Erratic Engine Coolant Temperature Gauge
 

Car is a 1995 MB SL320. The dash, engine coolant temperature gauge has decided to act erratically. It reads normal temp then suddenly reads max temp then back down to normal. Repeats this behavior about every 5 minutes. I've replaced the single pole temperature sender located on the thermostat housing next to a second, 4 pin, temperature sensor which I have not replaced. Replaced the thermostat with the jiggle valve facing the engine and replaced the coolant tank pressure cap. No improvement... still erratic gauge. Doesn't appear to be overheating but does seem to run a tiny bit rough some times. Anybody know how to determine what's the cause and repair? Can't run an ODB2 Scanner because ODB2 didn't start until 1996. Thanks.

On older style Benzes, erratic gauges lend themselves to bad grounds within the original wiring. The fix is to simply add a new ground strap to the chassis.

I would do a search within the tech forum with your vehicle chassis to see what the alternatives are for later model Benzes. The fix might be as easy as this.

Engine wiring harness been replaced? Your year and model is one of the ones affected by the biodegradable wiring harness, don't waste time and money swapping parts or rebuilding clusters until you've confirmed that it's been replaced with a known good one.

Have a look at my posts here and apply to your temp gauge.

https://www.peachparts.com/shopforum/diesel-discussion/400252-checking-coolant-temperature.html

Wiring in insulation is a problem with some of these cars so do a search on how to ID older harnesses. I'm pretty sure if the tag says DELPHI and a date later than your car that should be the good replacement harness. This tag should be near the engine computer end of the harness.