560SL overheat, no fan - PeachParts Mercedes-Benz Forum

rjk · #1 08-21-2007, 09:35 PM

My '89 560 SL is overheating. I've traced it to the electric radiator fan not coming on. Is there a relay I am missing or is the fan controlled directly by the temp sensor or computer?

I understand you can jump a connector at the coolant sensor to get the fan to run constantly while the ignition switch is on. Which connector is this?

I'm short on time so can't really do a thorough diagnosis right now which prompts my query for a quick fix to get me through to the weekend.

As an aside, the A/C cabin fan stops when the coolant temp rises above about 90C. Are they connected or is it just happenstance?

Bob

jako · #2 08-21-2007, 10:39 PM

well you have a viscos fan & electric fan.
viscos fan should take care of eng temp at all time, but if there is a problem, the electric fan will kick in at 105 deg. the electric fan also cools down the condenser for the ac.
engine runnig at 90 to 95 deg are normal.

goosd luck

rjk · #3 08-22-2007, 06:02 AM

Well, let me be more specific.
In stop & go traffic, the gauge will rise into the red zone but cool down once I start moving.
When I park & turn off the engine, the temp will rise to the point of boil-over.
Electric fan never comes on.
I assume I have problems with both fans?

ctaylor738 · #4 08-22-2007, 07:42 AM

Well, then you have more than one problem. My 560 has never gone much over 80 in traffic even on 95 degree days.

Suggest you check your thermostat, radiator (is it hot at top and cool at bottom?), the cap on the tank, too much antifreeze in coolant mix, air in system, and viscous fan coupling to get a handle on the basic overheating problem, .

Then you need to find out why the aux fan is not coming on. Temp sensor, fan relay, or the fan itself.

Lots of posts on these problems. Sorry it's not a quick fix.

dpetryk · #5 08-22-2007, 10:17 AM

As previously mentioned I agree you probably have two problems. 1. Low on freon. This will cause the air entering the radiator to be preheated and if the main fan is not functioning the engine will get hotter than normal. 2. The main fan clutch is probably not engaging properly. It is controlled by the temp of the air passing over it. AT the temperatures you mention it should be locked up tight.

If you want to test the electric fan, jumper the two wires connected to the pressure switch mounted on the receiver/dryer located in front left side of the radiator. There are two pressure switches. One has a wire lead attached about 4" long and it is the smaller of the two, the other does not have a lead wire. Jumper the switch with the lead wire attached. Disconnect the harness from the switch lead wires and jump them together. The fan should come on low speed.

Getting the freon to the proper level will actually make the engine run cooler since the aux fan will run when it should.

rjk · #6 08-22-2007, 06:28 PM

Thanks for the info.

T-stat is a month old (replaced when a "mechanic" told the wife that was her problem).
Radiator is less than 2 yrs old.
Pressure cap is good & system holds pressure (I have a Stant pressure tester).
A-Frz is new & properly mixed (done when t-stat was replaced, I checked it w/hydrometer)).
So that leaves the fan coupling...hmmm.

No bubbles in freon & air coming through vent is cold even w/blower not engaging. Nice and cold when it does engage.

I'm not sure which sensor is for which but correct me if I am wrong here.

The sensor in the T-stat housing with 3 terminals is for the dash gauge?
No continuity between any terminals nor between any terminal & ground.

The sensor sitting vertically in the water passage of the intake manifold w/2 terminals (front-drv side of engine under the air filter) is for the electric fan?
If so then the resistance of the sensor is 2.51k @ 90-95F ambient air temp. Tested it when I got home after a long hot day in the driveway, coolant must be at least that temp.
Resistance too high?

Where is the fan relay?

I see mention of an A/C lock out sensor in the parts section, is it what I think it is? Cuts off the A/C when the engine temp is too high? Or am I thinking too much?
I also see an Aux Fan Switch (1 terminal) mentioned that looks like the sensor mounted next to the 3 terminal sensor in the t-stat housing (it also looks like the A/C lock out switch). No continuity here either between terminal & gnd at 90-95F ambient air temp.

Thanks

K

rugby2 · #7 08-23-2007, 11:29 AM

I have the same fan problems with both my 560's. also several of my friends have with 380's and 450 sl's. i curse mb.eng. for not having an adjustable switch so I could make fan kick in at temp I want it too. not just prior to overheating. I found how to jump so go's on all the time. problem is it wont shut off when car /eng is off. I friend solved this by putting in a manual switch in /under dash so he could flick on when he was in traffic. I am not sure I want to drill a hole in firewall yet. has anyone found an aftermarket switch that kicks on at a lower temp.?

rjk · #8 08-24-2007, 06:01 PM

Is the sensor/switch in the T-stat housing with 3 terminals for turning the fan on/off?

No continuity between any terminals nor between any terminal & ground.

The sensor sitting vertically in the water passage of the intake manifold w/2 terminals (front-drv side of engine under the air filter) is for the electric fan?
If so then the resistance of the sensor is 2.51k @ 90-95F ambient air temp. Tested it when I got home after a long hot day in the driveway, coolant must be at least that temp. Tested again next morning w/ambient air at 75F, resistance up to 4.1k

From what I have seen on other posts the resistance is too high. Replace?

Where is the fan relay?

I see mention of an A/C lock out sensor in the parts section, is it what I think it is? Cuts off the A/C when the engine temp is too high? Or am I thinking too much?
I also see an Aux Fan Switch (1 terminal) mentioned that looks like the sensor mounted next to the 3 terminal sensor in the t-stat housing (it also looks like the A/C lock out switch). No continuity here either between terminal & gnd at 90-95F ambient air temp.

Thanks

K

regsatx · #9 08-24-2007, 09:56 PM

Two silver squares sitting side by side. I think one enables fan operation based on refrigerant pressure (low speed operation)...the other based on engine temp (high fan speed operation). In fact...if your refrigerant level is low, fan won't work properly. I AM NOT SURE ABOUT THIS...but I BELIEVE the relay on the right is the one for a/c pressure operation. And there is also a balast type connection near the fan that you should check.

Good luck.

CamelotShadow · #10 09-02-2007, 09:24 PM

Just got a 83 500 SEC
I'm trying to figure my out hot engine too
I have trouble digesting all that
I have a hot engine even wo the AC
So haven't run AC yet

Like to be able to use AC

Good Luck

rjk · #11 09-03-2007, 01:12 PM

After a closer inspection of the sight glass, I found it was dirty enough not to see through. That'll teach me to look at it without a flashlight!
Cleaned it off and a great deal of bubbles were readily apparent.
So, until I get the car in to an A/C shop, I jumped the electric fan and haven't had a problem yet. Temp sits around 80-85C.

Thanks for all the help.