OVP - where exactly is this thing? - PeachParts Mercedes-Benz Forum

VW1300 · #1 08-06-2010, 10:40 PM

Greetings:

Last June I got my A/C serviced (new hose and pressure switch, charge w/ R12) and it is working again. Shortly thereafter, my tach started getting flaky and usually doesn't work. My turn signals got flaky too although a replacement relay has seemed to help. The A/C now cuts in and out randomly, although that could be due to a leaky compressor, a known issue. But when it works, it works very well, which is about half of the time.

Reading previous posts has led me to wonder about the OVP relay - it seems this is tied to the Klima somehow as well as the tachometer. Questions:

1. Am I essentially correct in that the OVP is related to the Klima and tach?
2. Would a bad OVP cause these symptoms?
3. Where EXACTLY is this thing? I looked around the battery and the back of the engine compartment but couldn't find anything that looked like a relay. On the driver's side fender a black plastic box is mounted, inside is a small unassuming looking relay that says "Klima" but there are no fuses in there.
4. If I need to test the OVP, how do I do that?
5. Are the ones found in salvage yards usually still good, or should I just go ahead and buy a new one?

Thanks,

Jeremy5848 · #2 08-06-2010, 10:43 PM

In the '85 300D, the OVP is cunningly hidden on the right side of the dash. Remove the lower dash panel on that side. First check for a bad fuse (10A spade-type). In your car, the tach signal goes through the EGR electronics and then to the tach and to Klima. No tach signal, no a/c.

Link to thread with picture of OVP in '85 300D sans dash:

http://www.peachparts.com/shopforum/showthread.php?t=277159

See post #16 in the following thread for information on testing these components:

http://www.peachparts.com/shopforum/showthread.php?t=282324

Jeremy

VW1300 · #3 08-07-2010, 11:21 AM

OK, found OVP - thanks. It's easy to get to when you know where it is. The fuse tests OK. Which doesn't necessarily mean the OVP is good. But they're kind of expensive to just take a chance on at $75 a pop.

Jeremy, regarding your second link to testing the electronic idle speed etc - frankly it's a little over my head - can you translate the method to test EGR electronics into something a little simpler and more direct? And does my 123 have the same control unit/setup described for the W124 - specifically the box called "N8" in the procedure -where would mine be?

thanks,

Jeremy5848 · #4 08-07-2010, 12:10 PM

No, your 123 is not identical to the 124 in the pdf but it is similar. Much of the early 124 diesel electronics was introduced in the last 123 diesel -- your 1985. I had one of those and have an '87 124 so I know both and they are similar.

As to the pdf, I thought that the OVP test was done on the OVP relay itself, my bad. There are tests you can do on the OVP but I don't have time to find and post them for you this morning. I'll try to get back to this later today or maybe someone else can provide some help. OVP is really very simple -- it has a Zener diode rated at about 22 volts (my measurement, my OVP, YMMV). If the voltage from your car's alternator spikes due to some kind of a failure or if someone makes an error in jump-starting your car, the Zener goes into conduction, effectively causing a short-circuit and blowing the fuse. The Zener can go bad and cause the same effect. If the fuse is good then the Zener hasn't been "triggered" and the only other thing that can go bad is the relay itself.

The Klima relay also can be tested to see if it is doing its job of controlling the a/c compressor's clutch. I'll get back to this later.

Jeremy

VW1300 · #5 08-07-2010, 01:44 PM

Thanks. More info which may or may not be relevant - the tach usually sits somewhere between 500 and 1000 rpms no matter what the engine is really doing. But sometimes after coasting when I step on the gas, the needle swings up and reads around 2000 rpm, which is what the engine would really be doing. As soon as I lift off the gas, the needle swings down again. I'm thinking that the little increase in voltage when the engine operates above idle is making a difference here somehow - the tach works briefly in this situation - I don't know what though.

Also - the tach issue started right after the a/c service, and I know they were messing around down on the right side of the dash - could they have bumped or damaged something while working under there?

Jeremy5848 · #6 08-07-2010, 06:14 PM

Quote:

Originally Posted by VW1300

Thanks. More info which may or may not be relevant - the tach usually sits somewhere between 500 and 1000 rpms no matter what the engine is really doing. But sometimes after coasting when I step on the gas, the needle swings up and reads around 2000 rpm, which is what the engine would really be doing. As soon as I lift off the gas, the needle swings down again. I'm thinking that the little increase in voltage when the engine operates above idle is making a difference here somehow - the tach works briefly in this situation - I don't know what though.

That's weird! Can you put a voltmeter on (for example) the lighter socket and watch the voltage when you drive the car? It should be up around 14 volts unless you are really loaded down with heat or a/c on high speed, lights on, high-power stereo, etc. Even at idle the voltage should be well over 12 volts unless you have a charging system or battery problem. Speaking of which, others have had similar problems that were traced to low system voltage but those were usually later-model cars with much more electronics (W210, etc.).

Quote:

Originally Posted by VW1300

Also - the tach issue started right after the a/c service, and I know they were messing around down on the right side of the dash - could they have bumped or damaged something while working under there?

That is possible. The electronic box for the EGR system lives behind the kick panel on the right side of the front passenger seat footwell, not far from the OVP (which it uses for its power source). The EGR electronics almost never fails but a wire could have been torn loose, I suppose. If you get down in the footwell, you will discover that there is a magic door, a removable small piece that is part of the kick panel. Remove that and you will see the EGR electronic box.

Jeremy

panZZer · #7 08-07-2010, 06:20 PM

Quote:

Originally Posted by Jeremy5848

That's weird! Can you put a voltmeter on (for example) the lighter socket and watch the voltage when you drive the car? It should be up around 14 volts unless you are really loaded down with heat or a/c on high speed, lights on, high-power stereo, etc. Even at idle the voltage should be well over 12 volts unless you have a charging system or battery problem. Speaking of which, others have had similar problems that were traced to low system voltage but those were usually later-model cars with much more electronics (W210, etc.).

That is possible. The electronic box for the EGR system lives behind the kick panel on the right side of the front passenger seat footwell, not far from the OVP (which it uses for its power source). The EGR electronics almost never fails but a wire could have been torn loose, I suppose. If you get down in the footwell, you will discover that there is a magic door, a removable small piece that is part of the kick panel. Remove that and you will see the EGR electronic box.

Jeremy

I heard it's 3 things, Mainly the injection pump 85 rack position sensor computer. But also the tach signal goes thru it and it also has something to do with the egr. The magic door is supposed to give access to a screw to adjust the idle speed on an 85--but there's no screw there??

Jeremy5848 · #8 08-07-2010, 07:23 PM

I believe there's a plug containing resistors to vary the idle speed -- but the idle speed adjusting thing was not added to the IP until the next model, the 1987 300D Turbo (W124, OM603). You can pull the plug out and put it in in a different position but I no longer remember what (if anything) it is supposed to do.

Yes, the tach signal goes through the box and the IP rack position sensor talks to it. If you have disabled EGR, then the only thing it does is pass on the tach signal. Otherwise it could be removed.

VW1300 · #9 08-08-2010, 10:32 PM

Jeremy:

Thanks, found the EGR secret door. Nothing much to see, just the box and round thing. BTW, the idle speed on my '85 is controlled by a screw-type thing held in place by a nut, all on the side of the IP if I recall correctly. Only had to mess with it once, after a valve adjustment.

I confess I never did check the voltage output, even though I should have as a regular maintenance item anyway, but will look at that tomorrow and report back.

Some other things happened after the a/c service too, one of the power window fuses died. About 2 weeks after the service, another fuse went bad and caused the a/c to not work. I don't remember which one it was. But it seems to me something happened which is/was causing a stress on the electrical system.

Jeremy5848 · #10 08-09-2010, 01:49 AM

Correct, the '85 OM617 has a mechanical idle adjust only -- mind did. I suspect that Mercedes was "experimenting" with electronic idle and in the end decided to leave it off. It would be interesting to know if the OM601 in the W201 (190D) has electronic idle control -- a lot of the things that ended up on the W124 models in 1986 (gasser) and 1987 (diesel) were introduced on the 1985 W201 cars. The various "Introduction into service" manuals for the different years make many references to such things.

VW1300 · #11 08-19-2010, 05:23 PM

I am back from vacation, and tested the voltage at idle and higher revs - it varies bt 13 and 13.5V.

How do i check the components of the tach system? I can't stand it when the clock or gauges don't work - stuff I can't see doesn't bother me as much. The a/c has been good lately, even when the tach is reading 0.

Another question - is the tach gauge itself the same in the '82-85 W123s? (yes I know the signal is generated differently). There are some at the boneyard that I can get cheap and try out.

thanks,

Jeremy5848 · #12 08-19-2010, 07:53 PM

I cannot say with any degree of certainty but suspect that the meter movement is the same providing the RPM range is the same (so a gasser tach probably would not work). You should be able to measure the AC voltage signal feeding into the tachometer at the connection on the back of the instrument cluster. The signal should be a couple of volts at most; if it is absent, the problem is "upstream." If the signal is there but the tach needle does not move, the problem is the meter movement or its source of 12 Volt power or its ground connection.

VW1300 · #13 08-19-2010, 09:37 PM

Stupid question, but the AC voltage coming thru the wires should be directly proportional to engine revs? If I'm sitting in the car (running, in park) with the multimeter clipped onto the connector, the AC voltage should increase when I step on the gas, then fall to a lower value when the engine idles? Just checking so I know what to look for.

Thanks,

::matthew · #14 08-19-2010, 10:28 PM

the 85 OVP caught my attention, but I see that it was found and likely worked out.

I'm no expert in these electrical issues, but it seems that bad grounds are the most common cause for weirdness in the gauge cluster. Any chance the tach is suffering from bad grounding?

Jeremy5848 · #15 08-19-2010, 11:24 PM

I don't know if it's the voltage or the frequency that increases with increasing engine RPMs but I suspect it's the frequency. In another thread, a forum member reported measuring a frequency (in Hertz, or cycles per second) at idle which, when translated into RPM, came out at about 650 or so. I think the ring gear on the flywheel has 144 teeth so you get 144 pulses every time the crankshaft turns once. I've never made the measurement myself and am reluctant to claim anything one way or the other until I do. Clearly, if it's a variable frequency, you won't be able to see any difference with an AC voltmeter. In fact, you may not pick up anything at all if the frequency is out of the meter's sensitivity range. I need to put an oscilloscope on the tachometer sometime and see what the signal looks like.