Hi All -

I'm new here. Just picked up an 82 300td w/ 172,000 miles a couple of months ago. I've spent a lot of time researching these vehicles over the past year or so, and wasn't able to find a nice wagon until recently.

Anywho, everything works on the car as it should, except the tach which I know rarely work on these models at this age.

The tach works only when the A/C is running. A mechanic couldn't think of why such a thing would happen, though he didn't look at anything. I'm wondering if any of you have seen this, and/or able to point me in a direction.

Thanks for any help.

Take a look at the ground connections for the instrument cluster. Maybe when the AC is on, the tach circuit can find a ground.

Also try the tach amplifier fix... unscrew the amplifier and pull the circuit board out of the cover. put a flat cigarette butt or similar thing into the cap and then put the board back. This usually works.

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
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'92 Subaru
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'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad

Thanks. I'll check the ground first.

Other than a ground issue, the only other common thing I know of is the #14 fuse, it powers both the tach and the monovalve. You might want to take a look and see if it has a crack or a poor contact. I would try running a ground wire from the dash and see if that helps too. Very strange.

Funny you mention the #14 fuse as it blew last week - No Horn. With the fuse blown, the tach didn't work at all, A/C or no A/C.

After replacing the fuse, I honked the horn (without the engine running), and the tachometer went to 2,000ish rpms. I thought that was weird.

Definitely sounds like a ground issue to me...

Does the clock work? What about the temperature gauge? if it is bouncy that is another symptom of a ground problem

__________________
"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy"

Current
Monika '74 450 SL
BrownHilda '79 280SL
FoxyCleopatra '99 Chevy Suburban
Scarlett 2014 Jeep Cherokee
Krystal 2004 Volvo S60
Gone
'74 Jeep CJ5
'97 Jeep ZJ Laredo
Rudolf ‘86 300SDL
Bruno '81 300SD
Fritzi '84 BMW
'92 Subaru
'96 Impala SS
'71 Buick GS conv
'67 GTO conv
'63 Corvair conv
'57 Nomad

I agree, try adding a ground wire from the back of the tack to a ground point under the dash.

Next time when the tach does not work, try banging the dash above the tach and see it it brings it alive. If so, it may be due to a "sticky tach movement"

See post #80 in this thread for more info:

Tach amp repair attempt

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The clock and all other gauages work properly. Nothing is bouncy.

I would still try the ground wire, and double check the fuse. It could also be an internal problem with the tack or the tach amp, but it sure sounds like a grounding issue to me.

HUH I have the same thing happening. I have also tried the cigarette butt fix with no luck.

__________________
NC Benz
Fuquay-Varina, NC
1979 300D
1983 300D Turbo 260,000 Miles
1984 300D Turbo 345,000 Miles (sons car)
OBK #31
1998 Ford Expedition 5.4l (fer Haulin'!) 145,000
1973 19' SeaRay with 115 Mercury TOWER OF POWER!
Club Car Golf Cart 36V
Ex toys:
1967 Mustang 289 (First Car)
Fiat 124TC
1975 Honda CVCC
1980 Audi 5000 Turbo

Take a DVM and set it to the A/C millivolt scale and measure the output of the tach amp (I do not have the pinouts at the moment but I'll post later). With the engine idling, you should read 100 to 200 millivolts at idle and it should increase as you rev the engine. If you get no reading, the tach amp or the mag pickup or the wiring in between may have problems, or it may not be getting power. Next, set the DVM to the ohms scale and read the pickup resistance which should be around 80 ohms (I'll post pinout later)

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Thanks, but I'm not an electrician.

You do not need to be an electrician. Owning one of these cars does require certian "goto" tools. Mity vac and Digital Volt Meter are two.

What Funola is saying is a very easy test. A multi function meter will have the capability to do the test he is talking about.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

funola, still post pinout points, please.

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83 SD

84 CD