My 1993 300ce has an erratic water temp gauge. It reads normal then shoots
up to the highest point in a flash. If I tap the gauge with my hand it comes down to normal. This happens continiously during my drive.The gauge is part of my (fuel gauge assembly) Water,Oil & Fuel. I was told by a non-mb dealer
that it was the gauge itself and that it would have to be replaced at a cost of about $400. I was hoping that it could be a faulty ground wire behind the gauge itself. Are there any other members with this problem. I would appreciate any input here. btw: The fuel & Oil gauge readings are fine.

Thanks, Rick

Most all thermal switches, couplers and sensors work by going to ground gradually. The warmer they are the better the flow of current to ground.

That translates into a reading on the gauge. A fast jump to to full temp or gauge travel means it is grounding in the line (Wire) proir to getting to the sensor. Look for a bare spot on the wire in route to the sensor. This will most often be found while passing thru a metal plate such as the firewall. However most manufacturers have learned that lesson and pass it thru on a coupler to prevent this. Many manufacturers put a heavy sheath on the thermal couple wire to protect from chafeing problems. Also look for the looms and supports that guide the wire to the sensor, has it chafed into the wire?

I have found other wires or metal chafeing on the back of unprotected instrument cluster circuit membrains. I do not know if this applied to your vehicle. Does it have a film style or actual circuit board that is exposed on the back of the gauges? If so is anything touching against it? Was work done in the instrument panel, stereo or other systems behind the dash.

I have found some gauges that jump full over if they loose all connection to ground. Test this by unplugging the connector to the sending unit and turn on the key. If it jumps all the way you are looseing your connection. This is not as common. Then see if the connections are all tight, for a cause!

A instrument cluster with a poor ground on the system will also have the instrument lights dimming and blinking brite, dim, etc. The ground is common to all of the group.

The last is a shorting connection or wire in the instrument coil inside of the gauge.

Last question, any oil drips coming from under the dash above your feet?

P.S. >> The tapping on it to bring it down may not be undoing the problem but just assiting the gauge in returning to normal.

When the coil slaps it that hard over, into territory it is not normally in, it may need assistance relaxing, if I can use that term! Don't take it that the problem is in the cluster due to that!!

Thanks for your reply,

In response to your reply, I have no leaks under the dash. The instrument
cluster was removed about 6 years ago and I've had this problem for about 2 years.I changed the thermostat,sending unit,wire and connector.The other 2 gauges within this cluster work fine,( Oil & Fuel) I haven't as of yet disconnected the wire from the sending unit to see if it jumps.I will do that this week and let you know the results. If I tap the gauge with my finger
and it returns to normal then why can't I assume that the problem is in the cluster. Maybe a faulty wire or ground in the back of this cluster.

Thanks again for your prompt reply, Richard Tripi

Trippir -
how did this end up? I have the same issue with both the temp guage and the oil pressure guage in my 1992 400e. They both work fine - intermittantly and then suddenly jump/drop. The temp jumps to max and the oil drops to the bottom - at the same time. I took the assembly out once and replaces some bulbs, but did not open it or check the wires. I would love to see which wires these two have in common and check them at the connection! Share what you can...I will too.

John