1981 300 Sd

For my '87 300D, can anyone tell me where to find the main thermostat, and also the switch/thermostat that controls the smaller fan? What do they look like? Where are they located specifically? My small fan also does not come on even at 110 degrees. Just found out that I have the #14 head, so I'm extra paranoid about heat issues!

Thanks!

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine

Others will be able to help you more than me, but here is a short list:
Check your oil. Make sure that it is black and not creamy/emulisfied. (If you just changed the oil it will be black in a couple of blocks. ) Creamy emulisified oil means there is water/coolant in your oil. That is bad.

Park in a clean spot or put some card board under your car. This will at least give you a hint on where to start looking.

Umm, that is about all I can think of. Good luck with that!

-Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

Usually the coolant is leaking somewhere. I'd first advise a pressure test of the system and search for any leaks.

If you are sure that you have no leaks, then the coolant is disappearing into the oil via a bad head gasket or a cracked head. An oil analysis will confirm this. Don't delay in diagnosing this issue. Coolant is not healthy for bearings and camshafts if it accumulated to any significant amount. I had the same issue on my SD. Had a cracked head that needed replacement.

Thanks, guys,
The oil looks fine, on the dip stick anyway. The only coolant leak I have ever found on this car is what comes out of the over flow hose by the the reservoir cap.

Is a Blackstone oil analysis the only way to derermine if there is coolant getting into the oil?

Any tips on trouble shooting the aux. fan on this '81 300 SD so I can determine if it's doing what it should?

Forget the look of the oil. If you wait until the oil turns milky white, you've waited far too long.

Blackstone or the Mobil analysis that I use are fine for checking whether Glycol is positive in the oil.

Disconnect the fan at the plug and apply 12V and ground to the plug. See if the fan is working. Many times they are toast. I'm replacing the fan on the SD as we speak.

Maybe dumb questions, but if I'm losing that much coolant into the crankcase oil, wouldn't I see the oil level go up, or at least stay steady, rather than having to add a quart every few hundred miles?

Regarding the fan, does it matter which side of the plug I apply the 12 volts to?

The coolant boils off almost instantly, unless you're really leaking a serious amount and it gets emulsified. But, you won't see any increase in oil level if you're drinking a quart every 1000 miles or so.

Only if you care which direction it turns............

Replace the t-stat if it's not recent - use OE/OEM only (Behr, Wahler). It's located where the lower radiator hose connects to the engine. Unscrew the round sensor with one wire attached directly in front of the housing, this will make it MUCH easier to R&R the t-stat. (Trust me.)

About the overheating, assuming the t-stat is good, rad cap is good, there's no leaks and the system holds pressure, and the radiator & condenser fins are clean... you either have a bad fan clutch, internally corroded radiator, or both. The clutch is more likely especially if it's original. You can refill it with silicone if the bearing is good (search for details).

The electric fan should trigger at 105°C... it's controlled by the 3-prong switch shown in the photo below. With the engine running, unplug the 2-pole connector and short the two female sockets together... the fan should turn on. If so, replace the switch. If it doesn't, figure out why (and I'd still replace the switch on principle... I've yet to see one that still works after 15 years.)

__________________
Dave
Boise, ID
Check out my website photos, documents, and movies!

The lower radiator hose is looking like it's in need of replacement, so that will be a good time to do the thremostat also. Thanks for cluing me in on the location of the t-stat. I thought it was at the upper hose.

Since topping off the coolant, it hasn't gotten much above 80*, but I do plan to investigate the fan further. On my '81 300 SD, where is the 3 prong switch that you show the picture of located? In re-reading this thread, I think it's by the upper hose, where I originally thought the t-stat was. Am I correct? Is accessing it and changing it out as straight forward as one would imagine?

Before I do much, I want to further investigate where the coolant is going. With my next oil change, I plan to send a sample for analysis to see if it's a cracked head. I know the system is holding good pressure, as even after the car has sat overnight the radiator cap comes off with a "POP."

I find it odd that whenever I look at the bottom of the overflow hose, there is fresh coolant dripping out. Only a drop or two when I look, but it is visibly moist with coolant. As an experiment, I have just place that overflow hose into a plastic bottle in an effort to see how much is collected over the course of a few days.

Sorry, firemediceric, the info in my post is for the 1987 300D (original post topic), not your 1981 300SD. That 3-prong switch doesn't exist on your car, AFAIK. And, I think on your engine, the thermostat is at the upper radiator hose, not the lower. The 124/603 and 126/617 are TOTALLY different systems.

The radiator cap should not come off with a 'pop' when the engine is cold. In fact there should be zero water pressure. You could have a crack or head gasket allowing combution gasses to enter the cooling system. Cracks or failing gaskets do not always result in oil and coolant mixing.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Dave, you rock! Thanks for the picture and description of location. Yes, I'm pretty sure I need to replace that switch.

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine

gsxr,
Thank you for the clarification. I was looking at the wiring for the fan and it seems to run straight into the fuse box with only an in-line plug between the fan and fuse box, so I'm not sure how to see if the fan is getting the signal it needs at the proper time/temp.

rrgrassi,
given the car's symptoms, how do you suggest I proceed? Especially if a UMO analysis may not show coolant in the oil even if the head is cracked? The car, after it warms up, runs great. I hate to go looking for problems.

Do the oil analysis, as instructed, because even if there is no mixing, vapors do condense and can leave a tell tale residue the anaylsis can pick up. It possible to have mixing, but not enough to give the oil a "milk shake" look to it, like Brian Carlton said. Also do a compression and leak down test on the cylinders. You can also check for bubbles coming in to the coolant tank, with the cap off, and engine running. Take the cap off while cold, start the engine and let it idle until it reaches it's normal operating temp. Look for bubbles while the car warms up. When the engine is cold, it is possible for the cracks to seal due to the contraction of the metal, and then open a bit as the heat causes the metal to expand. I had a gasser Datsun F-10 do that. Started running hot, and losing coolant. The radiator showed bubbles when the engine ran. The Datsun also had some water on the #3 piston. Turned out to be a head gasket, and that was only seen after disassembly. Also repaired a Ford 460 that blew a head gasket. Only way to tell on this one was the water drooling out between the head and the block on the front of the engine. No visible oil/water mixing in either case. So do all the testing you can before throwing money at repairs.

__________________
RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K