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Electrical Gremlins - constantly running fan - temp sensor?

 
Old 12-31-2017, 01:40 AM
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Electrical Gremlins - constantly running fan - temp sensor?

Finally back home to have some fun with the car but now experiencing a weird electrical issue. The radiator cooling fan runs constantly even with the ignition off. I have a battery isolator installed so I can get the fan to actually stop. Have tried replacing the whole fan assembly but same issue with a new fan. Also noticed the ambient temperature seems to be reading high so have a new sensor on order to swap out and will also change the coolant temperature sensor. The coolant temp reads 98 - 102 c when driving, ambient temp seems to read from 10 - 30 C over expected but oil temp is rock steady.

Any thoughts or suggestions most appreciated.
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Old 12-31-2017, 08:22 AM
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I had similar symptoms with a faulty thermostat. No CEL either.
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Old 12-31-2017, 07:11 PM
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A/C pressure sensor was my issue. Fan would run full speed within 5 seconds from start.
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Old 01-02-2018, 06:27 AM
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and i had the same issue when the coolant temp sensor(attached to tstat housing) was left unplugged after head bolt swap....as soon as it started, fan came on and would continue to run after ignition off until the run-on power cycled off so my guess is going to be coolant temp sensor
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Old 01-02-2018, 09:08 AM
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Scan module codes and look at sensor data to pinpoint the problem. It's all just guesswork until you do that.
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Old 01-03-2018, 01:45 AM
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Thanks for all the feedback guys, I should have the new sensors and a new thermostat arriving before the weekend and will swap them one by one and hopefully sort the issue. No CEL code at this stage. Will advise which bit was faulty.

cheers.
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Old 01-19-2018, 01:04 AM
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Well tried swapping in a new engine coolant sensor, no luck. Replaced the thermostat, no luck. Dash did show the coolant warning symbol in white, seems to indicate Fan issue but that was also replaced. The air con did start blowing warm the same time the coolant symbol popped up and the coolant temp dropped from 92 C to 80 so will try and swap the A/C pressure sensor next.
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Old 01-19-2018, 12:08 PM
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What value is the A/C pressure sensor showing? When I had an A/C pressure sensor issue (was reading 0psi) the A/C button light would flash when you tried turning it on and it logged an "pressure too low" error code in the module.

With it unplugged, it read max value (6k psi or something) and the condenser fan would run constant.

If it is your A/C pressure sensor. It could be a bad senor, connector, or an open circuit in the harness somewhere.
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Old 01-23-2018, 06:13 PM
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Hey Jason,

I am having the EXACT same problem as you! I swapped out my battery, changed fan, and temp sensor. Still no luck. The fan runs at high for short intervals for about an hour or so when my car is off until my battery is drained. I am definitely leaning towards a bad/damaged relay of some sort. This started when we had lots of snow and Im thinking water got into circuit somehow. Driving me nuts! Have you had any luck? Thanks!
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Old 01-31-2018, 06:45 PM
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Finally had the issue sorted. For any others with fan constantly blowing issue this is the list of parts checked.

* Replaced Engine coolent temp sensor
* Changed to new fan assembly
* Replaced oil temp sensor (behind AC compressor)
* Replaced thermostat (also swapped in Mishimoto lower temp internal spring at same time)

None of this resolved the issue.

Had a auto elec specialist check the car over and after chasing the loom found a burnt in line resistor on the ECU signal wire to the FAN. Bypassed the burnt resistor and this fixed the issue

Couldn't find any information on this resistor value and Mercedes just say to buy a whole new loom so will run the car without the resistor and check to see if any issues.

Now the car is back up and running properly its time to hit the qtr next week!.
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Old 01-31-2018, 07:10 PM
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Originally Posted by Dave313 View Post
Finally had the issue sorted. For any others with fan constantly blowing issue this is the list of parts checked.

* Replaced Engine coolent temp sensor
* Changed to new fan assembly
* Replaced oil temp sensor (behind AC compressor)
* Replaced thermostat (also swapped in Mishimoto lower temp internal spring at same time)

None of this resolved the issue.

Had a auto elec specialist check the car over and after chasing the loom found a burnt in line resistor on the ECU signal wire to the FAN. Bypassed the burnt resistor and this fixed the issue

Couldn't find any information on this resistor value and Mercedes just say to buy a whole new loom so will run the car without the resistor and check to see if any issues.

Now the car is back up and running properly its time to hit the qtr next week!.
Hurry up man
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