For the first time ever my SD climbed slightly above the 100 mark (I think it is 100; the next mark below says 80). Although it did not come even close to the red 120 mark, I am wondering if my aux fan is working.

Anyone know how to jump something to make the fan spin?

Thx.

Start with the fan clutch, when they go south the cooling system suffers. The Aux fan is there to help the A/C and as a backup if the engine is overheating.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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

Quick and easy way is to turn on the A/C. The fan should come on regardless of the coolant temperature.

To check the temperature circuit, ground the sensor wire to the thermostat housing/ engine block.

Ignition on in both cases.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

AC unit locked up; belt removed so that is not an option. Assume the fan also works off a water temp switch too?

Do not know where the sensor wire is. Anyone got a pic?

If the A/C is not working, the Aux fan won't kick on until the high temp switch engages somewhere between 105-115˚C depending on the temp switch fitted to your car. If your engine driven fan is doing its thing and the cooling system is healthy it should NEVER come on under normal operation if the A/C is not in use.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

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

I'm not 100% positive but I don't believe the compressor has to be engaged/turning, yes the circuit needs to be activated but not the compressor for the aux. fan to engage. I'll go check on this '85 300SD I've got in my shop.

Just checked. Ignition on, engine not running, the fan comes on when the A/c or defroster is activated.

Diseasel might be on to something regarding the pressure switch. I'll check by disconnecting the pressure switch on the dryer in the morning. I'll also snap a picture of the over-heat connector.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

‘81 SD fan is switched only by the AC system. There’s no 105/115 switch. If the AC isn’t functioning, a bad fan clutch or other cooling system fault causes overheating.

Sixto
98 E320s sedan and wagon
02 C320 wagon

There is a "Auxiliary Fan Switch" listed for the '81 300SD. Seems to be the same as every other older diesel Benz.

https://www.pelicanparts.com/catalog/SuperCat/3335/MBZ_3335_WATPMP_pg2.htm#item6

Don't have an '81 300SD handy to check. Not having this sensor might be a quirk of the W116.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Guys - thanks for the help. Advice is conflicting but I cannot believe that MB engineers did not put in a overheat switch for water temp. The pic of the switch looks familiar and I will take a look.

The temp probably did not get to 115 yesterday but I still want to know if the fan will work if it does.

OK, went and looked. Yes, I found the switch plugged into the thermostat housing. However I am reluctant to cut that wire to short it against the engine. Looks like the wire screws into the top of the switch so maybe I can unscrew it and short it. The part in the pic shows a male end but mine has a plastic "cap" with wire in the middle and the cap appears be screwed onto the male end of the switch somehow.

This might help.

w116 300sd Temp Switch 212 - Mercedes-Benz Forum

There shouldn't be threaded connector but it might have the appearance of threads due to the way the cap is molded.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Yes, that is mine exactly. Many thanks.

I know you meant 115C regarding the coolant temperature but the thought of 115F ambient with the attendant humidity in Ga gave me the shudders!

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

On some models, they did precisely that. It's pretty clear to me that the aux fan on certain models was intended for the condenser, NOT the radiator.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

My SD days are too long ago to remember what the sender or switch is on the thermostat housing but it didn’t come from the factory as an aux fan switch. We’re talking about an ‘81 SD, right?

Sixto
98 E320s sedan and wagon
02 C320 wagon

Correct. '81 300


You guys are a lot smarter than me; what does the diagram say? In layman's language, s'il vous plait.