W124 tachometer is intermittent - why?? - PeachParts Mercedes-Benz Forum

old goat · #1 08-05-2010, 06:53 AM

I have an 87 300TD which has an intermittent tachometer. I works fine sometimes and other times just does nothing. Everything else on the dash works fine. I have pulled the dash and checked the connection and there appears to be no problem there. If it is working, it does not quit and if not working, it does not all of a sudden start.....well at least so far.

Does anyone have an idea or does anyone have experience with this problem?? Any suggestions would be greatly appreciated.

Thanks

cirrusman · #2 08-05-2010, 09:48 AM

I'm not sure if a W124 has an equivalent, but my W126 has a tach amplifier in the engine compartment. with time the gel holding the internal connections together shrinks. A quick fix is to put a cigarette butt in it to push it down. I don't smoke so I use a piece of shop towel and it has worked fine for the last 6 months...

old goat · #3 08-05-2010, 11:39 AM

Thanks for the response. I have two questions. First off, where is this amplifier in the engine compartment on a W126? The second is whether the 6 cylinder W126 diesel has the same amplifier or not? I have a wiring manual for my car and see nothing denoted as an amplifier for the tach.

If I can locate it in my 300SDL, then it might help me look for or locate it on the 300TD.

Frank

power7326 · #4 08-05-2010, 11:50 AM

i have the same problem in my 92 300d w124 !! but im in the middle of pulling out my dash right now so the tach problem has been put on the back burner for now .

bobodaclown · #5 08-05-2010, 12:02 PM

I'm pretty sure the Tach and Cruise control use the same source for RPM info. If your Tach cuts out but the Cruise control works, might point to a bad Tach.

Jeremy5848 · #6 08-05-2010, 12:28 PM

The tach in the 1987 300D Turbo (W124, OM603) is driven off of the EGR electronics. There's a sensor back near the flywheel, the EGR electronics is in the compartment behind the battery, and so to the tach. Note that the OVP relay must be working correctly for all of this to work. The cruise control does not depend on the tach signal (cruise looks at road speed via a sensor on the speedometer) but the air conditioning and the transmission kickdown does. Check the OVP (also behind the battery) for a loose or corroded 10 Amp fuse and for loose or corroded contact from OVP to its socket. Ditto the EGR electronics. The tach itself is simply an electric meter (like a voltmeter) and rarely fails.

Oldwolf · #7 08-05-2010, 12:34 PM

Is there an OVP situation where the A/C will work but the Tach not? I think I have read that these two failures can indicate a bad OVP.

old goat · #8 08-05-2010, 12:45 PM

Thanks Jeremy. I will proceed to check that out as I have not looked at that area.

Oldwolf · #9 08-05-2010, 01:11 PM

OVP Testing

Jeremy5848 · #10 08-05-2010, 03:01 PM

Quote:

Originally Posted by Oldwolf

Is there an OVP situation where the A/C will work but the Tach not? I think I have read that these two failures can indicate a bad OVP.

Caveat -- I'm sure of what I say only for the 1987 300D Turbo (W124, OM603) 'cause I've got one. Other 124s should be similar but I'm not sure how similar.

With that in mind -- the a/c compressor's clutch is controlled by a "Klima" (probably a German mfr's brand name) relay that looks at a number of inputs and "gives permission" for the clutch to engage. No permission, no clutch, no a/c. The climate control system will still work but you will have no cooling.

The tach signal gets to the Klima relay from the EDS (Electronic Diesel System) module, which lives, with the OVP relay, behind the battery. If the OVP relay has died or blown its fuse, the EDS electronics won't work and there's no tach signal for the Klima relay, thus no tach and no a/c. If you have disabled your EGR system, the only thing the EDS does is (a) control engine idle speed and (b) pass along the tach signal.

It is possible for the a/c to work and the tach not -- a wire could be broken/damaged/shorted or the tach itself could be damaged or dead. The tach itself is just an electric meter movement with some electronics to turn the signal from the EDS module into a current that makes the tach needle move up the scale and indicate RPMs.

Probably the most fragile part of the tach itself is the ground connection. If you take your instrument cluster out and then take it apart, you will find a strip of metal running along the base of the middle third of the cluster (the speedometer part). This metal strip connects the right and left thirds and provides ground for the tach and the clock. [See the attached photo.] If this metal strip has gotten dirty or corroded over the years or if one of the screws at either end is missing or loose, you will have a flaky or non-working clock and tach. In many old cars the clock is already dead and the driver uses the clock in his aftermarket radio. In that case, the only indication is that the tach does not work.

Jeremy

Oldwolf · #11 08-05-2010, 03:08 PM

So, if the clock is working fine, there is not a grounding problem that might be effecting the tachometer?

Jeremy5848 · #12 08-05-2010, 04:17 PM

Quote:

Originally Posted by Oldwolf

So, if the clock is working fine, there is not a grounding problem that might be effecting the tachometer?

If the clock works, the ground has to be OK. The power to the tach circuit and the tach signal are carried by a two-pin plug at the back of the tach/clock module. If the tach circuit is not getting power or signal, or if it has failed internally, it won't work.

JimmyL · #13 08-05-2010, 10:52 PM

OVP relay fixed the tach on my W124. It was intermittent, but more inop than op. AC and other items worked fine....

Oldwolf · #14 08-06-2010, 11:45 AM

Is the signal to the Tach the top or bottom prong? I just had my I/C rebuilt, most of the capacitors were in bad shape and were replaced. Now the fuel gauge works but the Tach is still flakey. Worked for about 20 minutes then turned off again. So I am assuming the I/C is in good shape. Now I must check the wiring to the Tach, and the condition of the OVP.

Can I take my multi meter and take a measurement at the tach plug to see if a signal is getting through?

Jeremy5848 · #15 08-06-2010, 12:17 PM

Quote:

Originally Posted by Oldwolf

Is the signal to the Tach the top or bottom prong? I just had my I/C rebuilt, most of the capacitors were in bad shape and were replaced. Now the fuel gauge works but the Tach is still flakey. Worked for about 20 minutes then turned off again. So I am assuming the I/C is in good shape. Now I must check the wiring to the Tach, and the condition of the OVP.

Can I take my multi meter and take a measurement at the tach plug to see if a signal is getting through?

On my spare tach/clock unit, the top pin (the plastic pin housing is round at this point) has a green wire on the inside and is probably the signal. You should see a low-voltage AC signal (millivolts) with your meter.

The bottom pin (plastic housing is square) has a red wire on the inside so probably is the power lead. You could check to see if it carries +12 Volts. Also, in the sheet metal on the outside near this pin is stamped a "+" sign, confirming that the bottom pin is the power supply pin.

The middle pin is missing. Ground is chassis. Note that the clock has its own 12V on-all-the-time connection but shares ground with the tach.

I need to put a scope on this tach signal, see what it looks like. I suspect that the tach electronics is simply a frequency to voltage converter but that needs to be verified when I get time.

Jeremy