Intermittent tach-idle-A/C issues (87 190D 2.5T)
 

Okay, I'm pretty sure I have a problem in my OVP / ELR circuitry which is causing a weird set of problems.

Frequently (but not every time) I start my car (an '87 190D 2.5T driveline transplanted into a '93 190E body), I get a condition where my tach does not function, and I can tell the idle speed is low. When this condition occurs, my A/C compressor will not engage, either.

I checked my Electric Troubleshooting Manual, and it seems that all of these circuits are tied to the ELR / OVP.

The strange part is that after a few minutes, the engine will often have a quick "sneeze", followed by the tach starting to work, the idle picking up, and the A/C compressor being allowed to work. Everything is fine after that.

Does this sound like an OVP relay problem? Perhaps the relay just isn't pulling in, gettting "stuck" in the open position until the right set of conditions (heat, vibration, etc) are met?

Could the problem actually be in the ELR adjustment pod? Is there a way to test the pod independently of the OVP?

Any advice on this issue would be appreciated.

SteveM.

I'm interested in the answer to this too. I would like to know what the path of the tach signal is, from the computer all the way to the green signal wire that plugs into the back of the tach.

OVP

Thanks guys,

Yep - it was the OVP. For now, I have actually managed to repair the OVP.

I took the relay out of the car, carefully removed the aluminum cover from the relay body, and started looking around. It didn't take long to find a lot of corrosion at the point where the #30 pin attaches to the bus bar inside the relay. The pin was loose enough that it could rotate in relation to the bus bar.

I managed to clean up this junction and flow some solder back into the joint to get everything working again.

I slid the cover back on, re-crimped it with a screwdriver and small hammer, put it back in the car, and voila - everything works the way MB intended.

It's probably not a good long-term solution, but should hold up until I get a new (to me) replacement OVP relay.

SteveM.

I think it is great when one can diagnose and repair the original part rather than buying new. Good job!

1989 190d OVP
 

Where is the OVP located in the car, I seem to keep looking and don;t see where it is located and what it looks like on this car. Thanks