My 84 300TD runs nice and cool on the highway, back roads, and through town all day but as soon as I stop it seems like it heatsoakes and the temp spikes. The temp comes right down if I start driving again.
My experience from other cars points directly to the fan clutch, no questions asked. Just wanted to make sure it's a good diagnosis on the 617.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Could be but a viscous fan really doesn't do much at idle. Now whether you want to pay $280 for a new Behr one which may or may not be good quality is a better question, but I think the answer for you whether or not the viscous fan is doing anything is to install an electric fan. I'm honestly not sure if it'll fit on a 300TD, but both on my brother's W116 300SD and my dad's W123 240D we put a Volvo electric fan on, plus upgraded the alternator. We have them come on only when needed, and removed the viscous fan.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

I actually have one of the high power two speed volvo fans kicking around somewhere from something else.
Even so, the car shouldn't be overheating when I get off the highway and immediately sit in stop and go traffic. It was about 65 degrees out, very humid, raining on and off (so not good for cooling). I drove for 3 hours on back roads then a couple miles on the interstate. I have a pretty heavy load in the car right now, maybe 1000lbs of tools, going up an 8% grade for 3 miles I maintained 60 and never went above 95 degrees on the temp gauge. That leads me to believe the cooling system is doing its job.

My only other thought is that the gauge might be acting up. It was over 100 degrees and when I stopped the aux fan on the AC evaporator wasn't running, it usually kicks on and off when I sit idle for extended periods of time... Whatever the problem is, I'd like it to be fixed.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

Sounds more like the electric fan to me.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I second this suggestion! My auxiliary fan stopped working a couple of years ago and I had heating issues exactly as you described.

Good luck!!

BTW, I replaced the stock 12" auxiliary fan with a 16" one from a 560 sedan (1987 SEL ??). Also installed a relay and wired so fan runs anytime the compressor is running. The AC cools better and the engine temp never goes up more than the width of the needle. I can run my AC at full blast in 95 degree weather while stopped in traffic "forever" now. All of this was based on posts by Rollguy. http://www.peachparts.com/shopforum/diesel-discussion/354646-mod-condenser-fan-compressor-working-together-adding-relay.html

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Baton Rouge, LA
1977 300D Non-Turbo

Sweet thanks for the link!

So two things:
1. I did the "newspaper trick" I read about somewhere on here. I used a paper towel roll to see if I could stop the clutch fan at idle, it didn't just stop, it took some force. But it didn't try to go again when I revved it up. So dunno there.

2. What powers the electric fan? I finally got home and don't feel like testing if the fan works in the dark. What sensor etc turns the fan on/off?

I was hauling up an 8% grade at heavy throttle for about 2 miles and had to come to a complete stop because someone drove off the road. Engine was so heatsoaked, without any good air flow the temp was climbing fast. Fortunately cranking the heat on full blast brought it down.
I actually have a big electric fan I might put on as a temporary measure. Can I remove the clutch fan completely and just put shorter bolts back in to hold the pulley on? I see no reason why not.

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

On my '84 300D the ice cube relay under the black plastic cover (driver's side) closest to the firewall is the aux fan relay. The sensor turning it on is the high side A/C switch down near the reciever/dryer behind the passenger side headlight.
Yes, you can just remove the fan/clutch and put the bolts back in and tighten them - a smaller diameter water pump pulley would help too, but don't have a solution for that just yet...

Are the stock bolts too long to put back in? I don't know what's behind there so I don't wanna hit something.

I'm pretty sure my clutch fan is bad. At idle with a cold engine I can very easily stop the fan from spinning (fans usually lock up when cold because fluid is viscus). So I'll take it apart and see if adding fluid helps, I've seen a few guides online for that. Unfortunately I have to go away for a week on a business trip, so it'll sit in a bus station parking lot until I get back next weekend...

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

I had the same symptoms last winter. I saw the temperature rising quickly to 115 C at a long stoplight when outside was ~70 F. Started running ~90 C (instead of normal 82 C) about Oct last year. If I rev'ed the engine at idle, it quickly started dropping.

Yes, you can change just the fan if careful. I did so, because loosening the alternator bolts is such a pain in these engines. Another used fan didn't help. I also changed to larger head fan bolts (Ace Hardware, look for "JIS" in trays), so I can use an open-ended wrench without risk of rounding off.

It seems both fan clutches are fine (hard to stop when hot and blows much air when rev'ed). I think being loose when cold is "as designed".

In my case, the overheating seems to have began after I added a tube of Aluma-Seal since appeared to be a tiny heater core leak. I am using Evans Waterless coolant, so perhaps the Aluma-Seal formed a clump somewhere that blocked flow in the head. A month ago, I changed to straight water + anti-rust as a summer test. It still ran hot for a day, then started running at 82 C all the time. A theory is that a sealant clump dispersed in the water. But, I changed the T-stat and water pump at the same time, and bypassed the heater core. Both T-stats tested same in a pot of hot water, but maybe the former one wasn't blocking off the Bypass fully (some here suspect new T-stats). The former w.p. looked fine. More testing to "run it to ground".

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

You can visually verify that the bolts are not hitting anything when you put them back in.

( 5/18/16 - I checked that data point and found it is not exactly true. The pulley covers the flange - so re-install the screws temporarily before attaching the pulley and spin the flange to verify that they won't cause it to bind. )

What you describe is normal behaviour for Mercedes fan clutches of that vintage. I have a new clutch & a second clutch that I added silicone oil to, and the new always spins free at idle, even when hot. I used a very high viscosity oil in the other clutch, and it basically locks up winter & summer. I ended up not happy with fan clutches, and went with an arduino controlled Volvo variable speed electric fan. The electric made me (and the 617a) very happy.

Which volvo fan did you use? I have one out of an older volvo (dunno which one) and it's WAY too big to fit, has the wrong style mounting flange.
What exactly turns the electric fan on and off? Both relays under the black cover by the brake booster are fine. One turns the AC clutch on/off and pulling the other seems to have no effect, but they both check out fine with a multi-meter. I took the fan off and put power directly to the contacts, while the bearings sound grumpy the fan works perfectly well so whatever temp sensor circuit is supposed engage the fan doesn't work.


I took the fan off (and put the bolts back in with thick washers so I can drive the car). The spring and pin thingy were packed up with dirt and grit so I cleaned it out. With the fan clamped in a vise there's pretty much no resistance, even when I heat up the spring so it bows out. Here's a video.
https://www.youtube.com/watch?v=jkxQlC1UsjA

Also, I don't think there's any way I could fit a bigger pusher fan infront of the evaporator, the current fan takes up all the space between the two support thingies. Pic:
https://lh3.googleusercontent.com/-9o5WRpWtyhY/V0IiuybI6CI/AAAAAAAAAbk/AXdGfRGUNtYey7XTsAhXwP1lM9ShcH-8QCCo/s800/IMG_20160522_171337320.jpg

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

So two things:
First, I pulled the connector off the aux fan temp sensor (the one on top of the tstat housing). It measured 5 ohms to ground and grounding the wire had no effect, even with the AC turned on and the engine running. All the fuses are good. Not sure what's up there, the wiring manual doesn't really show anything too specific for that circuit.

Second, there was a puddle under the engine in the garage and it tastes like coolant. The entire engine is covered in oil and grime (needs to be cleaned) so I have no idea where it's coming from, but bets are on the water pump. So since I need to make a massive pelican order for everything to rebuild the front end, I'll tack that on...

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

[QUOTE=chrisgt;3600718]......................................
Also, I don't think there's any way I could fit a bigger pusher fan infront of the evaporator, the current fan takes up all the space between the two support thingies. Pic:
https://lh3.googleusercontent.com/-9o5WRpWtyhY/V0IiuybI6CI/AAAAAAAAAbk/AXdGfRGUNtYey7XTsAhXwP1lM9ShcH-8QCCo/s800/IMG_20160522_171337320.jpg[/QUOTE

There is a way. I just installed this 16" aux fan in my 85 300D.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

You just bent those out instead of bending them in. Cool.
What is it out of?
What turns the fan on/off for you?

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1984 300TD -- summer daily driver
Many others that aren't Mercedes...

I have a thread going on the fan install if you want to know more. search for it. I'll be updating it with more.

It is turned on by the AC automatically and I can also turn it on manually with a toggle switch.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked