What is this fuse for

[The Ry Guy]

Hello all!

I have a 1993 400E that I am getting back in shape to be my daily driver. My question is what is this fuse for? It's a 30 amp strip fuse. I want to know what it's for so I could find out why the previous owner may have disconnected the wire from it. I've looked through all the wiring diagrams I could find for this car, but none point out what it is.



As a side note, hats off to all the contributors here at this site. I have been helped so much just by searching the forum, and finding answers to questions I've had come up while working on this car. Again thank you!

 

[captruff]

Looks like the fuse
For the HVAC fan motor

Jeff

 

[gsxr]

I think on a 1993 model, that fuse may be for the twin auxiliary cooling fans in front of the condenser. It's a 30A strip fuse.

There is a second, identical 30A strip fuse for the HVAC fan motor as Jeff/captruff mentioned above. This one is normally located at the front strut mount.

Assuming the HVAC blower is working normally, you'll need to see if the twin aux fans work... this very long thread may help diagnose issues with the fans:
https://www.500eboard.co/forums/index.php?threads/auxiliary-electric-twin-cooling-fan-troubles.5135/

 

[The Ry Guy]

Thanks for the quick replies. The HVAC fan works properly so definitely not that one.

Funny you should mention Aux fans, because those are on my list of things to diagnose. The A/C works (blows cold) but the fans don't come on. Shouldn't they be running when the a/c compressor is running? Also the car does run hot (about 3/4 on the gauge) when sitting and idling. I read somewhere on here about a resistor on the coolant temp sensor to help with that. I noticed the old (original made in 1992!) wiring harness that I took off the car had a resistor on the plug for the CTS. Perhaps I should swap it over to the new harness.

I'll read the thread you linked and do the diagnosis for the fans tomorrow. Hopefully that fuse will show up in there.

 

[gsxr]

The thread linked above has most all the information you'll need. Fans run on low speed based on refrigerant pressure, not just when the AC is turned on. Fans run on high speed if the engine temp exceeds ~107°C.

Don't swap the CTS resistor (aka "CoolHarness") over, this is usually a Band-Aid covering up a more serious issue like the mechanical fan clutch not engaging properly.

 

[The Ry Guy]

Update:

Yes, the mystery fuse is for the cooling fans. The component location list I have calls fuse F22/2 out for the telephone/terminal 30 (supplement 14, page 205). I'l have to update my wiring diagrams to reflect this.

I checked continuity between the disconnected wire and the relay (they sure don't leave much room to get that thing out!) as well as at the resistor.
The wires all had continuity, so that's good. I also checked the relay on the bench, it tests good as well.

Unfortunately the resistor is burned up, (maybe why the previous owner disconnected the fuse?) so until the new one arrives, I'll just bypass it for now. Why do cars have a low and high cooling fans speeds anyway? I'm sure there's a reason, since the low speed comes on from high A/C system pressure only.

Jumpering across the A/C high pressure switch with the resistor bypassed, both the fans came on, so at least the rest of the system works.

Good to know about the "Cool Harness" being a band-aid instead of a fix. Any good threads on diagnosing the fan clutch?


(Unrelated side note... the picture I uploaded on this thread wasn't blurry/pixleated until after I posted it. Did I do something wrong?)

 

[gsxr]

High speed fans are noisy, and the low speed is adequate for the intended function - to move some air across the condenser while the car is stopped or moving at low speeds, so the AC works better at low vehicle speed / engine RPM. A new resistor is cheap, if the wiring is undamaged it's an easy swap after pulling the driver side headlight out.

There are multiple threads on the fan clutch, some very long, with lots of info to digest. The only accurate method for testing is the "roar" test, and it's much easier on a hot day (very difficult, IMO, to properly test the clutch on a cool day). Search the forum and you'll find more than you ever wanted to know about the clutch fiasco.

Good question about the blurry image - Gerry? Whazzup?

 

[The Ry Guy]

I thought it might have something to do with noise. I'll get a resistor with my next online order to save on shipping. I thought about cleaning up the corroded post on the old resistor, but after I got it out of the car and on the bench, I think the threads are too far gone to be worth my while. The wiring got hot but it didn't melt but a little bit of insulation right next to the eyelet.

Agreed on removing the headlight though. I had to remove it anyway since the wiper motor for that light is seized up and I need to fix it too.

Speaking of fan clutch tests, I remember my instructor in shop class showing us a procedure from Mercedes to test the fan clutch where you had to have a cotton string of I think 3mm? You then tied it to one of the fan blades with the other end tied off somewhere else in the engine bay. Start the engine and bring it up to 2000 rpm? and if the string broke the clutch was good. I wanna say it was from the 60's? 70's? Seemed comical to me at the time.

 

[gsxr]

Speaking of fan clutch tests, I remember my instructor in shop class showing us a procedure from Mercedes to test the fan clutch where you had to have a cotton string of I think 3mm? You then tied it to one of the fan blades with the other end tied off somewhere else in the engine bay. Start the engine and bring it up to 2000 rpm? and if the string broke the clutch was good. I wanna say it was from the 60's? 70's? Seemed comical to me at the time.

I've never heard of that story! If true, it must be from 1970's or earlier.

 

[LWB250]

The fans come on at a given coolant temperature, I'm sure when gsxr rolls out of bed he'll post the value. The resistor is a modification people do to lower the temperature at which the fans come on.

Dan

Speaking of fan clutch tests, I remember my instructor in shop class showing us a procedure from Mercedes to test the fan clutch where you had to have a cotton string of I think 3mm? You then tied it to one of the fan blades with the other end tied off somewhere else in the engine bay. Start the engine and bring it up to 2000 rpm? and if the string broke the clutch was good. I wanna say it was from the 60's? 70's? Seemed comical to me at the time.

No more comical than the "newspaper test". I like it.

Dan

 

[gerryvz]

Good question about the blurry image - Gerry? Whazzup?

I don't know, but if you click on it, it comes in at full resolution.

For some reason, the original image did not compress to a thumbnail size upon attachment.

I edited the post, and re-sized the image to a thumbnail manually, and it is fine.

 

[gerryvz]

TEST IMAGE ATTACH.