First thing I would say is that the numbers on the readout are frequently not accurate. Perhaps the infrared thermometer will tell. If that is the case then you will know how far it is off. These problems can be real frustrating. I had an Isuzu Trooper that would overheat. Replaced everything except the radiator and still overheated (had to even replace cracked head) tried a flush product and no improvement. I ran a gallon of vinegar in the coolant for about 100 miles and then flushed it about 100 times (or so it seamed) till there was no odor of vinegar. Never overheated after that. Do not know if it would damage a MB cooling system. So follow the normal disclaimer, I cannot be held responsible... bla bla.

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)

did you put vinegar with water or vinegar in antifreeze??? Any one think of any damage to mercedes system doing this??? Won't the acetic acid etch the aluminum parts in moter and water pump??

I don't think I would be running something as acidic as that through my car. A new radiator is only about $220us, if it hasn't been replaced then it is well worth doing.

Matt.

I have seen a suggestion that if the engine was running lean that it would add extra heat load to the cooling system. Its possible if the mechanical mixture control was set lean and the oxygen sensor or Lambda control system was not working.

Its conceivable, but not too probable in my opinion.

To answer another question, I have my hood pad and I live in Texas and I have no overheating or over temperature symptoms described in this thread.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

I used about 3/4 gallon of vinegar and filled the rest with H2O. Kind of like you would use for cleaning out a coffee maker or even weaker. I left it in for about 100 miles or 2 days. I flushed it out very well. I think you could have drank the final flush water. I used a garden hose hooked up to one of the hoses and disconnected each of the other hoses including the inlet and outlet of the heater core.

Please note this was a last ditch effort. This is why I gave disclamers.

__________________
Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)

To summarize:
1) outside temp below 90F - air cond on or off - eng. temp NEVER goes over 95-100C.
2) outside temp over 90F - engine NEVER goes over 100c if air cond NOT turned on.
3) outside temp over 90F air cond. on - eng temp climbs to 110C at which point I turn it off and it returns to 100C or below.

Question : my mechanic said that for the temp switch in air cond circuit to kick in -FREON temp has to be over 117C - which can and usually will occur in real hot weather. It does NOT relate to the Engine temp. Question is: is there another temp switch in antifreeze circuit that should kick the aux fan in when engine temp hits 95C or over????
After talking to my mechanic this weekend he is down to two possibilities:
1) temp switch in antifreeze circuit (if there is one)
2) car runs so good - misreading temp gauge in dash.
As stated earlier, I have an IR non-contact thermometer that I will try this summer once heat returns up here in Canada and will post results. Only other question my mechanic has is: is the aux fan a one or two speed??
Thanks for your patience guys!!

For got to mention:
If I Hot wire the aux fan so it is on ALL the time - engine temp NEVER gets above 100C regardless of outside temp or if air cond. on or off - That is why I am down to the questions above.
Thanks Again

I think u have you internal headblown gasket

To clarify the fan issue: I referenced the '86 schematic I have, and the '91 car I have. I assume all are pretty similar.

- The electric fans have two speeds - low and High.
- Low speed ONLY comes on when the AC high-side reaches a certain pressure. It is not tied to coolant temp.
In warm weather with AC on, low fan speed will run most of the time during stop/go driving. The mechanical fan just cannot keep the AC condensor cool enough.
- High speed will come on when the coolant temp reaches 115, and turn back off when it cools to 105.
- If the AC is off, the low speed will never come on, so in stop/go driving, when the mechanical fan cannot keep the temp down sufficiently, the temp will rise to 115 before the electrical fan comes on.
- The mechanical fan has little or no impact above 40 MPH - the air forced through the radiator by the car's speed is greater than the fan could pull in. So, the fan clutch is designed to idle at these conditions.

Interestingly, under some stop/go conditions, the engine will run cooler with the AC on than it will with it off. With AC on, the elcetric fan is on low speed and helping cool the engine. With AC off, only the mechanical fan is trying to cool the engine, and the electric fan will allow it to reach 115 before it turns on to help.

Since new, my 300SE in 90-degree weather has run about 85-90 on the highway, or in light traffic. But, in heavy traffic jams, it will slowly creep up to 115 whether the AC is off or on, then cool to 105 as the hi-speed fan turns on.

I personally think a good-working system will not get over 115, as the high-speed fan should provide adequate air flow under virtually any conditions when it turns on at 115 (or thereabouts).

DG

My 1988 300SEL is an original Florida car and it has the underhood insulation. I know mine is an I6, but the highest I have ever seen the temp was slightly above 80°C. It will go there when the engine first warms up, then it falls back down to 80°C.

In traffic with the AC on, my 300SEL would normally creep up to the mark in the picture...some people said it was fine, others said something was wrong... :\

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

S-Class Guru has remarked on the thought that occurred to me as I read the symptoms you described, particularly that you could jump the aux fan and that would keep your temps under control. Low fan speed is triggered by a pressure sensor set to actuate the fan at 20 bars (or is it 15?) of compressor pressure, while the high speed fan is triggered by a temp sensor set to actuate high fan at about 105C. Given all of the other repairs to the coolant system, I'd go straight to the 2 sensors responsible for aux fan operations.

Additionally, Jim Forgione has devised a clever little device called Cool Harness, designed with Mercedes' known overheating tendencies in mind. Search this forum, there are threads. It functions by actuating the high fan speed at either 92C, 95C or 98C of coolant temp (depending on which variant you buy), regardless of what the A/C is doing at the time (i.e., whether the low fan is on or not). In other words, it starts the high fan some 8-15 degrees before the factory specs would start it. Installs very easily in place of the bipolar plug coming off the CTS. If you're interested, his website is at this URL:

http://www.k6jrf.com/MB_S500.html

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

Be aware that the a/c low fans are most commonly mis-diagnosed as a bad high side pressure switch. One jumpers the sw. and gets low fan ,verifying low fan circuit, so he assumes a bad sw. The actual cause is not the sw. at all , but rather a slightly low refrigerent level. No low fan is the first complaint on a car that has a small refrig leak. The a/c still works, so it is not suspect, but in fact the high side never reaches the aux fans cut-in spec b/c of low refrig level. VERY COMMON.
The complaint is as yours, with rising coolant temps in traffic and at idle w/ac , where airflow is needed the most. The condenser heat passively increases the Thermal Load on the radiator and you wind up waiting for the coolant High fan to correct. That should be at 105C cut-in, but by that time with no low fan , the coolant will continue to climb..
As stated , many choose to bridge the coolant sesnor to a lower cut-in, but the place you want to be looking is if your low fan comes on when the a.c is on and there is a high thermal load conditon..such as stop/go traffic.
If No low fan, simply jumper the high side pressure sw [ the one with the pig-tail leads] ..if the fan then works, the sw or refrig level is suspect..if still no fan, then the culprit is the fan circuit itself, with first check being the fuse, and next being the dropping resistor behind the headlamp.

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A Dalton

I would think that w/ the Aux Fans on continously, the eng temp shouldn't be higher than 90C even w/ the A/C on.

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Regards . . . . JimF
-------------------
'94 S500 Cpe

Visit my Mercedes Web Page

I agree, except under High thermal load conditions...
A good , old fashioned cond/radiator fins clear/clean might be also in order....
and a good flush. You would be surprized what crap gets in between the rad and condenser ....the more airflow you can get , the better.
A/C guys check those regularly...

Let me redo that ..

"GOOD " a/c guys .........

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A Dalton