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1984 380SL
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Discussion Starter #1
I was driving my 84 380SL last week and these were the conditions: 95 degree heat, middle of the day, unrelenting sun, constant light/moderate incline, 45 minutes of driving and my temps popped over 120 before I finally pulled over and let the car cool down for a while to 80 degrees and turned around to go home.

Is this normal? If so it's a bit unfortunate, because I'd love to take the car out freely in the summer.

If this isn't normal - where would you start in diagnosing the problem? It's been hot here in southern california for the past couple weeks and I've noticed the temps popping above 100 on longer freeway drives.

Thanks for any insight!
 

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1984 380SL
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In warm climates like ours, we need top-notch cooling systems. Assuming your radiator and water pump are good, check the switch for your aux fan, it should come on when indicated temp goes to 100, I believe. Also, to relieve a spike, you might turn on the heat which isn't fun but helps. If everything checks out, perhaps my assumptions are incorrect and you should have the rad and pump examined. Also, the cars are particular as to what type of coolant we use.

Others will pop in, good luck.
 

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1981 380SL, 2012 GLK 350, 2008 Corvette, 2002 Nissan Xterra, 1965 Dodge Dart Wagon
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253 Posts
Also check the thermostat.
 

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1984 380SL
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24 Posts
Discussion Starter #4
For the aux fan should I just let it get to 100deg and see if it kicks on?

Themostat would be simple to replace - just gotta drain the system?

Drove the car around today in the heat - stop and go all over the city - seems like the temperature only gets up there when highway driving or under load, if that helps diagnose at all.
 

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'88 560SL, '19 AMG E53 Cpe, '19 Audi e-tron, '53 MG TD, '35 Ford Cpe and a few more
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That would suggest a partially blocked radiator operating at reduced capacity or the thermostat.
 

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1984 380SL
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Discussion Starter #6
Thermostat is an easy initial fix.

But if it is the radiator - any thoughts on how to unblock? Or does it need to be re-cored?
 

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1983 380SL, 2000 S430, 1991 420SEL (retired) - RHD
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5,701 Posts
There are flushes available but your old radiator is probably full of crud.
 

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'88 560SL, '19 AMG E53 Cpe, '19 Audi e-tron, '53 MG TD, '35 Ford Cpe and a few more
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If it's an older all-metal radiator you may be able to find a shop to remove the end tanks and rod out the core. But if it's a newer aluminum and plastic one then a new one is indicated. Last time I looked a new Behr is ~$320 from AutohausAZ.

I should add that I've had plastic nozzles snap off easily on plastic end tanks on other German cars. I've given half a thought to swapping out the one in the '88 so I can pick the time and place for the replacement. It's a lot easier in the garage than some BFE place on the side of the road, assuming that I can find one.

It's old school and I haven't tried it in two generations, but one way to check the radiator is to feel the temperatures on either side of the core after driving. If they're close to each other then the core isn't flowing very much.
 

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1981 380SL
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I drove down from Maryland to Louisiana last week (A/C blasting the whole time), and hovered between 80-100. 120 is cause for concern, imo. I had cooling problems until I flushed the system, replaced the thermostat, water pump, aux fan relay, and filled with the correct coolant (zerex g-05).
 

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1984 380SL
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Discussion Starter #10
Ok, so just to follow up on this, I replaced my leaky overflow hose and the leaky radiator cap. The coolant is blue, so I'm pretty sure it's the right stuff.

It actually keeps good temps during long drives in the heat except when I'm going uphill. And I don't even think it is the added stress of going uphill - because it doesn't matter what speed I'm going. As soon as my car tilts upward, the temperature starts to climb above 100.

Any thoughts on what might cause this?

There is also the possibility that the uphill load - even if not very great - is causing the temperature rise. I have very little power going uphill - unless I hit floor it and hit the kickdown and it gets knocked into a lower gear, so something else might be wrong here.

Curious if there are any ideas?
 

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1998 ML320, 1975 280C
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184 Posts
Try the fan clutch. Mine was running hot going up hill, in the winter! I was driving back from Washington State to the Boise area after buying the car and even in the rain/snow mix, it was getting hot. I replaced the fan clutch and it helped a lot. Flushing the system seemed to complete the job, as it now stays at or just below the 200F range.
 

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1984 380SL
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Discussion Starter #12
Try the fan clutch. Mine was running hot going up hill, in the winter! I was driving back from Washington State to the Boise area after buying the car and even in the rain/snow mix, it was getting hot. I replaced the fan clutch and it helped a lot. Flushing the system seemed to complete the job, as it now stays at or just below the 200F range.
Thanks for this! What do you think the rationale is for why the fan clutch made a difference on hills? You think at the wrong angle the clutch was disengaging and the fan wasn't spinning? I could probably test for that, actually.
 

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1984 380SL
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Discussion Starter #13
Actually, I just checked my fan relay switch and it has the number 120 stamped into it. Which leads me to think the aux fan won't kick in until 120 - which is basically overheating. Is it possible to get a 100c switch? Is that what is meant to be on there?
 

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1998 ML320, 1975 280C
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Thanks for this! What do you think the rationale is for why the fan clutch made a difference on hills? You think at the wrong angle the clutch was disengaging and the fan wasn't spinning? I could probably test for that, actually.
To test fan clutch, try and spin the fan as soon as you turn the engine off. it should be at or just above the normal range. The fan should be hard to turn by hand. if it's easy, the clutch needs to be replaced.
 

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1985 380 SL
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131 Posts
Actually, I just checked my fan relay switch and it has the number 120 stamped into it. Which leads me to think the aux fan won't kick in until 120 - which is basically overheating. Is it possible to get a 100c switch? Is that what is meant to be on there?
If you look closely on the switch just below the 120 C you will see the word "max", this is the max recommended operating temp of the switch. The switch point is 100C. Based on what you describe the likely cause is the thermostat not opening enough due to corrosion or just plain age. Does the car warm up quickly?
 

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1984 380SL
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Discussion Starter #16
If you look closely on the switch just below the 120 C you will see the word "max", this is the max recommended operating temp of the switch. The switch point is 100C. Based on what you describe the likely cause is the thermostat not opening enough due to corrosion or just plain age. Does the car warm up quickly?
The car warms up normally, I believe. Operates right between 80 and 100c for long periods when driving around town or on the freeway on level ground.

As soon as I go uphill, the temp starts to climb and if it’s a long hill it’ll pass 100c and start creeping toward 120.
 

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Ivy : W114 280 1974, Tracy : R107 500SL, Mrs O’Leary: W108 280SE 3.5 1972
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Try turning on the heating in the cabin to max when it overheats. If that brings the temp down your rad needs work - a recore / flush / replacement. Thermostat is easy to swap and the fan clutch easy to test.


Sent from my iPhone using Tapatalk
 

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1984 380SL
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Discussion Starter #18
To test fan clutch, try and spin the fan as soon as you turn the engine off. it should be at or just above the normal range. The fan should be hard to turn by hand. if it's easy, the clutch needs to be replaced.
So I finally did this test. Ran my car up to temp. Popped the hood. Engine off. And the fan is totally freewheeling, no resistance.

I took a look at it when it's running and the fan is spinning - I tried to give it a little resistance by lightly sticking a piece of plastic into the fan while running. It was holding resistance when running.

But, yeah, when off and hot it just spins freely. So just to verify - I need a new fan clutch, yeah? That would make this fix so much more simple!
 

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1984 380SL
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