[rickangel]

Sorry for beating a dead horse but I just can't find the answer to my problem.
Last week my car overheated while stuck in traffic on a hot day. I changed the thermostat and checked the pulley driven fan clutch and the problem continues. While driving on the freeway the temp is about 85c but get into traffic and it will go into the red within 10 minutes. The a/c shuts down but the aux fan does not come on. Since the a/c does shut down, is it safe to assume that the switch in the a/c system is good? I hot wired the fan and it functions normally so I changed the single pole fan switch at the upper radiator hose housing. The fan still never comes on and there is no voltage at the plug near the fan when the a/c shuts down. Other than this issue the car runs great and is driven regularly for the last 6 months with no problems. There is no oil in the coolant or water in the oil and I don't see evidence of external leakage at the head gaskets.

 

[ChiefRider]

Hello-

First, fill in your profile. Everyone here likes to know what kind of SL you have and where it lives, as these bits of info can help us address your problems.

You have changed the thermostat, so it would seem your over heating predates that? WHen you refilled the coolant, are you confidant you were able to get all air out of the system? Is your radiator in good condition? Is the engine block clean and clear? Are you using the required coolant, or standard green?

I don't know specifics about what temp triggered switch kicks your aux. fan on, but I can tell you from exerience with my '88 560SL that I'm not sure I ever recall the aux. fan coming on, and I never had an issue with coolant temp. Depending on ambient temperature and driving conditions i would expect to see anywhere from 75 - 90c, with normal temp pretty much rock solid at 80. If your engine is running well with no pinging and your fuel economy is what it should be (anywhere in the 15-19mpg range) then I would be thinking about crud, scale etc. in the cooling passages. Also don't overlook a bad albeit new thermostat.

Don't forget to verify belt tension and fan clutch operation as well.

 

[petesmga]

I thought tht Aux fan came on when the A/C was on. As this didn't happen nor did it come on with excess heat this would be my first suspect. You say you checked your clutch fan, how ?

 

[abiby]

I thought tht Aux fan came on when the A/C was on. As this didn't happen nor did it come on with excess heat this would be my first suspect. You say you checked your clutch fan, how ?

My money is on the fan clutch. Best I can tell, there is no fool proof way to check it and the conditions point to that.

 

[GlueckAuf]

These comments are applicable to the '87 to '89 R107 only.

The Aux Fan first comes on at low speed when the AC's refrigerant hits a threshold pressure level, triggering a switch on the receiver-drier. This is normal behavior when the AC is used on a warm day.

The AC system is shut down when coolant hits 105 degrees C (221 F) via the 105/115C temperature switch.

High speed on the Aux Fan is ordered when engine coolant hits 115 degrees C (239 F) via the same thermal switch's secondary circuit.

These latter two stages should never occur if the engine and its cooling system are otherwise sound. They're like the lifeboats on the Titanic--the last line of defense against catastrophe, not a substitute for strong bow plates or a guy watching for icebergs.

I believe you need a more thorough, dedicated, "root cause" diagnosis of your cooling system. Is the pressure cap holding the prescribed pressure? Is the pump belt slipping? Are the pump's impeller blades corroded, broken, or missing? Is that new thermostat installed right and working properly? Is the radiator clogged or restricted? Are its fins allowing air flow? Are the engine's coolant passages free-flowing? Is there trapped air in the block? Is there an internal or external coolant leak? Is the engine generating too much heat to begin with due to ignition timing, an exhaust (or catalyst) restriction, or fuel injection problems?

Overheating often involves a combination of issues that individually, the cooling system could handle; but combined, they overwhelm its capacity.

If you don't have the equipment and/or expertise to measure, detect, and isolate these types of issues, it's time for professional assistance, in my view.

Good road,

 

[rickangel]

I'll try to answer all questions in this response.

1. I changed the thermostat after the 1st overheat.
2. To get the air out I topped off and then drove the car a couple of miles and repeated this until fluid level stopped going down.
3. The radiator appears to be in good condition. it flows through freely with a garden hose and there are no cool spots indicated with a laser temp sensor at normal operating temp.
4. While I had the thermostat out I also ran the garden hose into that port and water flowed freely from the bottom radiator hose.
5. At this point, I'm using water only. I'll get the system flushed and filled once this issue is resolved. The coolant system, as well as all areas of my cars, are properly maintained. The coolant was less than 1 year old and had no discoloration
6. Per our local dealership's shop foreman, I checked the fan clutch by looping a long, narrow length of rag around a blade and had someone start the car. the fan pulled the rag from my hand quite easily. According to the foreman, this indicates that the clutch is good.

 

[Ears]

These comments are applicable to the '87 to '89 R107 only.

The Aux Fan first comes on at low speed when the AC's refrigerant hits a threshold pressure level, triggering a switch on the receiver-drier. This is normal behavior when the AC is used on a warm day.

The AC system is shut down when coolant hits 105 degrees C (221 F) via the 105/115C temperature switch.

High speed on the Aux Fan is ordered when engine coolant hits 115 degrees C (239 F) via the same thermal switch's secondary circuit.

These latter two stages should never occur if the engine and its cooling system are otherwise sound. They're like the lifeboats on the Titanic--the last line of defense against catastrophe, not a substitute for strong bow plates or a guy watching for icebergs.

I believe you need a more thorough, dedicated, "root cause" diagnosis of your cooling system. Is the pressure cap holding the prescribed pressure? Is the pump belt slipping? Are the pump's impeller blades corroded, broken, or missing? Is that new thermostat installed right and working properly? Is the radiator clogged or restricted? Are its fins allowing air flow? Are the engine's coolant passages free-flowing? Is there trapped air in the block? Is there an internal or external coolant leak? Is the engine generating too much heat to begin with due to ignition timing, an exhaust (or catalyst) restriction, or fuel injection problems?

Overheating often involves a combination of issues that individually, the cooling system could handle; but combined, they overwhelm its capacity.

If you don't have the equipment and/or expertise to measure, detect, and isolate these types of issues, it's time for professional assistance, in my view.

Good road,

I Agree with above...

http://www.benzworld.org/forums/r-c107-sl-slc-class/1278562-overheating.html#post2021412

 

[Weather Man]

Twice this summer I had to turn the garden hose on the radiator to clear the bugs.

 

[Aussiemerc]

My 420 SEL always ran hot, then I found out the radiator was full of crud inside. New radiator core fixed the problem. It was 17yo when I changed it. I also changed my mechanic. ( I'd been complaining for a couple of years.) My 107 radiator just rusted out but worked fine right up to it's demise at age 26. I don't think the correct coolant was used. You know, top up with water and charge for coolant.