[sethwm2]

I pulled down the kick panel on the passenger side. Removed the tach relay, sure enough the red 10 amp fuse on the top of it was blown. I replaced the fuse. Put the relay back, it still did not work. The relay said west-Germany on it so it is original. All the the fuses are brand new from the dealership in the fuse box. Should I replace the entire relay as my next step? Then after that replace the sensor in the bell housing? Thanks.

 

[mach4]

I pulled down the kick panel on the passenger side. Removed the tach relay, sure enough the red 10 amp fuse on the top of it was blown. I replaced the fuse. Put the relay back, it still did not work. The relay said west-Germany on it so it is original. All the the fuses are brand new from the dealership in the fuse box. Should I replace the entire relay as my next step? Then after that replace the sensor in the bell housing? Thanks.

If the fuse did not blow, I'd pull the plug on the EGR controller and check for 12v on pin 1

The sensor in the bell housing is about the last thing to check, but check the connector between it and the EGR controller for corrosion, etc. It's almost never the problem. Check all your wires for continuity and fraying. Check the green/black wire between the EGR controller and the tach for continuity.

 

[sethwm2]

The new 10 Amp fuse did NOT blow out after installing it. Will check the EGR plug soon. Thanks.

 

[sethwm2]

If the fuse did not blow, I'd pull the plug on the EGR controller and check for 12v on pin 1

The sensor in the bell housing is about the last thing to check, but check the connector between it and the EGR controller for corrosion, etc. It's almost never the problem. Check all your wires for continuity and fraying. Check the green/black wire between the EGR controller and the tach for continuity.

Do I use a volt meter and put the positive on pin 1 and put the ground on pin 14 to look for the 12 volts? How do I check that green and black wire for the tach?

 

[mach4]

Do I use a volt meter and put the positive on pin 1 and put the ground on pin 14 to look for the 12 volts? How do I check that green and black wire for the tach?

Yes, just put a voltmeter on pin 1 and the other end to ground and look for 12v. As far as the tach wire, I'd do a quick preliminary check for continuity. Pull the plug out of the back of the cluster and check for continuity between pin 25 and the tach plug.

If all that checks out then you can think about putting a voltmeter on the tach connector and looking for a change in voltage with engine rpm.

There are only 5 components to the system - the VR sensor (82) on the bell housing, the OVP relay (85), the EGR controller itself (80) and the tach movement (a)... and of course the wiring and connectors.

Here's the entire system

 

[sethwm2]

Thanks to everyone who responded!

Well, most of the time I do not follow up with things that I have fixed or solved. But thanks to what you guys have posted in this post an along with other posts I have found the culprit of the Tach failure. It was the 10 amp 12 volt overload relay located under the far right vent on the passenger side under the kick panel. After finding out that the 10 Amp fuse located on the top of the relay was broken, the next easiest thing to check was the relay itself. I went to Mercedes and they had one IN STOCK! :thumbsup:

I installed the new one and sure enough the tach works beautiful!

I have attached photos of the old relay on the top of the box that the new one came in. You can see the part number so you can get one as well if you are having a hard time with your tach.

Looking at the schematics posted here and on the side of the relay, I can see that there is a diode in the relay that converts the signal from the sensor to DC so that the gage can read it I think? The old relay has something rattling around inside of it as well.. must be that diode...

Thanks again!

 

[mach4]

Well, most of the time I do not follow up with things that I have fixed or solved. But thanks to what you guys have posted in this post an along with other posts I have found the culprit of the Tach failure. It was the 10 amp 12 volt overload relay located under the far right vent on the passenger side under the kick panel.

Great job.

 

[Bill12632]

Need the address and phone number of Bob's Speedometer,or any other shop that doesen't charge an arm and leg for their services. Bill12632

 

[Jareb]

Need the address and phone number of Bob's Speedometer,or any other shop that doesen't charge an arm and leg for their services. Bill12632

Here ya go..look under contact: Bob's Speedometer

 

[QXwerty]

Is the relay itself usually the culprit? I just went outside and pulled mine, and the fuse is perfectly intact. I've suffered through two of these tanks with effed up tachs. My '83 was fixed by sticking a few foam earplugs up into the amp cap under the hood. My '83 300CD... lots of familiar parts. Lots of unfamiliar parts. But I really would like another working gauge aside from the speedometer lol

 

[gymbeau]

I have been having trouble with my tachometer and air conditioner compressor not working. They seem to be tied together because they both fail to work at the same time. After much research on multiple forums, I think I uncovered a solution.

Gain access to the connector at the EGR controller. I think most of us have disabled the EGR for one reason or another, so the controller is basically useless. Make up two jumper wires. Install one from pin 20 (RPM sensor) to pin 25 (to Tachometer). Install the other one from pin 14 (ground) to pin 16 (RPM sensor). I just did mine and everything is working again. I put the connector on top of the EGR controller after I wrapped electrical tape around the jumpers to make sure they don't come out.

By the way... is it taboo to post links to other forums here?

 

[Kampzite]

Where is this egr controller?

 

[gymbeau]

EGR controller is behind the right (passenger) kick panel. Just forward of the door. You have to remove the "hush panel" under the glove box and then the plastic kick panel. I also had to remove all of the carpet in order to get the kick panel out.

 

[Kampzite]

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Thanks. Good to know for the future.

 

[QXwerty]

Any tips on getting the kick panel off? Aside from taking out the carpet?

 

[gymbeau]

Not sure how far you got. The hush panel has to come down first. There are 4 screws with 3 being behind covers right under the glove box door. The kick panel has one screw right on the floor. It eluded me at first. The strip next to the car door needs to be pushed outward and there is a hidden clip there near the car door. After all of that, it's a matter of wiggling it until it realizes that you are going to win.

 

[QXwerty]

Guess my terminology is off. Hush panel - the panel I had to remove to replace the blower motor? If so, it's off. Kick panel - the plastic panel that made me cuss in German on the right-hand side of the PS floorboard? That's the one I'm going to go back and examine today. I've checked everything else re: tach not working. Aside from having the gauge cluster rebuilt, which I will probably do anyway, this relay is the only component I haven't checked.

Thanks again.

 

[QXwerty]

Okay, got the kick panel off. Thanks again. Found the EGR controller. Fuse on the relay is fine. Looks new. Going to test the jumper wire fix, but cannot seem to get the plug loose. I removed a small flathead screw. Do I have to hold my mouth a certain way to get the plug loose?

 

[QXwerty]

Got it. Both ends of the plug were corroded. What would usually cause that? Pics added. Jumper wire fix didn't work.

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[black85benz]

water from bad windshield gasket, or rust from around battery tray, they will corrode if heated up too much, but a fuse should have blown first if that was the case