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Question? My 1995 SL320 has developed an erratic dash engine coolant temperature gauge. After warming up the gauge reads normal, then after a few minutes the gauge jumps to extreme hot then after a few minutes it reads normal again. This erratic gauge behavior repeats about every 5 minutes. So, I replaced the engine coolant temperature sensor (a single electrical pin sensor) that's screwed into the thermostat housing. Then I replaced the thermostat and the coolant recovery tank pressure cap. No change in gauge erratic behavior. Now I'm thinking I should replace the second engine coolant temperature sensor (it's a 4 pin electrical connection) which is also screwed into the thermostat housing right next to the one I replaced. However I can't seem to find anyone or anyplace that will provide me with the exact OE replacement sensor for my VIN number WDBFA63E7SF124282 Plus, I'm not sure if this car has two of these sensors and where would the second one (maybe the second one is for the fuel injection calibration???) be located and which of these sensors sends the signal to the actual gauge. No dash alarm or check engine lights are on so my guess is the engine is not overheating. So, I really don't know what or why is causing the erratic gauge behavior but obviously it needs proper diagnosis and repair. And no, I don't have any confidence in the local MB dealers service centers. Yes, I live in Vegas. Sure would appreciate some advice and help with this issue and maybe even decoding my VIN number. Thanks in advance, Dave.
 

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Now I'm thinking I should replace the second engine coolant temperature sensor...
Take the money you'd spend on that sensor and put it on the Patriots to win this Sunday. You've already replaced the sensor that controls the cluster temperature gauge.
 

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You have a 95 so have you have a checked the engine harness to see if it's falling apart cause this may be a short in the harness.
No, I have not analyzed the Engine Management Wiring Harness. I did replace the whole harness two years ago due to all the wiring insulation degrading and exposing the actual wires. Why? Because MB, in their infinite engineering wisdom (they think it is), used a biodegradble insulation which didn't work long and degraded due to the engine compartment heat. Replacing the harness was a seriously laborious and tedious job made worse because each wire is individually soldered to its connection lead on the controllers... instead of just terminating each wire to a connector which would mate to a connector at the controller. Thanks for thinking about the wiring possibly being the cause of the gauge being erratic.
 

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I had the same problem, tried all the repair suggestions. Purchased a used cluster off E-bay with a broke lens, then I swapped circuit boards and problem fixed. Mileage if off by 4k, so I took photo for proof.
 

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I had the same problem, tried all the repair suggestions. Purchased a used cluster off E-bay with a broke lens, then I swapped circuit boards and problem fixed. Mileage if off by 4k, so I took photo for proof.
Hi rp930. Thanks for the temp gauge repair solution. I've been avoiding the potential, obvious source of the erratic temp gauge issue being the gauge itself. Why? Because it looks to me like to R&R the whole gauge cluster is one major effort... and knowing MB will require some special tool to unfasten the critical part after one has the cluster half way out. Anyway, which circuit board are you referring to and exactly where did you find the used cluster and what's your take on the end-to-end R&R? Thanks, Dave.
 

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Take the money you'd spend on that sensor and put it on the Patriots to win this Sunday.
The Patriots won by a score of 43-0. So if you didn't buy the sensor and you bet the game, then you are doing well.

My next move would be to spend under $10 on an ELM327-based OBD-II interface like this one. This can read engine coolant temperature data directly from the engine ECU and log it to a phone or PC. While logging data, when I see the temperature gauge act erratically I would compare the gauge representation to what is being logged. If there is agreement, then you know the cluster gauge is not your problem.
 

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I had a problem with my Volvo 740 glt some time ago but as well as the temp gauge fluctuation my fuel gauge was doing the same . Turned out to be a small part in in my speedo
 
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