Question on jumpy tachometer on '85 300D - PeachParts Mercedes-Benz Forum

MercFan · #1 08-13-2010, 05:08 PM

I found a number of posts on this topic - apparently this is most likely related to tach amplifier problem which some fix using 'cigarette butt' technique...

On my car, I believe I've located the tach amplifier can but when I unscrew the screw top the plug inside is missing - there is nothing plugged into the socket - I never removed anything from there so it must have been missing when I purchased the car some 4 years ago. How is it that the tachometer was working fine until just this week?!

There is always a chance that I'm looking at the wrong thing here. I'll post a photo of what I'm looking at so you can tell me if this is it or not.

Meanwhile, if someone has some pics of what it should look like I'd appreciate it.

THanks,
James

techguy512 · #2 08-13-2010, 05:31 PM

Nope. Your car is not equipped with a tach amp. The empty cover you mentioned is exactly that, simply a cover for the "diagnostic port".

Your tach is controlled by the EGR computer, but most of the time problems with this later system are traced to other things like the OVP relay.

MercFan · #3 08-13-2010, 10:43 PM

Quote:

Originally Posted by techguy512

Nope. Your car is not equipped with a tach amp. The empty cover you mentioned is exactly that, simply a cover for the "diagnostic port". Your tach is controlled by the EGR computer, but most of the time problems with this later system are traced to other things like the OVP relay.

Help my out please - what's the OVP relay do, or where is it? What's my next step in diagnosing the jumpy tachometer - it just started acting up this week.

toomany MBZ · #4 08-14-2010, 08:28 AM

Check post #22 for a pic.
http://www.peachparts.com/shopforum/showthread.php?t=256636&page=2

MercFan · #5 08-16-2010, 06:50 AM

Quote:

Originally Posted by toomany MBZ

Check post #22 for a pic.
http://www.peachparts.com/shopforum/showthread.php?t=256636&page=2

So if the tach is jumpy (sometimes dropping, somtimes jumping uncontrollably) - would I start by checking the connections to that relay? I'm assuming from the picture that I'd have to remove the glove compartment insert to get at it?

toomany MBZ · #6 08-16-2010, 08:45 AM

I don't know, as I haven't had to tackle this problem.
Sounds logical tho.

MercFan · #7 08-23-2010, 09:35 AM

I think I found the wires that send the tach signal from the transmission bell housing - the two wires (white and brown) are attached to the firewall right in the middle of the engine bay (behind and above the engine). I connected the voltometer and set it on the AC settings, I started the engine and I could watch the voltage climb and fall proportionally as I stepped on the accelerator, so I think I'm getting a good signal.

I then took out the side panel on the passenger side to reveal two EGR devices - one looked like a little access port - was able to remove that without breaking anything - see picture 1. And the other in pic2 is the EGR computer (whatever that does). Is there anything servicable in those two pieces I've pulled off of this poor car?!

toomanyMBZ - I saw the pic you included, not sure I've got to that relay yet - didn't see anything with the red cap on it. Perhaps I have to remove the glove box to get to that relay?!

MercFan · #8 08-23-2010, 03:46 PM

I've located and removed the OVP relay (red top) - checked the 10A fuse - checked out ok with continuity check.

I see the four different prongs on the bottom of the relay.... could someone give clues on how to test the relay by applying 12V and measuring output at various pin combinations?! Looks like a $47 dollar item to replace... I wanna be sure.

Thanks - plz help

toomany MBZ · #9 08-23-2010, 04:29 PM

I picked one up from the JY, number 126 540 47 45, (1985 123) when I put power to prong #31 and a ground to #30, I hear a click.
Fuse is good.
I'm using a 12v 3 amp power supply.
This test was NOT scientific by any means.
I can't read an electrical schematic, but there are hieroglyphics on the side.

MercFan · #10 08-24-2010, 02:58 PM

Quote:

Originally Posted by toomany MBZ

...but there are hieroglyphics on the side.

LOL...

Appreciate the laughs.

I think I got lucky this time... I pulled out the OVP relay, cleaned the prongs, checked the fuse and re-seated it all back into place, and so far so good.... I hate to call it fixed, but it's good at least for now.

Thx,
James