Radiator stays, but there are a couple of hard ones to get to.
In the Archieves someone did a DIY tutorial..do a Aux Fan Search.

Might even be in the DIY section...........Click on that 'Resources" box

Arthur,

Found a DIY by Chris W. and pulled the fans off this evening (just before it got too dark). I reassembled everything less the fans due to the need of a commuting car. I figured it would not hurt seeing the fans were not working anyway.

After I got the car running, I tried starting each fan. A direct jumper to each fan resulted in ZIP - nothing. Next, I inspected each motor. On the lower side of each fan housing, there was debris in the form of bug carcasses, plant material and dirt poking out of each motor. obviously sucked in with the cooling air for each motor. I spun each motor tapping lightly on the housings and "stuff started falling out.

After a few minutes of carefully removing the debris, I tried a Jumper and VIOLA! The fans ran! So I suspect the primary problem in my case is the fans were non functional at the motor housing due to sucked in debris fouling the works.

Someone's attempt to "fix" the motors resulted in the damaged wiring that will be repaired this weekend when I reinstall the fans.

Hopefully, I am over the hump on this troubleshooting episode.

I will report on progress when the fans are reinstalled and tthe systems rechecked with operational fans.

Give them a good blast of compressed air and some WD40 for the brush holders..they will come back OK..............
Might even reverse the polarity and run them backward a min or two to get the brushes to clean themselves..

Cleaned and reinstalled fans. Repaired damaged wires from fans to engine compartment.

Jumper from battery operates fans.

Test 1 - Key on - Remove blue tstat connector - No low fan activation

Test 2 - Key on - Jumper wire on AC sensor behind Drivers headlamp. - No High fan activation

Examined 30A and 15A relays under fuse holder - Both fuese intact.

Suggestions on next step?

Good

{ *one note ..you have high/low fan backwards, but that does not mean anything for diagnosis..High fan is blu sens..low fan is a/c ]

So, behind the driver headlamp area .. you mentioned you do see that Resistor I posted earlier ????

If YES , then here is the test:
.. take a jumper from the battey pos+ and jumper over to each side of that R [ one side at a time , of course]
One one side you should have low fan, on the other you should have high fan

Jumper to resistor by headlamp

One side - Low fan RUNS

Other Side - High Fan RUNS

Good

Change fuses 'D" and # 7 and then redo test 1 and 2 , both w/key ON

Well Arthur. Success!

Replaced the fuses (after a quick trip to Autozone) along with some dielectric grease on the tips.

Pulled the connector off the Tstat and FANS!

Jumpered the connectors off the AC and FANS!

Back to normal.....

One additional surprise, I found the wiring diagram and saw the trigger connection from the AC switch on the dash so I tried that as well and much to my surprise my Blower came on and AC started coming out!

Guess my AC blower motor is still holding ground. I will need to replace the solid jumper in the fuse holder over the drivers wheel with a proper fuse now that I know what it is.


Thank you for all your help Arthur.

Very Good.

This is a classis example of the posting of some very simple, sequenced tests and an owner reporting the results back for further procedure to come to a final and correct diagnosis.. I had your schematic up and I simply needed you to perform the test in the same order that I would be doing if I were at the vehicle. ...I wish all the posters would do as you did. I usually have to twist their arm and repeat it 5 times over.
In your case, you will notice we did the prcedure backwards b/c someone had done some jerry-rig wiring..where I would usually start my test on the supply side vs going back from the motors.
Ya did GOOOOOOD.............

NEXT !!!.....Number- 57............................:)

I have read too many posts where your replied 4,5,6 times or more and made sure not to ignore your guide. My many thanks!

I am glad you have seen those post..it gets frustrating...sometimes I just sit here in amazement and wonder what the hell am I going to have to do to get this clown to pull that connector off the blu sensor ?????????
[ "yeah..but is I do dat, how's da fan gonna run..huh??" ]
One guy kept PM me [ and I hate PMs] to be more specific , so I finally told him to wait 'til the kid got home from soccer practice and he could print it out and give it to him..."

They ask for help and want a Magic Bullet sent thru the computer or something.......hahaha

"Sir, Yes Sir..This is a Call Center , Sir "..May we fix your Mercedes,Sir?"

There was one post the other day and the guy was Bumping it in less than an Hour ...well, you know I answered that one right off , don't cha ???

Just comes with the Territory.................

15 years later, your legendary tests are helping me diagnose my Aux fan issues, so I tip my hat to you kind sir! I hope you're in the best of health!
I'm actually surprised that people had these issues that long ago, mine's only developing these corroded contact related problems in 2023 (or maybe I'm only just beginning to look properly).

Anyhoo, my '91 300E 4matic (converted to 2wd) sedan with Automatic Climate Control and dual fans has high-side fan issues. My question is, did this year model have 2 relays or one? I noticed you mentioned there are two systems at play, an earlier version with one relay (for both low and high side) and a later version with 2 relays (separate for low and high side). I couldn't find any reference pics so I pulled the relays and took some pics. If anyone could identify what they see, that'd be great! (Yellow 40A relay goes into slot A in the relay box, Silver relay 20A? goes into slot B).

Yours has 2 relays: one for the pre-resistor and one for the high speed fans.

Am I holding up the right ones in the above post?

So far, I did the blue thermistor connection off with ignition ON test to check if Fans would power on high, they DONT. I can hear the relay 'click' as I take the thermistor connection off though. That should mean the relay is fine, right?

Tonight I shall remove the left headlight and inspect the resistor ballast area for corroded contacts.

Also, here's a pic of the state of my blue coolant temperature sensor/thermistor for aux fans.

Just put +12v to the fan and see if they actually work for starters. You may be unnecessarily digging around.