tachometer fluctuates wildly when radio volume turned up
 

Hi Mercedes dudes,

I need som help with this one:

Recently my tachometer has started to fluctuate wildly, with the engine RPMs rock steady. So it it is clearly an electrical problem.

I have noticed that it is clearly correlated to the volume of the radio. If i turn the volume all down, it is fairly steady. As sooon as the volume is turned up and reaches a certain threshold, the tach starts to bounce up and down.

I replaced the OVP with a another, just to rule that one out. It didn't make any difference. Another problem with an intermittent ASD light turning on made me suspicious about the OVP.

The voltage over the battery terminals during idle and with the radio turned up is varying in the range 14-15 V.

My assumption is that it is must be related to the draw of amps that increases when the radio volume is turned up and somehow starves the tach, but the battery voltage as mentioned, doesn't drop significantly.

Do any of you have any suggestions?

The car is a W124, 300E 12V, 1990.

Thanks
BR
Viggo

Answer
 

My best guess:
* The radio ground or power are connected to the wrong circuit.
* The radio ground is not connected or damaged = back feeding ground through any available circuit.

.

On my '83 CD, the tach dances a jig when I honk the horn, , , yes, I use it quite a bit.
-corne-

Viggo, does it have the original radio?