Greetings.

My aux fan refuses to cooperate.

About a month ago the fan on the car blew a fuse and turned out to be locked up. I swapped the fan with one from my 420SEL parts car and had no issues. Nothing notable happened since, but this week it that decided it'd rather not function.

I have been reading around and have done some fiddling.
1. bench tested the fan and it does function off the battery
2. jumped the 2 wires to the ac condenser with a fuse to no avail
3. checked the fuse and swapped with the one for the door locks
4. put my test light in both ports on the plug for the fan, not hot

I haven't put any effort into investigating the high-speed switch, cause obviously that shouldn't be needed in current operation. It still doesn't come on in excess of 105c though. I imagine that the low speed switch in the AC system is also not the problem as bypassing it doesn't make a difference. My next thought is investigating the relays. I have found discussions talking about jumping wires in the relay ports however the instructions are based on the wiring diagram that I do not have. Perhaps the fan is no good but again, it works straight off the battery.

I guess my ask is if anyone can think of anything that I've not done or if anyone has a wiring diagram file/link. Thanks!

Check ETM and jump pins in high speed relay socket so that it runs on high speed. Also check if both relays get power.
Alternatively, with the receiver-drier cables jumped, check if you are getting power to the pre-resistor. Maybe the pre-resistor is broken.
The high-speed setting bypasses pre-resistor and puts current directly to fan.
The relays can be bad, newer versions are equipped with replacable fuses.
On my friend's car, one of the receiver-drier pigtails had a bad ground. Problem was resolved when I made a new wire.

Lastly, post some pics of the car and what you would like for it

The pre resistor sounds like it might be the piece to the puzzle I didn't realize was there!
I just read that it's located behind the quasi-firewall deals and I'll get right on that.

As for the car, I'm sure you'd want something a little less broken in haha. I obtained it on the cheap about a year and a half ago with 358k on the clock and I'm now nearly at 375k miles. It routinely gets driven down to Dallas from Cincinnati and next week it will do just that but take a "quick" detour in Montana making for a 5300 mile round trip. Needless to say, I'm quite excited.
(not sure if these will upload correctly but here are some photos. The second is the diesel next to the stripped 420

https://mailuc-my.sharepoint.com/:i:/g/personal/florohwn_mail_uc_edu/EZFQcDjRf-9Ip7Kf-fX0i4IBkxFu2gYzg_IDF0CARmVRhA?e=GXosS1[IMG][/IMG]

If the fan runs on high, but not on low, you have a bad relay or a bad fan resistor. The resistor is a real PITA to get to, it's mounted down below the brake booster. If the old fan was seized up, it very likely burnt out that resistor, sorry to be the bearer of bad news!

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Continuity can be checked with a multimeter, relay socket and wire on the fan

Thank you both!

I went ahead and pulled the pre-resistor (not too bad once the firewall type deal is partially removed). As I understand, with the two leads coming from the ends of the resistor jumped, the fan should run at high speed when ignition is on. I tried this with both a wire and then a pair of vice grips holding the two ends together and neither provoked any movement from the fan. What you said about burning up the resistor is a little hope-inducing but I’m still a little concerned about the lack of activity, even with the leads jumped. As far as visual inspection is concerned, my resistor looks fine. This very well not mean anything but I found a photo of one that was visibly charred and broken.

After this I played musical chairs with the relays. Each one was in a new position and still nothing from the fan. I found a 560 thread that said the relay base labelled C was the low speed relay. It also said that by jumping the top left pin and the centre (only) pin on the right, the fan should come on with ignition and once again this provoked no movement.

I shall head back out to see if I can get any power from any pins in the C relay. Is there any point in jumping the connectors on the high speed switch when bypassing the resistor produced no high speed spinning?

Exciting news!

In the fuse base labelled C the two pins in the center had power. When the two foreword most pins (on the side with 3) were jumped the fan cave to life. When the fan came on there was a puff of smoke from the fuse so I disconnected my jumper immediately. I take this as good news despite the fact that I’m not really sure how to interpret it.

Note: when this was done, the two resistor leads were clamped together with vice grips. I also tested to see if the fuse had power with ignition, and it did.

(With regards to the multimeter comment, I addmittedly do not have one and subsequently am clueless as to its operation. As far as I understand it is used to measure current? I hope that my test lighting and jumping has bypassed the need for a multimeter but I obviously am not at all sure.)

Be aware that the wiring to the fan resistor is not rated to carry the full amperage of the cooling fan. If you bypass the resistor, the wiring will overheat and can melt the insulation off or cause a fire.

The torpedo fuses are known to have connection issues. If you have the original aluminum fuses (silver or grey colored metal strips), hit the trash with them ASAP and replace with brass or copper ones with ceramic bodies. Clean the contact points in the fuse box with a wire brush or something before installing the new fuses. The majority of electrical "gremlins" originate in the fuse box solely from crappy fuse connections.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

You can buy a cheap multimeter at Harbor Freight or Home Depot.
You should learn the basics of electric circuits if you want to proceed diagnosing electric circuits in your car - don't want to be mean, just stating a fact.

I don't know which pins are which, but you can check that in ETM which is a part of service manual of your car. High speed operation bypasses the pre-resistor, that's why it's high-speed.
From what you've written, it means the relays are getting power. It also means wiring from fusebox to fan is good.
I'm not sure but your relays may be bad. I'd replace your relays with a set of newer ones from junkyard, it should cost a few dollars.
If pre-resistor is intact visually, it's usually good.

That is simply not true. See ETM wiring diagram.

I agree with your comment about fuses.

I should still have the set of relays from my parts car 420, I’ll slap those in and see if the fan cheeches to life. As far as fuses go, they’re all brass/copper, however a new set and a contact clean certainly wouldn’t hurt anything.

I’ll reinstall the resistor tomorrow and proceed with the relays. I hesistate to be hopeful as I swip swapped the ones already in the car around but there’s nothing to lose.

Lastly, regarding the multimeter; you are, of course, entirely correct.

All cube relays aren't alike. For some, perhaps most, Mercedes the high speed relay is internally fused. If you substitute an plain relay, the fan will have no fuse at all. The low speed relay is fused normally through the fuse box, but is sometimes a double pole relay.

My comment is based on the ETM. 1.5mm wire is good for ~5 amps. The cooling fan on high speed draws roughly 8 amps if the bearings are healthy and the alternator is making ~14V.


Main power wiring to the high speed relay and from the fuse to it is 2.5mm wire, good for ~14-15 amps. The wiring jumpered to the low speed relay and to/from the pre-resistor is 1.5mm wire. See drawing 128/2 in the ETM. Your car, you do as you see fit.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

At lunch here I just swapped out the relays, again to no avail. To your point, @Mxfrank, all the relays have the exact same part number so I’m not really concerned about using the incorrect type.

So here’s what I know, I guess. There is power to the fuse, so everything in front of that is A-OK. I have power to the relay so everything in between those two is A-OK. If I jump the relay connections the van turns on so the fan motor is A-OK. Jumping the low speed switch does nothing so it is A-OK. The resistor looks fine and with the leads jumped there is still no spinning with ignition so it’s A-OK. The relays have been swapped around with ones of the same part number to no avail, so the relays themselves are A-OK. With the relay points jumped there is movement so the wiring between everything is A-OK.

The only thing I’ve not tested is the high speed switch, but that shouldn’t stop the rest of the system from functioning even if it were no good, should it? I did not reinstall the resistor for my little relay experiment, but I doubt that would cause an issue, regardless I’ll reinstall it later and try again.

Here’s a thought. With the ignition on, when I jump the leads to the low speed switch, should I hear a click from the relay? I’ve not tried this yet and maybe it’ll provide some insight?

Yes you should hear a click.
Jumping the high pressure switch pigtails off the receiver drier should turn on the low speed fan. The act of jumping with ignition on should make an audible click. Maybe there is continuity problem between this switch and fusebox. Maybe bad ground at this switch. But you said you have power in relay socket once this switch is jumped?

Also, once you are in a junkyard, I suggest you go through any 90s Mercedes and upgrade to switches which look like this: https://www.amazon.com/Mercedes-Benz-Auxiliary-Relay-Stage-SL600/dp/B01BQMADS4

they are blue and green. They have easily replacable fuses.

So, to test low speed relay. Provide ground to once of the wires that connect to receiver drier pigtails. Fans should come on low.

To test high speed, locate your 105/115C temperature switch and apply ground to pin no 2 on its connector.