Intermittent problem: Tach dies and idle drops, gets rough. - Page 2 - PeachParts Mercedes-Benz Forum

DieselCJ · #16 10-23-2006, 11:48 AM

Where is this relay located so I can check the fuse and contacts?

Thanks,
John

Quote:

Originally Posted by barry123400

If it is an over voltage protection relay you might be able to monitor it with a meter once you have a look at a schematic. And understand how it is integrated into the circuit and it's apparent function. If you have a friend with one you may be able to substitute it as well for a test. The last is not too likely unfortunatly. You could buy a new one for the best price you can get and ebay it if it is not the problem. Everyone has heard so much about them you might even make a little money. Stranger things have happened.

If there is a fuse iside or outside of it clean the connections and check or substitute the fuse.

thang · #17 10-23-2006, 01:48 PM

It's behind the battery on the center side on my 1987 300D. It's white and square with a 10 A fuse ont the top.
I had the same thing happened on mine, changing the relay did not help. I pulled the sensor out, cleaned it, put it back, it did not help. Then suddenly it fixed by itself for a while now. I think a bad wiring/connection is the cause.
Good luck.
thang

DieselCJ · #18 11-02-2006, 06:39 PM

Is the red top unit the OVP relay? If so, the fuse and contacts look fine, where are the soldered joints that go bad?

Thanks, John

DieselCJ · #19 11-03-2006, 08:43 AM

gsxr · #20 11-03-2006, 11:34 AM

Quote:

Originally Posted by DieselCJ

Is the red top unit the OVP relay? If so, the fuse and contacts look fine, where are the soldered joints that go bad?

Inside. You'd have to take it apart if you want to try and fix it.

DieselCJ · #21 11-03-2006, 01:25 PM

gsxr - is this the unit?

I recall a thread about soldering a unit that has a few relays inside?

BTW, I removed the fuse and the engine ran the same (not well), does this prove the relay is bad?

This looks fine to me:

DieselCJ · #22 11-03-2006, 01:35 PM

The number on my unit is: 210 540 08 45

Most of the OVP threads indicate part #210 540 37 45

fastpakr · #23 11-03-2006, 03:16 PM

It's just a guess, but I'd suspect that the '37' unit is an updated part.

DieselCJ · #24 11-03-2006, 03:30 PM

Quote:

Originally Posted by fastpakr

It's just a guess, but I'd suspect that the '37' unit is an updated part.

I just called benz, they are both OVP relays, but for different cars. Mine is $83 in stock.

DieselCJ · #25 11-04-2006, 09:36 AM

Woohoo! Problem solved!

Thanks for the input everybody!

This morning I got the part at the counter, then went to the cashier who asked for about $128 (with tax).

I went back to the parts counter to tell the man it was $83 on the phone yesterday... after typing at his computer for a minute he said: "ok, you can go back to the counter" Total was $89.

I then went back to the parts counter and asked why there was a price difference since I will prob be buying parts in the future, he said: "They gave you a discount and didn't tell me", hmmm... all I did was call up yesterday, they had no idea who they were talking to.

gsxr · #26 01-03-2007, 04:55 PM

My "new" 1993 300D has a similar problem... intermittent dead tach. No ABS light though, so I replaced the crank sensor ($50 + a couple hours labor). Same deal, worked OK at first, then back to intermittent operation. D'OH! I thought a flaky OVP would trigger the ABS light. I was about to swap OVP's with my '87, which is working fine. Then, I discovered that the 1987 OVP relay and 1993 OVP relays are different! The newer one has more pins, and 2 fuses instead of 1. Grrrr. Sounds like I may need to replace mine too.

Question: Did you have ANY other symptoms besides the tach dying? Was the ABS light ever lit when it shouldn't have been?

Part numbers & photos:

201-540-08-45 = 1987 relay

201-540-37-45 = 1993 relay

gsxr · #27 01-09-2007, 12:28 PM

UPDATE:
With the tach not working and idle speed low, I tapped on the OVP relay with a screwdriver. Bingo, the tach and idle speed instantly went back to normal. I replaced the relay, and the problem was cured. Wish I had tried that *before* replacing the tach sensor at the crank/flywheel.

Lesson learned: On the older cars, a bad OVP relay typically appears as dead tach, low idle, AND the ABS light coming on. (If there's no ABS light, the sensor is probably bad, not the OVP.) On newer cars, the tach can be out, OR the ABS light on, and the OVP may be bad... but you won't necessarily see both symptoms at the same time.

Anyone want a good, used, OM60x tach/crank sensor cheap? I've got an extra now...

iceman805 · #28 07-18-2007, 05:18 PM

Quote:

Originally Posted by gsxr

UPDATE:

Lesson learned: On the older cars, a bad OVP relay typically appears as dead tach, low idle, AND the ABS light coming on. (If there's no ABS light, the sensor is probably bad, not the OVP.) On newer cars, the tach can be out, OR the ABS light on, and the OVP may be bad... but you won't necessarily see both symptoms at the same time.

All of the sudden a few days ago the idle on my recently purchased OM606 became rough, and the tachometer started intermittently failing (at this point I didn't note the A/C was probably out as well). I decided to check the vacuum system and after returning some vacuum lines that came apart under the manifold to the miserable state in which I purchased them (I intend to go back in armed with all measure of vacuum elements!) I noticed that the tachometer had completely stopped functioning, it was still idling rough, the A/C was out and now it was shifting hard as well. In searching rough idle, hard shifting, intermittent tachometer I just found the OVP threads.

I used the test ovp thread to see if I could determine if it was definitely bad. Since this was my first time using a multimeter I don't know if my methods were sound so I will be performing this again after work today.

Three of the pins looked to be a bit discolored/darker compared to the others. I used both red and black leads to connect to each of the pins while plugged into the connector I noted various levels of current anywhere from 3.x to 9.5 and I swear that some reading increased from what they were the previous moment on the same pin.

After this "test" my idle problem disappeared, my tachometer returned and my A/C is back now. I couldn't tell if the shifting issue has been resolved yet but I'm sure hoping so.

Questions:
Is there somewhere I can look up what the proper readings were supposed to be when testing the OVP and what settings on the multimeter should I use? I bought a 12 range one with a little case from radio shack.

Is it possible to clean off the pins and continue using or is that not advisable.

Thanks so much to those who've gone before me posting on this issue and any help with this is very much appreciated.

gsxr · #29 07-18-2007, 05:28 PM

I haven't found any easy test with a multimeter. However in my case, the test was simpler. With the problem present (low idle, dead tach)... tap the OVP relay with a screwdriver lightly. If the symptoms suddenly disappear, the OVP is bad.

The discolored pins really don't mean much unless they're really corroded, or have green crud on them. For your VOM, measure the battery voltage, select a range that allows easy reading of 12V from the battery.

iceman805 · #30 07-19-2007, 04:29 PM

OMG! I would believe it's too good to be true if it wasn't for the fact that it happened to me.

Replacing the overload (overvoltage protection) relay fixed all these problems in my 95 E300D. Unreal...

Now about those Russian EPC schematics I've been pouring over and my vacuum leaks...this list rules - UBER ALLES!

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