I have a 1978 300d and recently when i come to a stop light the temp will climb to the red......... then when i start driving again it goes back down.... I see that I have a electric fan but i have never had it turn on. Any Ideas?????

Do you also have a mechanical clutch for your engine fan ?
That is a classic symptom of failure of that clutch to be working correctly...

i imagine if it were the engine fan, you would notice it not turning whenever you popped the hood with the engine running, that would be easy to spot.

the electric fan is probably busted. if you live in a hot climate, if it is above 85F you NEED the aux electric fan to work to keep from overheating at idle, even if the AC isnt on. It should come on whenever the engine temp gets to about 90C or something.

also, you could have too much antifreeze in the coolant or a million other things.

good luck.

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

San Diego, as an oceanside community, doesn't get that hot (unless you are inland, of course). Overheating at idle suggests one of two things is happening to prevent the engine from shedding its excess heat --
(1) (more likely) not enough air moving through the radiator due to clogged fins and/or a worn-out mechanical radiator clutch and/or a failed electric fan or its thermostat, or
(2) (less likely) not enough coolant moving through the block due to a worn water pump

If the water temp is getting into the red (110C+), the auxiliary fan should be coming on. You can test the fan by tracing its leads to a connector and jumping directly to 12 Volts. There's a switch in the water jacket that turns on the fan when the coolant gets above about 108C. That switch is at least as failure-prone as the fan and maybe more so. If the fan tests good, the switch may be bad. You can test the switch with an ohmmeter when the engine gets hot or just replace it.

Test the mechanical fan by opening the hood when the engine is very hot. Stop the engine with the "Stop" lever on the IP while watching the fan. The fan should stop almost immediately, because the thermostatic clutch activates when it gets hot and makes a hard connection between the pulley and the fan. If the fan continues to spin for a few seconds, the clutch has failed. You can also test the mechanical fan with a rolled-up newspaper while the engine is running and hot. Carefully stick the newspaper into the fan blades; if the fan slows and stops, the clutch is not engaging. Learn the difference by doing this test with the engine cold so that the thermostatic clutch is definitely not engaged. Watch your fingers!

Dirty radiator fins are hard to see because you have two radiators and two fans that get in the way. If the fins get clogged with dirt and insects, the radiator can't shed its heat, especially at idle. A high-pressure spray is good for cleaning the fins. Best to remove the fans and the radiators, although that's a lot of work.

If you fix all of the above possible problems and still have the overheating, you may want to check for a worn water pump. A worn water pump, although less likely to happen, can also cause your symptoms. You may be able to test for this by running the engine at speeds above idle while at a standstill, especially after fixing any fan or clogged radiator problems.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

FYI, only the W116 300SD has the electric fan come on at high coolant temperatures. The W123's only come on with the A/C.

If it is the original old radiator it may be partially blocked....so at a complete stop you are not getting the extra volume of air flow that keeps it cool while moving. If everything else is working properly (ie. fan clutch, thermostat etc.) then you may want to try back flushing the radiator....or may be replacing it.

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

Wash it out with a pressure washer too. Mud and debris from the road builds up over time and can block the radiator airflow.

I must respectfully disagree.

The FSM "Service Manual, Heating, Air Conditioning, Automatic Climate Control, Model 123" (publication 2339, date 1988) shows that most versions of the 123 climate control system have two thermo switches for the aux fan -- one in the water jacket set at 100C and one in the refrigerant (receiver-dryer) set at 52C or 62C.

The only exceptions I can see are in the manual system (used almost exclusively in USA-version 240D cars). There, the Delco compressor version ("1st version") has only one temp switch for the aux fan. Interestingly, it's in the refrigerant, not the coolant. The York compressor versions ("2nd version" and "3rd version") have two switches as above.

The ACC systems II (the dreaded "servo" system) and III both have two switches for the aux fan. These two versions cover all 300D models from 1977 through 1985 (including mine), with the exception of Euro cars and special orders that have the manual system.

Therefore, it is almost certain that Baja Abre's 1978 300D has both a coolant switch and a refrigerant switch for his aux fan.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

try this to check your rad - run the car to operating temperature - then run your hand over the front surface (ie facing the road where the air enters the rad) - if you find cold areas then the rad is partly blocked, try flushing it and if not then you need a new one.

Incorrect. The aux fan will come on even if the a/c isn't when the temps get to 100C.

__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

I have yet to see one that works that way.

Mine does, and it is all stock. Why do you think they put a 100C aux fan sensor on the head?

__________________
1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg

You have a 280, this is the Diesel Discussion forum. There is only one sensor on the head and it is for the dash gauge.

I have not seen the coolant switch on any '83 or '84 vehicle with the 617. The switch is only present on the dryer.

That switch definitely was present on the 603 commencing 1986.

Which is what I've seen. Only the W116 300SD has a temperature switch in the t-stat housing to turn on the fan.

My idea is by the time the coolant hits 100*c the heat convection from the engine/coolant has heated up the A/C freon enough to trip the dryer switch. I hit 100-120*c a few times in my 82 300TD and the electric fan never once came on. It also didn't have working A/C.

There is the coolant temperature sender on the head, the temp switch on the t-stat housing to stop HVAC coolant flow with a cold engine, vacuum switch for the EGR, and the overboost pressure switch. None for the fan.