Auxiliary fan question '87 300SDL - PeachParts Mercedes-Benz Forum

tcom · #1 07-29-2019, 02:15 PM

Good morning,

Where does the auxiliary fan get its signal. Appears to be from the high pressure switch. I don't see a switch on the radiator or head. I know the later models with two speed fans,(ala BMW) get the signal from the compressor lead and run in low whenever the A/C is on and switch on the radiator to engage high speed when coolant hits a spec'd temperature.

I don't want to get the head pressure to 290-300psi to test it if not absolutely necessary. No point in straining an older system unnecessarily.

It's still running R12 so I don'r want to evacuate the system unnecessarily either.

Tony

Diseasel300 · #2 07-29-2019, 02:37 PM

The Aux fan has 2 control inputs. The pressure switch on the filter/dryer will turn the fan on "low" when the high side pressure on the A/C system is >200PSI. The second input is on the head near the coolant outlet, it will turn the fan on "high" when the coolant temp is >105˚C

Before jumping to conclusions, test the fan if it is receiving power. You can short the wires to the pressure switch together to enable low speed, or short the 2 pins on the 2-pin connector on the grey sensor in the head together to test high speed.

On a hot day, head pressure will be high, even with the Aux fans on. R134a runs even higher, it's not uncommon to see pressures >300 on a really hot day with a hot car. The system can take it.

tcom · #3 07-29-2019, 03:26 PM

Thanks for the quick reply.

The fan is original. It wasn't working when I got the car. I spun it by hand and it sounded like a box of rocks. I connected it to a power source and of course it sounded like a bigger box and dragged.

The fan connector has two lugs, hot and ground. The female side also has two leads hot and ground. THe wires are from the high pressure switch.

I replaced the fan with a Meyle which is a good compromise between URO and OEM. There is no signal at the female plug fan with the A/C on. The head pressure is 185/38 @70* so I can't tell if the signal is initiated by head pressure or some other way.

I haven't seen a two speed automotive fan without at least a three wire connector. I'm sure they exist but I haven't seen one. My '96 BMW 750iL has a two speed unit which runs on low whenever the A/C is on and goes on high when the head pressure goes above a certain point. Same for the coolant temp. The switches are in the same locations as you described in your reply.

I like R12. Much better cooling and I believe the compressors live longer with mineral oil rather than the PAG oil used with R134. PAG is nasty stuff. Eats paint, is tough on o-rings, is hygroscopic and an irritant. Ester oils are marginally better. I know this is off topic but........

Diseasel300 · #4 07-29-2019, 03:36 PM

The 2-speed fan uses a resistor to drop the voltage to the fan motor. There are 2 relays in the fuse box to control it, one goes through the resistor, one bypasses it for full speed. Mercedes used the same system for a couple of decades, very simple.

If you're only at 185PSI head, the fan won't be on. As the head pressure rises, the fan will kick on and off as you reach the switch pressures. At 70˚F ambient, I wouldn't expect them to be on at all unless the cabin is extremely hot. This is not a BMW, MB used pressure to cycle the fan rather than leaving it engaged all the time that the A/C is used. It prevents excessive sub cooling in cooler temps for better A/C performance in low ambient conditions.

R12 does work better, but 134a works well in these Gen II systems as well due to the pressure switch for fan control and the small tube dual-circuit condenser. Run PAO oil if you ever have to convert, it doesn't eat O-rings like PAG, nor is it hygroscopic.

filp · #5 07-29-2019, 05:11 PM

I strongly suggest looking into ETM in Service Manual. It helped me diagnose my issue a lot.

tcom · #6 07-29-2019, 07:53 PM

OK, That explains it. Damn I should have thought about resistor controlled fan speeds. THanks. Good possibility the resister has failed. I'll check it out. I have the factory manuals for the 124s. That should get me in the ball park.

I'm going to download the manuals for the car. This is the second 300SDL I've owned along with several 240Ds which should be designated 240 KL, (kurtz, langsam). But they are tough.

Here the temps seldom get above 80. so the fan won't get tested. There is hot weather south and east of here and heavy traffic so I want the fan operating correctly. Actually I want it working properly regardless.

Diseasel300 · #7 07-29-2019, 09:37 PM

You can try blocking off the radiator with a piece of cardboard to force the head pressure high. It should go up pretty quick if you have some heat load in the car (parked in the sun for example). Throw a gauge on the high side and see if the fan comes on when the pressure goes above 200PSI. It probably will. If your clutched fan is working properly, the aux fan will probably never come on in 70ish degree weather unless there's a high heat load on the A/C system.

tcom · #8 07-29-2019, 10:16 PM

That is my thought. I'm going to give it a try but will be a few days as I have other projects.

New viscous drive fan clutch so I've got some engine protection without the auxiliary fan operating properly.

We have a white house with a south facing white garage door sheltered from the wind. At noon and in the afternoon on a 70* summer day it can get up to thirty degrees above ambient with the car nosed toward the garage. None of our cars have heavy tint on the windows so with the greenhouse effect it can get pretty toasty in the cabin.

Since my last post I looked around for my manuals. I found them in a cabinet in the shop along with several covering long gone cars I have owned over the years. Sometimes pack ratting pays dividends. The manuals cover up to '86 including the 300SDL. Switch, pre-resister and resister relay locations are pictured. Same for the wiring schematic. Glad I didn't sell the manuals years ago.

Again, Thank you.

Tony

tcom · #9 08-06-2019, 08:01 PM

Good evening,

Sorry that it took a few days. I got it. Borrowed the HVAC manual from the independent shop I use from time

Model specific for the '87 SDLs and more detailed.

Traced all the current paths and it turned out the relay was bad. Looked good until removed from the block. Burned and cracked in one corner. New OEM relay fixed it.

Thanks for all the replies.

TOny