Temperature Needle Floating Mystry

[ijibon]

Dear Members,

Problem:
Instrument panel temperature needle float up to red zone and back down to normal whenever the car temperature reaches the fan-on temperature. It is normal sometime and the fan kicks into high speed. Most of the time, fan not engaged to high speed.

New engine harness, new radiator, new temperature sending unit, and related relay also changed. This problem still exist.

Please help and advise what may be the cause to this problem.

 

[gsxr]

It sounds like an intermittent short circuit to ground. Normally I'd say to check the engine harness, but since that is new, it isn't likely. There is no relay, the sensor connects to the engine harness, which connects to the body harness, which connects to the instrument cluster. I'd suspect a possible issue with the body harness and/or wiring behind the cluster, or maybe a defective gauge in the cluster. Click here for a resistance curve of the sensor, this is not in any of the factory documentation.

The twin electric fans should not turn on high speed until 107C, which is between 100 and 120 on the gauge. If the car is frequently running that hot, you may have other problems with the cooling system. Since the radiator is new, I'd suspect the fan clutch, and pressure-test both the system and the cap.

BTW, welcome to the forum!

 

[Klink]

Sometimes problems similar to yours are simply the result of undervoltage in some areas, and / or feedbacks from areas with higher source voltage back through areas of lower source voltage. It is easy to waste a lot of time and money chasing electrical components when the actual fault is very basic.

Unless you've already done so, start with these inexpensive and easy steps:
Remove the large bare copper grounding wire between the transmission bell housing and the body. Clean the contact surfaces well with a wire brush and reinstall it. Do the same with your battery terminals, and pay a little extra attention to the additional smaller wire attachment point at the positive battery terminal bolt. Similar to my suggestion for the engine ground wire, remove the body mounted end of the negative battery cable from the body and clean that connection as well. Then find one of the many threads in this forum relating to corroded and/or cracked fuses. Most of us simply replace all the fuses, preferably with the copper version. They tend to be less micro-crack and corrosion prone.

 

[Maui]

I had the same symptoms on my wagon and it turned out to be the gauge itself. I swapped the instrument cluster with a spare I had and the problem was solved. Obviously if you are trying to preserve the original mileage on a E500E swapping the cluster may not be the best solution, but as easy as they are to swap it would confirm a bad gauge. I do believe a reputable speedo shop can set the mileage to match your original cluster. Thanks to Gerry for making the suggestion in my case.

 

[Klink]

I had the same symptoms on my wagon and it turned out to be the gauge itself. I swapped the instrument cluster with a spare I had and the problem was solved. Obviously if you are trying to preserve the original mileage on a E500E swapping the cluster may not be the best solution, but as easy as they are to swap it would confirm a bad gauge. I do believe a reputable speedo shop can set the mileage to match your original cluster. Thanks to Gerry for making the suggestion in my case.

It can indeed be the gauge cluster itself, and it often is. For what it's worth, the separate instrument sections visible in the cluster are actually separate largely self-contained units. You can remove the Phillips screws from the rear panels and the instruments can be removed from the housing and replaced separately. So it is possible to replace the unit with the coolant gauge while retaining your original speedometer.

 

[gerryvz]

My 1990 560SEL (V126) had a bad temperature gauge that flickered and went up to max/red zone and back down to low and all points in between. This happened particularly when the engine was warmed up.

It was the gauge itself. I pulled the cluster, removed the needle, and replaced the temp gauge with a spare W126 unit that I had. Replaced the needle, did a bit of work to make sure the gauge/needle were accurate, and was good to go. With a flickery temp gauge, it's usually the gauge itself. The temp senders are pretty reliable and as long as the wiring between the sender and gauge is in known-good condition, it's most likely a wonky gauge.

They are easy enough to change out on the W124 as well - you don't have to replace the entire round gauge. You can replace just the temp gauge portion if you have a spare. You have to be VERY careful with the needle removal and replacement, though. They should be the same for any W124 model. I would feel very confident of using any E320, 400E/E420 or other W124 temp gauge from about 1990 onward.

Also, before you remove the instrument cluster (disconnect its connections), you MUST remove the negative battery terminal to cut power. This goes for instrument cluster removal on ANY W124 or other model.

Cheers,
Gerry

 

[Klink]

Also, before you remove the instrument cluster (disconnect its connections), you MUST remove the negative battery terminal to cut power. This goes for instrument cluster removal on ANY W124 or other model.

Cheers,
Gerry

Excellent advice, Honch! Failure to disconnect the battery is one of the surest ways to damage the instruments either upon removal or on reinstallation. It is the first step in the factory removal instructions for any instrument cluster, yet it is frequently overlooked or ignored...

 

[ijibon]

Received all the informative advises. Thank you all for quick responses. It looks like the temperature gauge in the instrument panel that need to be look at.

 

[ijibon]

Dear Gerry,

Thank you for the helpful advise. Have not look at the instrument gauge itself. Most likely it is the cause.

 

[Stevester 500E]

My story with temp. gauge.
I bought my first MB, a 1993 300ce from a used car lot, the guy was honest actually.
On the way home I thought I was scammed because the temp gauge went to the 100 mark, and then it would bounce up and down on occasions. I called the car lot and was told that the car did not have an over-heating issue.

I was sure that I had bought a car with an overheating issue. I took it to MB for a general look at the car. They said I needed a water temp sensor and a temp gauge. I bought a new temp sensor and installed it, but it didn't solve the issue. I then bought a use cluster and installed it and it solved the issue. I replaced the whole cluster because it also had an odometer click that bugged the heck out of me.