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.

I had similar symptoms with a faulty thermostat. No CEL either.

A/C pressure sensor was my issue. Fan would run full speed within 5 seconds from start.

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

Scan module codes and look at sensor data to pinpoint the problem. It's all just guesswork until you do that.

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.

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.

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.

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!

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

hey guys sorry to bring this post back, but i am having an issues with my oil cooler fan running all the time and no oil temp reading on the amg cluster selection. i changed the thermostat, changed the oil temp sensor behind the ac compressor, and still running. i hooked up a launch diagnostics nothing, i hooked up the star nothing, i hook up a cheap obd 2 reader no codes but when i hit erase the fan shuts off, i get a read out of 320 degrees oil temp and then 30 secs later the oil fan kicks on again. im stumped

320 degrees is the default for maximum of that sensor. Meaning you likely either have a short circuit or short to ground, or could be unplugged. I find it hard to believe you used star and have no faults for this. Since it's not emissions related it wont throw a cel or hard fault.

Thank you for the response, I changed the oil sensor 2 times with an oem and cleaned the connector and made sure it clicked in.

I put the launch diagnostics and have no stored or hard faults went into the oil cooler fan option and shut on and off and shuts off and turns on.

ur saying it might be a short to ground?

Your help is greatly appreciated

short to ground or an open circuit yep. I dont have access to a wiring diagram at home but i'll see if i can get you pin numbers etc...

Thank u so much this driving me crazy, I supposed the c63 is the same as the e63 2011 same setup.

My 05 E320 did the same thing a year or two ago - start it and the fan ran high speed all the time. Shut engine off and fan stopped. I was told it was "powers and grounds." I can't remember exactly except I thought I went to the fuse box left end of dash, the lowest two fuses on the right were corroded. I sanded them and their contacts slightly with 220 grit and I think that solved the problem.

The past couple weeks it's doing the same thing but that procedure didn't help. Plus, the CEL came on last week. I WD40'd the gas cap but the CEL is still lit. I'm going to hook up my scanner, investigate and keep you in the loop.

Hi, sorry to bump an old thread but I’m having this same issue with my 2011 e63. Cooling fan runs at full blast and there is no oil temp showing up on the dash. No CEL or anything and no other issues. Did you ever resolve it? Hoping for something simple as my car had already been in the shop for a month for another unrelated issue.

Control module is broken this unit is running both electric cooling fans.
Bad module will drain your battery when the fans still ON and the ignition is switched OFF.