How do I test the tach? w124 - PeachParts Mercedes-Benz Forum

locry · #1 01-20-2011, 09:35 AM

I had my gauge cluster off, when i reinstalled it the tach was inop. I figured it was just a loose connection...but after fiddling with it some more it still isn't moving... I'd like to test the connector for signals... what readings should i be getting? Can i also test the Tach itself? what voltage should i apply to make it budge/jump?

punkinfair · #2 01-20-2011, 11:42 AM

i can't tell you how to test it but i've experienced the same thing where re/re of the cluster results in the tach going no-op. then it magically started working again. check the connections, surely in my case it was a bad connection which 'fixed itself'. it's not likely it broke because you re/re'd it.

Jeremy5848 · #3 01-20-2011, 12:23 PM

The tach itself is just a milliammeter plus an electronic circuit to turn pulses into current. It is unlikely to fail (but not impossible). The three-pin connector (only pins #1 and #3 are used) on the back of the tach is for +12V (pin #3) and the tach input signal (pin #1). Ground is through the chassis; the same ground is used for the clock. Since you have had the cluster apart, this ground is the most likely culprit.

There is a strip of printed-circuit wiring that runs along the bottom of the middle section of the instrument cluster. It carries ground between the left and right section. If this ground is defective due to corroded pc traces or missing screws, the tach and clock will not work properly or intermittently. This ground path is shown in a picture in this thread, post #10.

If the ground path is OK, the next thing to check is the tach sensor test point. It is pin #2 in the test jack, located in the engine compartment. Some cars have a black plastic cover over the connector; mine is missing its cover.

See this thread, post #19, for a picture of the test connector, near the battery, with a red arrow pointing to pin #2. With the engine running, there should be an AC signal at this point. It can be measured with an AC voltmeter, a frequency counter, or an oscilloscope. I'll check it later today and update this post with some real numbers.

The same tach signal goes to the Klima relay (pin #2) and the Electronic Diesel System (EDS, behind the battery), pin #25. If there is no tach signal, the electronic idle control and the a/c compressor won't work. If they work, the tach signal is present, no need to test.

Jeremy

locry · #4 01-20-2011, 08:31 PM

ey thanks for the helpful info guys! I have the cluster apart right now, might as well clean it.
Yes, the AC still works, the compressor is engaging, so i guess there's a tach signal before the tach. I'll check the ground circuit... my clock stopped working less than a year ago... but the tach just stopped working yesterday.
again thanks... will update once i have results

locry · #5 01-22-2011, 12:10 AM

quick question: where does the cluster get its ground? what's the source connector? coz it seems the tach cluster doesn't have ground. if it isn't the printed metal strip circuit thing what else could it be. i did clean the tach cluster side of the contact but i missed cleaning the other side. If it isn't that strip where else could the ground circuit have been cut?

Jeremy5848 · #6 01-22-2011, 03:05 AM

The cluster is grounded through pin #1 of the round 15-pin connector. The ground wire is brown. The ground point is W1, a stud in the chassis behind the left side of the cluster. Exception: the temperature display has its own ground wire. The metal pc board strip connects the left (gauges) and right (clock & tach) thirds of the cluster and provides a ground connection. The middle third (speedo, etc.) "floats" -- it has no ground except for the metal strip and is made from plastic anyway.

Jeremy

locry · #7 01-22-2011, 04:42 AM

thanks jeremy! you've been very helpful...scanned a bunch of tach threads, most of which you were onboard. hehe

to clarify... if the temp, fuel, and oil gauges work...its safe to assume there's ground through the brown wire correct? it's just not getting to the tach section via the metal strip?

locry · #8 01-22-2011, 06:04 AM

solved! I found a break along the printed circuit board... the ground path from the round connector to the metal plate where the metal strip attaches to has a burned out gap. I think I may have accidentally touched the positive connector of the clock to the metal plate of the tach cluster... don't know why i bothered to attach it since the clock was busted to begin with. anyway... i'll be attaching a separate ground wire to the tach section.
question: is the UNUSED pin on the tach 3 prong connector a possible ground for the tach?

Jeremy5848 · #9 01-22-2011, 11:38 AM

Quote:

Originally Posted by locry

question: is the UNUSED pin on the tach 3 prong connector a possible ground for the tach?

In all of the cars that I've seen, the middle pin is missing, there's only a hole in both the plug and the socket. Yes, you could fabricate something or perhaps replace the existing plastic plug/socket parts with new ones but it would be a lot of work. Easier to attach a ground lead to one of the screws holding the tach section and jump it across to the other side of the cluster. If you use a piece of solid wire, you can form a loop and slip it over the #1 pin of the 15-pin connector. Push the plug on and it will hold the jumper in place.

Jeremy

locry · #10 01-22-2011, 12:42 PM

Thanks Jeremy, that's exactly what I ended up doing.

I do have that 2nd pin in the cluster though...

To formally end my tach mini-saga: It was a simple missing ground issue that was traced to a broken PCB path due to a "positive", in my case the clock connector, making contact with the metal backplate of the tach cluster. Fix was as simple as fixing the looped end of a wire to PIN 1 (12 o'clock position) and sandwiching the other end to the metal "bridge" strip.

btw... i tested the tach gauge when it was still separate from the rest of the cluster by attaching it to its connector and grounding the metal backplate to chassis ground. This should've been my first step, could've saved me a lot of time and frustration.

oh well