Tach fixed - but now some sort of electronic whine at 2000 rpm - 85 300D fed - PeachParts Mercedes-Benz Forum

dieseldan44 · #1 02-19-2009, 01:03 AM

I sucessfully fixed my tach as outline here:
85 300D OVP Relay - old one has 5 pins, new one has 6
by replacing my over voltage protection relay.

but of course there is a catch. Now I have a very distinct whine that comes right on at about 2000 rpm until 2300 rpm (when it instantly turns off) when the car has fully warmed up. It sounds electronic in nature, but i need to be revving the car to make it happen, so I have not been able to pin point where it is coming from - somewhere behind the passenger side dash area.

anyone hear of this? before and after i replaced the OVP relay the car runs great.

I would think this sounds like EGR fishiness but i dont know where to begin, other than pulling that relay - the whine is super annoying - more annoying than not having the tach by far :-)

thanks (again),
dd

JimmyL · #2 02-19-2009, 01:08 AM

Can't imagine what that could be. The only whine in any of mine happens on those rare occasions when the wife rides in them.......

Jeremy5848 · #3 02-19-2009, 01:23 AM

The '85 300D (W123, OM617) has the EGR computer behind the right front passenger's kick panel, close to the OVP relay. There should be a "door" in the kick panel. It's easy to pop out the door and see if the noise is coming from there, harder to remove the entire kick panel. With the OVP relay gone bad, the EGR computer was not working. Since your whine started at the time you fixed the OVP, it is a good possibility that the noise is coming from the EGR computer. The tach signal goes through the EGR computer; you might be hearing a harmonic of it. Depending on how loud it is, you might choose to ignore it.

johninva · #4 02-19-2009, 08:01 AM

if the egr computer was totally fubar the tach would not be working, but it may be slightly fubar ? Seems like water gets in somewhere, maybe windshield gasket, gets on the cabling to the egr computer, runs down the cable and collects in the connector and the computer module itself. All the 85's I have come across in the salvage yard except for 1, have had liquid in the connector area or evidence of liquid once being there. check your computer connections.

harpua00 · #5 02-19-2009, 09:10 AM

I had this on my '85 as well. As stated, the whine/squeal goes away when you unplug the OVP. I've replaced all vacuum connectors & elbows on the two vacuum control valves mounted on the left front fender just above the glow plug relay (one elec. operated, one vac. operated), as well as on the connection to the EGR itself. This has minimized the whine but I still get some, at light throttle only, 2000-2500rpm or so.....exactly when you would expect the EGR to activate.

The whine on mine is pretty minimal now, but once the weather warms up I'm planning on tracing it down by unplugging the electric connector to the fender mounted elec. operated vacuum valve, unplugging and capping the vac line to the vac. valve right next to the EOVV I just mentioned, and lastly capping the line at the EGR connection. By doing this one by one it should be possible to isolate the culprit making the whine (which in my case at least clearly sounds like a vac leak).

dieseldan44 · #6 02-19-2009, 11:17 AM

thanks for the ideas. looks like i will be going into the EGR computer today to see what i can see. i get very nervous working on anything called a computer from 1985 :-)

im pretty sure my whine is electrical, ill find out for sure today.

so how do i go about disabling a 1985 fed EGR? anybody has a procedure? ideally id like to remove those extra vacuum components, i want the space for an upcoming two tank WVO conversion.

dd