Tach not working on 1985 300TD - ALDA related? - PeachParts Mercedes-Benz Forum

corchard · #1 04-08-2008, 10:45 PM

I've been enjoying these forums for over a year now, but have not yet found an answer to the last lingering problem I'm having with my 300TD.

Last December when I first bought my Federal (first sold in Texas) 1985 300TD, the tachometer was not working (along with a number of other things I've since fixed through advice in these forums).

Since it's a 1985, the cigarette butt trick won't work, and I've already replaced the fuse in the OVPR, as well as the OVPR itself.

So, I started thinking the problem must be in either the tach itself or the rpm sensor cable.

Then, after I replaced the very sooty banjo bolt and ALDA vacuum line to the switchover valve, the tach started to work- much to my pleasant surprise!

But it was all short-lived... because a few weeks later when I replaced the old, beat-up ALDA unit with a used one in better shape, the tach stopped working again.

Bummer... especially since I'm not sure how the tach "magically" started working when I had previously replaced the vacuum line. I've checked the OVPR and its fuse, tried jiggling all the lines I may have jiggled when I got the tach to work earlier, but to no avail...

Any ideas would be very much appreciated.

Magoo · #2 04-08-2008, 10:50 PM

Must be coincidence. Alda and Tach are not even close to being related. You have a loose connection somewhere.

DeliveryValve · #3 04-08-2008, 10:57 PM

Yes, It could be the wiring is amiss. Look at this diagram and follow it.

Then check the RPM sensor using this method.

corchard · #4 04-09-2008, 06:16 PM

Thanks for the quick replies! I figured it was just a coincidence between the ALDA changes and the tach operating those few weeks, but thought I would double check here.

The rpm sensor appears to be operating within the ranges in the test illustrated above, though I've now cleaned the contacts at both ends of it anyway, and am trying to follow the control unit wiring diagram posted above.

Where in my 1985 300TD would I find the control unit, #80 in the above diagram? And where is #86, the trimming plug?

Thanks in advance.

winmutt · #5 04-09-2008, 06:41 PM

No its probably not a coincident. The switchover valve and the tach ARE closely related. You might want to check the control boards. I believe #80 is the cruise control unit which is what controls all of this. Is the turbo boost kicking in? Eg is the switchover valve venting or are you getting a turbo rush?

corchard · #6 04-09-2008, 10:37 PM

Hmmm... I replaced the switch over valve about a week ago, and a few months ago I replaced the cruise control unit with a working one I picked up in a salvage yard.

As far as I can tell, the turbo kicks in- that is, when I mash down on the accelerator, I seem to have a boost in power.

DeliveryValve · #7 04-09-2008, 11:21 PM

Quote:

Originally Posted by corchard

...
Where in my 1985 300TD would I find the control unit, #80 in the above diagram? And where is #86, the trimming plug?.....

#80 the control unit #86 trimming plug is found on the passenger side kick panel. There is an access door. The trimming plug looks like a dial with 7 contact points inside it.

#80 is not the cruise control unit.

Jeremy5848 · #8 04-10-2008, 01:13 AM

The 1985 W123/OM617 models are unique in that the tach signal goes through that #80 controller, located behind the right front side panel in the footwell. As was previously mentioned, there's a "door" cut in the plastic panel. If the engine is otherwise operating well, the controller is probably OK. The culprit may be the tach sensor (#82) or the wiring somewhere between it and the controller or between the controller and the instrument cluster.

Check for damaged wires and then try replacing the sensor. I've never had to do this particular fix (knock on wood!) and don't remember where the sensor lives -- someone will tell us -- don't believe it is in the same place as the older cars.

Jeremy

JimmyL · #9 04-10-2008, 01:24 AM

Quote:

Originally Posted by Jeremy5848

Check for damaged wires and then try replacing the sensor. I've never had to do this particular fix (knock on wood!) and don't remember where the sensor lives -- someone will tell us -- don't believe it is in the same place as the older cars.

Jeremy

The sensor is on the tranny bell housing, drivers side down from firewall behind oil filter cannister.
BTW, what is the "trimming plug"? I've seen it in there with the EGR computer but have no idea what it does.
Never have had my tack working.....

DeliveryValve · #10 04-10-2008, 02:02 AM

Quote:

Originally Posted by JimmyL

The sensor is on the tranny bell housing, drivers side down from firewall behind oil filter cannister.
BTW, what is the "trimming plug"? I've seen it in there with the EGR computer but have no idea what it does.
Never have had my tack working.....

It's an idle control. Lift and turning the knob should raise or lower idle in 25 or 50 rpm increments.

corchard... does your A/C work? If not look into the Kilma relay or revisit the OVP relay.

johninva · #11 04-10-2008, 07:11 AM

hey, my tach doesnt work nor does my ac. Ive had some doubts about my turbo also, dont know if its turbo'ing or if its just kicking down when i mash on the pedal. Where is the ovp relay located ? How does the kilma relay play into the scenario ?

corchard · #12 04-10-2008, 04:49 PM

(JimmyL, thanks for asking about the "trimming plug"- that was going to be my next question. And thanks to DeliveryValve and Jeremy5848 for answering and guiding us all to it and the control unit.)

I was able to clean out both ends of the rpm sensor (a grimy, awkward job at the south end of it). This didn't change the multimeter readings I am getting from it.

Yes, my a/c works, as does the cruise control and everything else- just not the tach.

This morning I cleaned the contacts in the control unit (part # 002 545 6032)- they had some green oxidation. I also pulled out the instrument cluster and cleaned the tach contacts there, too- but still no working tach.

I posted earlier that as far as I can tell the turbo kicks in, though perhaps the tranny is simply dropping down a gear (this being the only 300TD I've ever driven, it's hard for me to know how the turbo should feel).

Since this tach worked briefly a few months ago (as described earlier in this thread), I'm now at a loss of what to do... as far as I can tell, all of the component pieces are working or are able to work, just not collectively.

I haven't found any cut or frayed wires, though I can't trace them entirely since they are wrapped as a bundle in plastic sleeves or pass behind the center pods in the dash.

winmutt · #13 04-10-2008, 04:54 PM

Does your cruise control work?

Jeremy5848 · #14 04-10-2008, 05:51 PM

Hmmm, this is an interesting one (more frustrating than interesting, you say?). The cruise control and tach have no related components (well, the engine, the battery, etc., but you know what I mean).

AFAIK, the only function performed by the "control unit" that lives behind the passenger footwell panel is to control the smog stuff. That's why the wire from the tach goes first to the control unit -- it needs to know engine RPM.

If the RPM sensor fails, the smog stuff won't work correctly. Depending on what you have done to the smog stuff, you may not even notice. In any case, the RPM sensor is supposed to have a DC resistance of 1.9 +/- 0.2 K Ohms. With the engine idling at 700-800 RPM, the sensor is supposed to produce at least 4 Volts AC. There's a connector on the firewall that you can unplug to do this test -- see the picture that DeliveryValve posted.

If the sensor works, another possibility is the tach itself, which is simply an analog meter. You can check it with an ohmmeter. The current that any common ohmmeter produces should be enough to bounce the needle of the tach. Be careful to touch the probes for just a second so you don't damage the movement or bend the needle. If the tach is good you'll know right away. If the needle doesn't move, that meter movement is bad.

If both RPM sensor and tach test good, the only other possible culprit is the wiring or the control unit itself, because I think the RPM signal goes through the control unit to get to the tach. The control unit looks to me to be an electronic board in an aluminum box, so it might suffer from the same kinds of cold solder joint problems that the cruise control module has.

The "trimming plug" in the 1985 W123/OM617 is for the smog stuff only. There is no electrical connection to control the IP (like there is in later cars), only a "rack position sensor." The "trimming plug" is wired directly to the "control unit" which has some control over the vacuum lines -- but only for the various smog devices, AFAIK, as shown in the circuit diagram posted earlier.

Jeremy

DeliveryValve · #15 04-10-2008, 06:46 PM

Quote:

Originally Posted by Jeremy5848

.....
The "trimming plug" in the 1985 W123/OM617 is for the smog stuff only. There is no electrical connection to control the IP (like there is in later cars), only a "rack position sensor." The "trimming plug" is wired directly to the "control unit" which has some control over the vacuum lines -- but only for the various smog devices, AFAIK, as shown in the circuit diagram posted earlier. ....

Regarding the trimming plug Jeremy. I understand what you are saying about the rack position sensor but in this post, M-Shop member Bstef says otherwise on post #11…. Maybe controlling the emissions output also has some effect on the idle. (Pic #1 of the trimming plug is from Bstef)

Quote:

Originally Posted by johninva

…. Where is the ovp relay located ? How does the kilma relay play into the scenario ?

The OVP relay is found behind the glove compartment. Check the fuse there. See pic #2 from Bstef on the attachment.

The Kilma Relay controls the A/C compressor. It starts and shuts it down. My theory is a possibility the Kilma disrupting the OVP Relay which causes the Tach and A/C not to work.