Over Heating 300e 2.8 only with AC on
 

Dear Members,

After searching, reading, and fixing most of the issues on our 1993 300e 2.8, I have to now ask for guidance from the brain trust.

Without AC on, the temperature gauge is horizontal, ie. normal, 80 C. Temp holds steady, even in hot ambient temps.

With AC on, the temp climbs when slow in traffic or city driving.

New electric aux fans (both), they come on at low speed under thermal load of compressor. Jumping the connectors at the pressure switch also sets them on low speed. Jumping the high speed temp sensor also sets them on high speed, and they come on when temp climbs.

Replaced fan clutch as it overheats when in slow or stop and go traffic, no difference.

Water pump is less than 1 year old, changed due to leak.

New pressure cap and thermostat (behr)

New radiator-replaced due to crack

Compressor comes on and the low pressure side is in the blue zone on the gauge (25-35 psi). No funny noises from compressor, does not cut out, except when temps get excessive.

Bled coolant out of thermostat housing by loosening temp sensor, fluid flow, no bubbles.

Temperature climbs when AC is on, aux fans kick in, and temp still climbs to 110-115, at which time I freak, turn off AC and turn on heater to dump heat which is effective, but it seems as if it takes a considerable time to dump the excess heat. There are no bubbles in the coolant, no boiling in the reservoir, no loss of coolant. I am stumped, it is not even that warm outside. The fans suck air in through radiator, cleaned the condenser and air path when replaced aux fans and radiator. I am at a loss.

Thanks for your advice.

Martin Wolf

Are the electric fans running in the correct direction?? Do they blow air thru the radiator?

Thanks Doc,

To complete this thread, the recent link on aux fans running backwards, explains it all so very well.

The two new aux fans were both running counter to the stamped arrows on the fan blade hubs. Despite the keyed electrical connectors to prevent reverse installation, both fans indeed were spinning backwards.

As described in prior link, these fans apparently are used in other applications and must be checked to be sure of proper rotation.

The overheating was due to the reverse air flow caused by the aux fans which came on with AC pressures. At low speed, the temp climbed until high speed kicked in, and things went from bad to worse.

With fans running properly, everything is operating as intended, what a relief to see a stable temp gauge.

Fortunately, most new parts were required due to individual failures, ie. cracked radiator and aux fans not spinning. The fan clutch and T stat are probably good to change out anyway, so I still feel the love.

Martin

Good catch MB Doc!

MW, he obviously knows by experience (and by the threads posted here) that this 'reverse' flow seems to be a a regular event.

One easy thing you can do is to ensure that your af/water ratio is 50/50%. If you live in a real hot climate, then 40/60 would be better. Of course, use only distilled water.

You'd be surprised to see that most af/water ratios are 70/30. That's enough to cause a elevated engine temperature. A simple af tester described in Menu#17 will ensure that your af/water is correct and your engine is properly cooled.

Distilled water is necessary for the cooling system? I thought the manual stated that any water good enough to drink was fine?

I wouldn't put tap water in the MB's cooling system but you can. Do some reading and you'll find that your radiator will be in much better condition a few years down the line if you use distilled water.

If you do use tap water, then make sure you flush and replace it every 1 to 1-1/2 years. Good af can 'handle' tap water's impurities for that long but then it breaks down and turns corrosive to the alum.

Why even use tap water to begin with?? Distilled water is cheap and it's a great "investment" for your radiator.

i live in sunny so cal and use a mixture that is probably 80/ 20 water to antifreeze.

antifreeze does not dissipate heat as well as water does.