Hello,
My 85 300D has an interesting new problem. The tachometer and the AC stopped working at the same time. I never knew they were related! Some symptoms:

At idle (relatively smooth): tach reads zero, AC clutch does not engage
Step on accelerator, revs up to about 1500 rpm: AC clutch kits in, tach reading goes up to about 1000 .
Hold engine about 1500rpm for a few seconds: tach drops back to zero (not gradually) and AC clutch kicks out. It's like a relay somewhere gives up.
Repeat with same results.

I've visually inspected the KILMA relay. Looks fine
I've resoldered the OVP relay.
I resoldered the CCU a couple years ago and it has worked fine ever since.

Other info:
AC works great when the clutch will stay engaged.
Tach was always fine until a month ago. Then for a few hundred miles it just read low. Then it started reading VERY low for a bit longer. Now it does as I've described above.
AC compressor clutch started being very erratic about the same time tach started acting up. I've witnessed it clicking on/off very rapidly. It was making a noised that reminded me of a loose spark plug wire.

Any help will be greatly appreciated.

Brian

I can never remember which years had a compressor speed sensor to determine if the belt is slipping, but it appears that your has it.

The tachometer and compressor speed are compared, and the clutch is disengaged if they differ. This is to release the load on the belt if the compressor locks up, but often raises its ugly head due to other issues, like yours.

You are describing the KILMA relay, the '85 300D was the first year to have it. If the tach is erratic, the KILMA relay is probably doing it's job correctly and cutting off the AC. I believe fixing the tach will solve the AC problem.

Thanks guys. Based on these symptoms, anyone have a recommendation on where I start with the tach?

Sorry, I don't know much about the tach on the '85, it's unique. Anybody?

im interested in seeing where this goes, no tach or ac on my 85

__________________

1996 E300 110,000
1985 300TD 212,000
1980 300D 238,000

So far I have checked the rpm sensor and it appears OK. All I could accurately check was the resistance because my cheapo mulitmeter can't read the pulses. With the engine running, I did get miscellaneous low millivolt readings from it though.

I don't think the tachometer gauge itself is the culprit because of the effect I'm seeing on the A/C compressor. Please, someone chime in if my logic is off here.

I've got a Used OVP relay on its way from Dan Braschear and I'm hoping that will fix it. If it doesn't I'll have to see what's left in the circuit to check.

Will keep posting as I have more results.

Well, I got my tach fixed. It was the connector for the EGR Control Module. See my post here:
1985 300 D Tach Problem

So, did that also fix the AC?