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

MTUpower · #7 12-21-2004, 09:51 PM

You may need a new tach amp- and I have several working units if you need one- or you might need to check your Over Voltage Relay Protection fuse, which is located in the fuse box. It's a red topped relay with a fuse in it. Check that fuse.

pawoSD · #8 12-22-2004, 03:07 AM

My tach amp is new, I bought the one thats in there brand new off a parts website back in the summer, but installing it made no difference, I didn't check the overvoltage protection relay fuse, I'll look into that, I did try pulling out/cleaning/reinstalling all my regular fuses, nothing changed from that either. This is on the 82, so my tach amp is on the fender, and the new tach amp part was identical to the old one, regular 6 pin amplifier. Tomorrow I'll check out my fuses and the sensor to see if I can get it to wake up, the tach on our 83 works perfectly, its not fair.

kmaysob · #9 12-22-2004, 03:22 AM

borrow the tach amp off the 83. its possible the new one was defective. or do just the opposite and put the "new amp" on the 83 and see if it works

crazy banana · #10 06-25-2005, 07:27 PM

PawoSD, did you ever get your tach fix??

Johnhef · #11 06-25-2005, 08:10 PM

Quote:

Originally Posted by SYRacing

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

Mine would do the same, hit the horn and the tach would bounce.

Mine turned out to be a fried resistor on the tach circuit board. I do however, need some help from anyone who has the faceplate off their tach at the moment. When soldering new resistors in, I lost track of one of the contact points, so I'm not sure where one goes. If someone could take a picture of the circut board of the tach/clock, I'd really appreciate it. Mine was from a 1982 where the amp was still on the fender of course, 84's and up had different pins on the rear of the tach so I'm not sure if one of those will work. I have an 84 tach filling the empty space while I'm repairing mine...

SYRacing · #12 06-25-2005, 09:12 PM

Yeah mine is still doing it...without a functional speedo I rely on the tach for an estimated speed calculation...
It's intermittent but I can live with it.

crazy banana · #13 06-25-2005, 09:25 PM

the check pin on the counter weight right above the oil pan. if the threads for the oil pan bolt is messed up. you'll have to put a nut on top of the bolt to tighten the oil pan bolt. if that bolt is too long...it'll knock off the pin on the counter weight. the Tach senser read off that pin. that's what happened to me. i have to drill it and make a new pin in order for that tach to work again.

pawoSD · #14 06-26-2005, 01:40 AM

No I never got my tach to work....

It hasn't so much as twitched in over a year, its completely dead....despite the pins going into the (replaced with known to work) gauge having 13.6 volts at idle.....and a brand new tach amp, and all the contacts cleaned.....I've never been able to figure it out so I gave up. I'm guessing the problem lies with the crank/pickup, since there is voltage at the gauge, there must just be no signal for it to read/amplify. When I first got the car it would work at random, sometimes it would jump up and stay at max rpm for a few seconds, twitch around, then fall dead. So I replaced the amp....no difference it was still dead and no longer did the twitching....so I replaced the gauge pod with one from **************/ebay, and no go with that either....right around then I gave up. At least the clock on the new gauge pod works....the old one didn't.....

trontek · #15 06-26-2005, 11:22 AM

socket that the amp plugs into is some sort plastic/nylon(?) and has disentegrated so that it no longer holds the pins properly. I have yet to order a new crank sensor with a new cable and socket but will do so shortly unless someone knows where I can get a new socket to solder on the end of my cable.