Weird Tach Behaviour
 

Now the warmer weather is here and I have been using the A/C I have noticed that my Tachometer reads low when the A/C is on. It is stable and moves with engine speed, but it is reading very low. It shows 2000rpm at 75mph where normally it should be close to 3000rpm. :eek: I haven't seen it flicker when the compressor switches in and out but when I floored it into kickdown (which I believe does disengage the compressor) the tach jumped back to normal until I backed off.

Any ideas anybody :confused:

I have a similar tach problem with my '83 300D, the tach often reads correctly, but goes to 0 every time the turn signal light illuminates (will literally toggle between say 2500 RPM and 0 in time with the flashing of the turn signal) and will peg high when cruise control is engaged. Not sure I want to tackle electrical work... so I'm anxious to see the solution for your problem.

Check your gound connections and also fuses. It sounds like something is funky though on the back of your instrument cluster. Make sure the 4 screws that hold everything in are nice and tight and the connections are clean. Some grounds run through the cluster under screws and they have to be good to make sure everything works.

Update
 

My problem may be ambient temperature related rather than anything to do with the A/C. Just coincidence that I am running the A/C when it is hot and the Tach is misbehaving.

First thing in the morning (with no A/C on), the tach performs normally even after a 25 mile run it is still showing the correct rpm. After the car has been sitting in the sun (70-80 degF the last few days) the tach will misread from start-up. At idle it shows the normal 500-600rpm but is way off as road speed increases in every gear. Switching the A/c on or off does not change the tach reading.

All other instruments are normal so I don't think it is a grounding issue since I would expect them all to be affected.

Don't rule out a bad ground. Everything expands and contracts with a change in temperature, and this could cause a change in the grounding. There is a central grounding point behind the cluster that the ground for the tach runs too, and possibly the turn signals. I would check that to make sure they are clean and tight, even loosening the grounds, moving them around, and tighten. A bad ground could cause errors due to the higher resistance.

Point taken.
I have been meaning to remove the Cluster anyway to replace the dash lighting bulbs and replace the Ignition Switch, so now I have another reason to do it.

Also the tach has its own ground. The wire comes up in the connecter that hooks onto the back of the speedo and then the ground, via the strips/screws on the bottom, splits one way to the gauges and then to the other side for the tach. Its very plausable that the split to the tach needs a little tightning up. Also check the press on power connecter on the back of the tach...that thing can get loose.
Let us know how it goes :)