am going to guess a Rambler with a vacuum tach and the electric pump that runs the AC compressor is using tooo much vacuum.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

It is a 1983 Mercedes Benz 300 S D .....ya know, th royal blue un with th camel interior Sorry, I fergot to mention that lil tid-bit

Regards

RUN-EM

Ok there are three components of the tach system. One is the sensor at the crank pulley. The second is the ampifier on the drivers fender which is usually the cause of tach failure. Three is the tach itself. Try looking at each item and do a search here to see what to do.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C

like a voltage drop issue to me. when the air is on the electrical is stressed to the max. if the voltage is low i am guessing that the tach takes a little vacation. my unscientific analysis.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

a voltage regulator going bad can have different symptoms, but I personally experienced a bouncy tach as my regulator was giving up

I experience a bouncing tach and speedo whenever my cellphone is checking my email.....

I know this doesn't help you, but it does show that a GSM phone can interfere with the electronics.

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1998 W202 C230 - The money pit of late.
1984 W126 300SD (356,800 miles) Gone to the wrenchapart
1984 W123 300D Gone to the wrenchapart
1972 W108 280SE 3.5 (sold but not forgotten)
1986 Buick Grand National 3.8l Turbo (86k miles)
1966 Glassic Model 'A' Replica http://www.glassicannex.org

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