Tach intermittent-87 300TD
 

Bought this wagon @ 6 mths. ago with bad head gasket.Replaced gasket,now tach is inop.,though if I tap relays next to battery tach may work and engine surges.Though I've been a tech for many years,I'm not real knowledgable with M.B..Would appreciate any input. BTW,I've already taken some of your suggestions with good results,such as ALDA adjustment,removal of AFM,etc..Excellent site.

One of the relays behind the battery is the overvoltage protection relay. It's a tall square section relay with a blade fuse on top. Internal solder joints might be loose if tapping restores function. The rpm signal is distributed to the tach, EDS, cruise amp and Klima relay (maybe others). Idle rpm might be low coincident with no tach signal. There is a base idle adjustment on the IP but that's typically set at a very low 500-350rpm with EDS bumping it up to 630rpm or whatever the trim pot is set to through the ELR actuator, the red plastic thing on the tail end of the IP.

Sixto
87 300D

Tach issue
 

That's interesting since the relay near it has been modified.I took it apart for inspection,but not the other.Back to the drawing board.Thanx for the info,sir. Rick

OVP!

Probably the Klima relay was modified to ignore an errant interrupt signal.

Sixto
87 300D

Tach issue
 

Sixto,you were right-on.I removed the OVP,opened it up to find rust on lower section(cowl drains were plugged).Cleaned it and found solder had blown out of a pin on vertical cicuit board.Re-soldered,sprayed w/WD-40,now works perfectly.I have a few other minor issues,but I'll post later.Thanks very much for your wisdom. Rick