DIY: Tachometer Service and Repair on 1987 300D Turbo - PeachParts Mercedes-Benz Forum

filon102 · #1 04-16-2010, 12:23 AM

Taken from "Alldata" back in my school, since Im trying to fix my tach problem, I wanted to share with you guys

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Quote:

The electronic tachometer (E-tacho) comprises an inductance transmitter on transmission and an indicator in instrument cluster.

The inductance transmitter on transmission translates the speed of the transmission main or input shaft into an electric signal. The number of signal impulses is in accordance with vehicle speed. The signals are converted in tachometer into a mechanical readout (speed and odometer).

On vehicles with automatic transmission the inductive transmitter (1) is combined in a harness together with the electric connections of starter lockout and backup lamp switch and can be exchanged only complete with harness.

If the E-tacho does not indicate, the following checkup can be made for diagnosis "inductive transmitter with line" or "E-tacho" defective.

Checkup

1 Remove inductive transmitter on transmission, unscrewing screw M 6 on inductive transmitter for this purpose and pull out inductive transmitter.

2 Switch on ignition. Place a wide screwdriver against underside of transmitter. 3 Pull off screwdriver with quick movements and put back again. This fast pulling and pulling back will generate a signal on intact transmitter which will cause the tachometer needle to perform a slight deflection.

4 If not, remove instrument cluster and pull off 4-point coupler (2).

5 Connect a voltmeter (measuring range 0-3 volts, AC) to jack 1 and 3 and repeat test.

If the needle on voltmeter deflects when the screwdriver is pulled away from transmitter, the transmitter with its line is in order and the E-tacho is defective. If the needle is not deflecting, the transmitter is defective or the line to instrument cluster is interrupted.

Good luck!

biopete · #2 04-16-2010, 01:34 AM

Nice. Thanks. Having same problem with tach on 87 TD . I changed ovp and it did not help. I can bump the dash hard with fist and tach jump up to 500 rpms and goes back down. So it wants to work.

filon102 · #3 04-16-2010, 01:45 PM

Quote:

Originally Posted by biopete

Nice. Thanks. Having same problem with tach on 87 TD . I changed ovp and it did not help. I can bump the dash hard with fist and tach jump up to 500 rpms and goes back down. So it wants to work.

lol dont break your cluster now

I hope this helps, I haven't test it out yet, but its a start. Good luck!

jmann.arch · #4 05-17-2010, 12:36 AM

Has anyone tried this to figure out tach issues?

I have an 87 300 td and the tach died after it was sitting for a while - at least I am pretty sure the tech worked before.

Since everyone asking about it had a 87 300 td - I was wondering if there was some particular issue why all our tach's have died?

Also - is there a better picture of the unerside showing where the inductive transmitter hooks to the transmission? I need more shown to locate it.

jmann.arch · #5 05-26-2010, 11:20 PM

filon102 - great post!

I pulled it out and did the flat screwdriver test to nothing. The end is a magnet (didn't expect that - but it makes sense). I pushed it off real quickly and there was no movement on the tach. I was kinda dissappointed and got ready to put the inductance transmitter back on. That's when I noticed the gashed wires - either mechanically, or gnawed nearly through. Pic posted(http://www.peachparts.com/shopforum/attachment.php?attachmentid=80952&d=1274929033)

SO the new transmitter and harness is on order.....

Jeremy5848 · #6 05-27-2010, 12:25 AM

I don't recognize the picture of the instrument cluster in the Alldata post but it isn't an '87 300D or TD. The '87 has a three-pin connector for the tach with only two pins used.

The tach signal goes through the electronic diesel system (EDS), which is in the component compartment behind the battery. If the EDS module dies, your tach won't work. The EDS module is protected by the OVP relay so that has to work also.

Other than damaged wiring, which is more common than you might think, especially in cars with missing belly panels, the ring gear sensor and the tach itself rarely fail.

Jeremy