Another try....at fixing my tach - PeachParts Mercedes-Benz Forum

pawoSD · #1 12-21-2004, 01:28 AM

I have always wondered why my tach won't work, when I first got my car it would occassionally do a sporatic bouncing and rapid movement all over the place. This was rather rare however, so I replaced the amplifier on the fender, nothing, I cleaned all the contacts and checked the wires, nothing, I used an electric contact cleaner on the contacts, nothing, I then replaced the tach/clock cluster pod, still nothing.....using my multimeter on the cable that plugs into the tach in the dash revealed that it supplied 12.8-13.4 volts, depending on engine speed.....still no tach readout however. I put so much effort into fixing it, I just wish I could get it to work.....any other ideas of what could be wrong/what I could do? I tried the search....but didn't find any issues where the "cigarette butt trick" didn't work, I tried that and more...

Help?

pawoSD · #2 12-21-2004, 02:23 AM

Anyone? I just don't know what else to try in attempt at getting it working again....

Pete Burton · #3 12-21-2004, 03:35 AM

By amplifier on the fender, do you mean the RPM sensor with 6 cylindrical prongs thatplugs into a socket on the left front fender? I think that is what fails the most. However, the sensor itself can also fail for a few reasons. The wire can get frayed and/or partially cut. Or the sensor position can be misadjusted at the crankshaft. Try this: with engine not running but ignition on, have a buddy watch the tach while you wiggle the wire all along the RPM sensor. Using a stubby, flat blade screwdriver, move the screwdriver blade toward and awat from the sensor rapidly. There should be some tach movement

pawoSD · #4 12-21-2004, 03:34 PM

The sensor is the only thing I haven't looked into....I did wiggle its wire quite a bit, but didn't do the screwdriver test, I'll try that later today....the amplifier on the fender is brand new, well in July it was, but I don't even know if the old one was the problem, I am guessing its probably a problem with the actual crankshaf sensor then right? I'll try your suggesstion and try realigning it too, it seems a bit far away from the crank, but its so hard to reach in there to get at it/adjust it....oh well, I'll try.

R Leo · #5 12-21-2004, 03:50 PM

Does your 'new' tach amp (approx a 2" tall cylinder out on the fender) look like the old one? '83 was the last year for the tach amp to live out there. After that, the diagnostic plug is still there with a simple screw on cover about 3/4" tall, and the tach amp is somewhere else in the car.

SYRacing · #6 12-21-2004, 08:59 PM

had the same problem with a sporatic tach on my 300D....ended up being my lack of a fuse cover was causing the tach to be sporatic....a simple beep of the horn would cause it to start working again. Cleaned all fuses and contacts and got a cover...all is well

yuke · #7 12-21-2011, 10:30 AM

Took me 9 months to figure out why my tach would not work on my 85 300 SD turned out to be the simple OVP relay even though the fuse looked good on top of relay the relay itself was bad replaced it and tach worked great. I did encounter a problem this past summer where the tach would bounce when it was at about 3000 rpm but lately has not been doing it so forgot about for now. Happy Holidays

pawoSD · #8 12-21-2011, 08:10 PM

Oh it was resolved, I just never updated the thread.

It was the amp....had to clean the terminals and replaced it with a brand new one ($85) back in 2005....been working flawlessly ever since.

Wshew....7 years since I posted this thread!

Still drivin' the same car though!

106,000 miles later!

tyl604 · #9 12-22-2011, 11:39 AM

pawoSD - my 81 SD tach has not worked since about 86; when it does it exhibits the same strange pegged or zero reading or uncontrolled bouncing. Then a few months ago I got a rebuilt tranny installed. The tranny would not shift but the tach now worked perfectly. I drove it about five miles back to the tranny shop with the tach working fine. They installed another rebuilt tranny and tach has not worked since. Except for a sporatic 100% peg or a few bounces. So what happened between the first replacement tranny and the second; why did it work with the first replacement and not the second? Almost sounds to me like whatever attaches to the tranny could need tightening up, loosening, or something.

Ideas?

pawoSD · #10 12-22-2011, 11:54 PM

Quote:

Originally Posted by tyl604

pawoSD - my 81 SD tach has not worked since about 86; when it does it exhibits the same strange pegged or zero reading or uncontrolled bouncing. Then a few months ago I got a rebuilt tranny installed. The tranny would not shift but the tach now worked perfectly. I drove it about five miles back to the tranny shop with the tach working fine. They installed another rebuilt tranny and tach has not worked since. Except for a sporatic 100% peg or a few bounces. So what happened between the first replacement tranny and the second; why did it work with the first replacement and not the second? Almost sounds to me like whatever attaches to the tranny could need tightening up, loosening, or something.

Ideas?

The pickup for the tach is on the front of the engine on an 81SD, and the tach amp is on the driver side fender. Transmission swap shouldn't affect it at all unless they jarred something loose or did not connect something. Depends how they did the swap....engine pulled? Or from below?

tyl604 · #11 12-23-2011, 03:19 PM

They pulled it from below. I am leaning to something needing tightening down near/on the tranny but do not know what.

hey_allen · #12 12-22-2011, 10:35 PM

Having replaced my tranny (and engine) in my SD, I can say with some certainty that there isn't any electrical tach signal from the transmission, though that does seem awfully coincidental in the timing.

What I did to get mine working again was to clean the face of the sensor down at the 2 o'clock position of the harmonic balancer, and then took a very small screw driver to the back side of the socket that the tach amplifier plugs into, using it to tighten each of the sockets that the pins on the tach amp touch.

Once I had done this, I've had no issues with the tach other than a couple brief jiggles of the needle.

I suspect that the cleaning of grease and grime off of the front of the sensor did almost as much good as the contacts being tightened, as dirty grease plays havoc with the magnetic sensors on the airplanes that I work on occasionally.