I know this must hace been covered before, but i was not able to find anything for my issue when I did a search.

Here is the deal. The car will not overheat (runs at about 85-90ºC) while moving with or without the AC on the highway or around town or without the AC when stopped or in traffic. When I turn on the AC and am either stopped or in stop and go traffic the temp will creep up to over 100ºC pretty quickly. I can generally put it in neutral and rev it to about 2-3K and it will come back down to normal relatively quickly as well.

Heat will not get hot out of the vents unless the engine is reving? Otherwise if the engine is above idle the heat works great.

Good fluid level (MB coolant) with a flush last summer. No oil in coolant. radiator hoses are soft while running and in the morning. No smoke and such, the engine seems to run perfect so I don't think the head is cracked.

The electric fan does not come on at any temp up to about 105ºC (as high as I let it get)? I can jump the low speed switch at the AC dryer and it will come on. I can jump the high speed three prong switch (new and replaced with the grey one) and it will come on.

I can stop the clutch fan with a newspaper while the engine is at idle. The fan clutch does not engage when the temp goes over 100ºC. So I think this is a good place to start, but...

Why does the electric fan not come on? I think the red switch on the dry may be bad, but I just changed the three prong switch, so that one should still give it a high speed signal when the temp gets up there. Unless there is something else going on.

Any help appreciated.

G

Does 100C concern you? Seems to be a fine temp under certain circumstances.

As to no heat at idle, perhaps your aux water pump is shot.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Agreed ^^

100C is normal in warm ambient idling, fan clutch should engage just above that, running down the road should stay below 100C but not necessarily down to 85C. There is a cooler thermostat IIRC, but the engine is supposed to run hot to be efficient, if your radiator and cooling system are in good condition you should be fine stock for near-desert temps with the A/C on full and full-throttle running (been there, did testing, temps never got much above 110C IIRC).

I am not sure what temp is supposed to trigger the electric fan, but the A/C triggers it on pressure (again IIRC) not temperature, is your refrigerant at normal pressures?

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Gone to the dark side

- Jeff

100C doesn't bother me. The problem is when I am in traffic or just parked with the AC on it does not stop at 100C it will keep going up.

It sounds like your viscous clutch is dead. One thing you can do in the meantime is add a manual switch in the dash to turn on the electric fan whenever you want. There are a number of simple hacks that can be done to improve the cooling situation on the OM603.
colder fan temp switch
colder thermostat
electric puller fan behind the radiator
smaller water pump pulley
change the coolant/water ratio

The problem is the cooling system on these engines is marginal when everything is in perfect working order. Add one worn out viscous clutch and you're over 100c in no time. There is no backup, no margin for error. OM601 and OM602 even tho they make a less power got larger viscous fans because there was more room between the engine and the radiator. Theres too little room in front of the OM603 for a properly sized fan.
For comparison with the above modifications I can sit in 105f stop and go traffic all afternoon with AC blowing and my temp gauge never goes over 90c. Viscous clutch fan can't do that.

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CENSORED due to not family friendly words

Even with the nutty MB fan controls, 100C is abnormal. I'll take all arguments on that. That should improve when you replace the clutch.

The low speed fan deploys based on pressure in the a/c system. I believe the trigger point is 300 bar, which isn't going to happen too often. But idling in traffic is a situation where it could. There are three components involved: the switch on the receiver dryer, the relay, and the fan resistor. The resistor is on the left inner fender, somewhere down behind the headlight. Make sure it has continuity. If not, replace it. I don't remember how the relay was set up for my '87 300d, but on my '87 190D it's a silver cube in the electrical center. The relay in the 190D was underdesigned @ 15A, if yours is the same you can and probably should upgrade to a 30A relay.

I'm not understanding your post mxfrank. If the cooling system is marginal, then what about 100C is abnormal? Seems contradictory, but I might be missing something.

I drove these cars when they were new, and spent some time testing in 95F ambient, we were doing NVH testing with vibration (accelerometer) measurement equipment one day, running the A/C hard, I drove from stop to full-speed at max. acceleration, back down to a stop, save data, do it again, change accelerometer locations with the car idling and A/C on, do it again, ... spent the whole day out in the hot sun testing with an early 300TD and it NEVER overheated. Yes the electric fan was running constantly, yes the temp gauge spent a lot of time around 100C, yes the viscous fan roared every time I pulled onto the track. The cooling system when new (or in excellent condition) is designed for desert conditions, pulling and hauling loads, running hard, etc. If your '87 diesel runs hot sitting in traffic it needs maintenance!

My bigger worry at the time was drawing down the battery, running a motor-generator unit in the car (from stock alternator) to provide 117vac for the (now antequated) equipment while the A/C and engine fan ran.

I did blow a heater hose once during testing, cut it shorter and re-installed to continue testing, it turned out to be a defective hose. Engineers used to have fun.

__________________

Gone to the dark side

- Jeff

In my experience 100*C is nothing to worry about on a hot day with the A/C running, but it probably shouldn't go too far above that without looking at the root cause. These things will run happily for years at 100*C without a niggle, but it does sound like the OP's fan clutch is beginning to give up the goat. Better to change it now, vs. 200 miles from home in a cloud of steam. I'd try and find a Sachs clutch as well, skip the Behrs that are made in China all together.

I've actually found my engine gets its best fuel efficiency in summer out in the desert, when even on the interstate it's running about 90-95*C for extended periods at high speeds.

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1995 E300 Diesel
~300k

I have been dealing with the same in my 1985 300D all winter. Exactly the same, it would run ~90 C normally, even on cold days, so I assume that was the T-stat's setpoint. But at long stoplights when ~70 F outside, it would quickly creep >100 C which scared me, and reving the engine in neutral quickly made it drop. It always ran at 82 C for years previously. It seemed to start running 90 C last Fall, after I added 1 tube of AlumaSeal in the top hose. It appeared to have a small heater core leak (smell, no liquid) and that has worked for me well in other cars where the leak is from sound metal. Another guy posted that his heater core leak was from the crimped gasket and crimping tighter fixed it.

What I tried:

Tested block temperature switch and swapped with one in my 1984 300D. No difference in either car. Also, shot area around sensor w/ IR gun and appeared gage indicated correct. Also, when >90 C, the T-stat appeared to no longer control so must be wide open.

Swapped clutch-fan w/ used one that seemed tighter when cold. No difference. But, the new one doesn't seem to offer enough resistance when stopped or move as much air as I would expect when >100 C on the gage.

Wwapped T-stat w/ new one, but both stamped "87 C" and silver, w/ MB logo and appeared from same factory. It did appear I got better mileage at the hotter temp, recording 25 mpg one week with many drives on a cold engine.

Since concerned about a cracked head, I recently changed from Evan's Waterless coolant to straight water (recovered Evans) and bypassed the heater core since that is hardest to purge. I also went back to an older copper colored T-stat and new water pump (old looked fine). No change for a few days, but then it started to run at 82 C all the time, though hasn't been as warm lately. One suspect might be that the AlumaSeal clogs in Evans and the water freed it. Happily no indication of a head leak - i.e. gases bubbling out or "oil mousse" in coolant (Evans is normally "greasy" so hard to tell).

After a few weeks of OK, I will swap back to one of the new T-stats since one suspicion is that they don't fully block off the bypass opening. As most know, these T-stats don't just open flow to the radiator, but must also close a bypass around the radiator. While using straight water (w/ 1 bottle anit-rust), this is easy. I would also like regulating ~90 C instead of 82 C, as the new ones appear to do.

If I make it thru hot July on the water, I will re-flush w/ citric acid, dry the radiator and block (w/ purge air for days) and swap back to Evans. It can't be just the Evans since I have used in both 300D's for several years w/ no overheating on our hottest summer days. Worst-case before was 103 C climbing the steep I-5 grade to L.A. on a very hot day.

If the straight water w/ hot summer creeps up, I will focus on the clutch fan more.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Since the 87's #14 head Is so sensitive to overheating you NEED to make sure everything is right.

FIRST you need to drain and flush the cooling system. Everything needs to be clean inside!

Next you should pull the fan and clutch and upgrade to the 98om606 fan and clutch.

While it's out, pull the radiator and clean the outside of it, and the ac condenser. Use coil cleaner after the leaves are cleared.

Inspect the belt tensioner and shock and spring... if old and clunky, replace all three. The tensioner has a HUGE washer under it... dont leave it on the old one. You can rebuild the tensioner... I've heard.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I put a new fan clutch on my '98 E300td about a week ago. One reason I did that was a simple test with an infrared thermometer. On a hot 90 degree day with the air conditioner running, the temperature of the coolant going into the radiator at its top was about 100 degrees C. The temperature of the coolant leaving the radiator at the bottom and going back into the motor did not drop; it was almost 100 degrees. The radiator wasn't reducing the temperature.

Today with the temperature in the sixties and a new fan clutch, the temperature of the coolant going into the radiator was about 80 degrees, and the temperature of the coolant in the bottom hose going back into the motor was 69 degrees -- a more than 10 degree drop in temperature.

You should consider doing that test on your car.

OK, your radiator hoses should not be soft when running, or it sounds like your cooling system is not holding pressure. If it doesn't hold pressure, I think your cooling system will not operate properly.

I recently changed out my blue switch for the red one, as well as changed my viscous clutch (twice). I've not heard the roar, but I've discovered once recently, when climbing a long hill, that the temps went down to about 85 (from 100) a short time after I crested the hill. I've not been able to get the car past 100 since.

In your case, have you checked the rad cap for pressure? That's the first thing I would do. Then jump the wires (I can't remember which) for the electric fan to ensure it runs. Finally, if you haven't done so already, change the viscous fan clutch. I've done it twice already and discovered that the main ingredient required is patience and dextrous fingers. I only used an allen key.

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1987 300TD
1984 300D
755,000 KM and going strong
BC Canada

Working on the clutch fan is much easier if you upgrade to the 2-piece fan shroud. If you remove the fan, IMO it is worth finding a 2-piece from a later 124 (E320 will work), helps a lot if you need to service anything on the front of the engine including the belt.

__________________

Gone to the dark side

- Jeff

Didn't want to start a new similar thread, but I am experiencing similar problems...

Recently I have noticed my temps are hotter than they were last summer. I paid particular attention today....driving home from work, with stop and go traffic, the temp went up to 100 no problem with the a/c off. When sitting in traffic it would climb to just over 100. Outside temps were at 70' max.

After getting home I took it on my hill I use for testing. Again, no a/c on and when I reached the top the temp was just over 100 with the outside temp around 68. I then took the car on the freeway, can home, let it idle for a bit, and the temps stayed at or above 100.

Needless to say, I am nervous about the summer, with temps consistently in the 90s, plus I go camping and take her over many passes...

Does this sound like a fan clutch problem? My clutch is e original. The system is holding pressure, coolant is the correct type and topped off (last changed 1.5 years ago), and the radiator is 2 years old. I have the same car as the op, 1987 300d

The only thing I have done to the car that might affect it is a turbo wastegate adjustment, as I brought the psi from 9 to 11 full throttle...I don't think this is the problem because the coolant temps got to 100 with simple, non turbo usage heavy, driving

Certainly sounds like classic symptoms of a failing/failed fan clutch. See if it offers any resistance when hot with a "newspaper test."

Also, skip the Behr junk clutches and get a OE from parts.com.

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1995 E300 Diesel
~300k