coolant system overflow problem ahead of road trip

[vtmbz]

I recently replaced my old weepy radiator and all the hoses on my 84 190d.
I checked the vanes on the water pump; cast iron and in good repair.

I tested the thermostat; it opened at 80c. I dont remember if I ran it up to 100c, but I figured open is good. I installed it with the detent in the right place in the coolant neck. ( I think!!)

Car runs right at 85c and after the new radiator, climbs a long hill at 95 with WOT. That was 10 deg lower than previous, so all good.

Today I ran a stress test in the garage; ac on and 3000 rpm in place.

The system ran up to 105c and the magnetic fan didnt lock. Then all the coolant pumped out the overfill tube. I would say about a gallon. The heater brought the temp down, but why would I lose so much coolant? Should I be concerned?

0verheating is hardly ever a concern in this climate but I was appalled at how much coolant was lost.

[optimusprime]

.Will your fan clutch have a fuse for it..Check power supply to the fan clutch.

[optimusprime]

You can run a pair of wires form + and - from the battery.Then with engine running touch the + and - pins on the fan ,if it pulls it in and turns the fan clutch will be ok .Now check fuse for the fan clutch ,this i think is number 8. But if unsure check all your fuses. Let us know how you get on please

[Mxfrank]

Definitely make sure your fan is working. It's possible that your radiator cap is bad. Once the cap opens, then the coolant WILL boil over.

The technical reason for this is that fluid boils whenever it's vapor pressure is above prevailing pressure. With the cap in place, system pressure can rise by 120 or 140 centibar. This has the effect of increasing the boiling point, because the fluid would need to be around 120c for vapor pressure exceed the cap's trigger pressure. However, once the cap opens, the system pressure drops to atmospheric. At that point all the fluid will instantly boil if the coolant temperature is a bit over 100C, depending on coolant concentration. This chart show the relationship of vapor pressure to temperature for pure water:

[vtmbz]

This is a repost...

I followed your advice and built a jumper for the sensor harness. With key on and engine off I shorted the harness and got a loud click at the fan. Running it locks up perfectly. So either the sensor is defective or the harness is. There are a stack of plugs on this one sensor, which is three pole red, with the top one being the electromechanical fan.

Think I can permanently jumper the fan at this sensor or will that melt the wiring?

Also, does a lower cut-in sensor exist?

[vtmbz]

Mxfrank : that explains that! The coolant didn't get to 110 before the cap bailed out I'm guessing. For the moment I'm carrying my jumper wire with me and can use it if needed. I'm headed to northern que and it's colder up there than here if that's possible. Doubt it will over heat in motion.

I still would like a lower temp sensor-- it looks like they come green 105/120, red 100/110 and black 100 but why the black ones only list one temp is a mystery. Anybody know about this?

[Mxfrank]

If you permanently jumper the switch, the fan will be on all the time. It's not an issue with the wiring, but you will be wasting a great deal of power and fuel running the fan.

I know of no "cheap" replacement switch. The extra pole on the switch is usually used to turn the A/C compressor off above a certain temperature, which is why there would be two temperature ratings. I'm not familiar enough with the early cars to know why the black switch has one rating. I think this is easy...read the part number off the original switch, and order the same one. And they you should be fine.

[vtmbz]

Update: a great trip into deepest French Quebec but got held up at the border returning to the U.S. and dumped coolant. I was carrying water so no big deal although they wouldn't let me close my own trunk and broke the latch.

Anyway I decided once home that when renewing my cooling system I used a defective radiator cap which was releasing early. Also the temp sensor shows "1" when testing continuity so replacing that as well. Should be able to perform the stress test after that is replaced; the correct cap lets me heat up to 110 without a blowout, so that's progress. The goal,is to able to survive a long traffic tie up in hot weather with the ac on.

[vtmbz]

Successful restoration of cooling system:

The final fix was the aux fan: it ran from its harness ahead of the sensors.
Jumping pins 30 and 87 on the relay made it run. Installing the relay and jumpers in the ac temp switch and it wouldn't run. I thought bad relay, but no..

The final problem was too low pressure in the ac system; adding freon made the aux fan cycle off and on and now it works as it should: I can sit at idle for long periods of time and not reach 100 deg c.

It's noteworthy that I had some ac, but not enough to engage the aux fan.

[vtmbz]

we drove down in the 190 to see the red sox play like little leaguers on acid. In the process of getting to Fenway Park we got stuck between two duck boats in a truly chaotic traffic jam of a Boston sort-- three cow paths meeting three other cow paths and then all six merging into one rutted track, which was being paved at the time.

The ambient temp gauge read 40d, which I have never seen before (maybe in my whole life in New England), we had to keep the windows up because of the duck boats spewing fumes, so we sat in the middle lane with no hope of escape with the a/c on just idling. For about half an hour.

I'm happy to say that everything worked perfectly. So just to spread a little hope out there-- it is possible to get your old MB to cool like it used to.

PS by the middle of the first inning the fans were heckling the home team and asking Pedro Martinez if he wouldn't like to pitch just one more game for the sox...Fenway is a tough crowd.