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I don't rely your average factory temperature gauges to have pin point accuracy but not a big gap between them either way. Do they read any different when the car has warned up and moving?
You can simulate temperatures with resistors via the gauge and sensor plugs if you want to find out what accurate.
 

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Discussion Starter #3
I don't rely your average factory temperature gauges to have pin point accuracy but not a big gap between them either way. Do they read any different when the car has warned up and moving?
You can simulate temperatures with resistors via the gauge and sensor plugs if you want to find out what accurate.
Its the same from stone cold upto around 95ish then gauge will stay the same even if the ac coolant temp read 101 or 105. I noticed also low fan kicks at 100 med. At 105 and high at 108 in ac diagnostic.
 

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Yep. Well its all quite simple to test if you want to but id check that your cooling system is up to scratch first if its getting that hot. Your viscous fan should be really going strong at that temp.
 

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Yep. Well its all quite simple to test if you want to but id check that your cooling system is up to scratch first if its getting that hot. Your viscous fan should be really going strong at that temp.
I think i found the problem why my temp is high. My expansion tank has few leaks. And system i think is not pressurizing properly. Still, is it normal for the reading to be different? Gauge and ac diag.?
 

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Maybe not normal but probably not unheard of either being Mercedes, i dont really have much experience on that front, mine reads roughly the same but sometimes drops to 0 in the a\c panel.

Depends how bad the leaks are the location but sounds like you check the whole system and make sure its functioning properly first. Im guessing you are on original 1998 parts based on your mileage?
 

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Discussion Starter #7
Maybe not normal but probably not unheard of either being Mercedes, i dont really have much experience on that front, mine reads roughly the same but sometimes drops to 0 in the a\c panel.

Depends how bad the leaks are the location but sounds like you check the whole system and make sure its functioning properly first. Im guessing you are on original 1998 parts based on your mileage?
Just changed the water pump since the origanal pump was making noises. Together with the thermostat. Changed the rad. Too since the plastic are brittle. If this still persist i might need a head gasket job 😞
 

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If those items were bad then i would expect fan clutch would likely be worn too, your aux fans are going to be coming on a lot at low vehicle speed to try and compensate. Expansion tank caps can go bad also. I know these engines have headgasket problems but you'd want to make sure its the case via pressure testing.
 

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Discussion Starter #11
If those items were bad then i would expect fan clutch would likely be worn too, your aux fans are going to be coming on a lot at low vehicle speed to try and compensate. Expansion tank caps can go bad also. I know these engines have headgasket problems but you'd want to make sure its the case via pressure testing.
Would a very small leak in the expansion tank be the cause of high temp? Coolant level seems still spot on
 

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Coolant leak or pressure leak? Either way should replace the tank anyway as a small crack in a closed system can give way very quickly when under pressure.
 

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Coolant leak or pressure leak? Either way should replace the tank anyway as a small crack in a closed system can give way very quickly when under pressure.
Hmm. Might be pressure leak since coolant level is still the same. I ordered the new oem tank and will be arriving nextweek. Can this cause high temp.?
 

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Possibly. Have you pressure tested the cooling system? Is it running the same temperature on highway\open road?
 

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Possibly. Have you pressure tested the cooling system? Is it running the same temperature on highway\open road?
Can't seem to find a shop with a pressure tester for the cooling system. On open road temp is around 88-91 on the ac coolant temp
 

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Still, is it normal for the reading to be different? Gauge and ac diag.?
Doesn't seem like anyone addressed this specifically, so here I go:

Yes, but you should rely on the diagnostic value from the climate control. That takes the voltage direct from the sensor or some other witchcraft:dunno:, and then displays a number for how that value should read. The gauge cluster takes the same input as the climate control, except the voltage is sent to a potentiometer (the moving gauge). Over time, the brushes in the potentiometer can become dirty and worn, not leading to a true representation of that voltage.
Sometimes my gas gauge acts funny when it gets to 1/8th tank (moves all over the place). Kent Bergsma from MercedesSource did a video on this phenomenon, that's where I got my info. He also talks about how to clean those potentiometers and test them I think.

I've also had a funny experience when Michael Jackson was playing in the car on the radio. The coolant gauge was bouncing to the beat, I even have video somewhere:grin
 

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Discussion Starter #18
@Merc96 thank you for clearing that up. @NZJay fan clutch is locked and is loud. Ill wait for the new expansion tank since now i know im having pressure leak thru 2 very small crack in the tank. @102 i can hear boiling in the tank and radiator Hose isnt that hard which indicates pressure isnt holding up. If after changing the expansion tank it still has high temp. Problems, then im all out of ideas. Changed water pump 2x, thermostat 2x, rad., Locked the fan clutch, aux fan starts to scream @106.
 

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Discussion Starter #19
Update. Overheat no more! I it was the expansion tank. Coolant stayed full but pressure is leaking is the cause. 45mins. Heavy traffic and gauge stayed 90-96'c.
 

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My '94 S320 overheated. Temp gage not registering. Going to replace thermostat and sensor. Which of the sensors on the housing is the coolant temp to the dash gage ?
 
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