1972 280SE 4.5 Cooling

[rkroel]

Okay,

I've asked a couple of questions. Now I'm going try and contribute by documenting my trials and tribulations of cooling a 280SE 4.5 in the Arizona summer. I have seen other threads dealing with cooling issues on these cars, hopefully this one can help others in the future.

My car currently will do 65-70 in the 110 degree heat with the temperature gauge sitting slightly above 3/4's. If I turn the AC on or try to go faster the needle begins to climb. This happens slowly with increased speed and semi-slowly with the AC on.

If I come to a stop or slow down to parking lot speeds the needle will also begin to climb.

My goal is to get the cooling system and A/C up to the task of functioning well enough to sit on the freeway in 5:00 traffic and 110 degree heat while keeping myself and the car cool.



After evaluating the cooling system I have come up with a plan of attack:


1) Install the correct radiator cap. The car currently has a 13lb non-factory cap. EPC says the car needs a 1.2 BAR cap.

2) Trouble shoot the pusher fan mounted in front of the condenser. I can't get this fan to come on with the AC running and the car hot.

3) Flush the cooling system with some Prestone flush and refill with the correct mix of Mercedes coolant and water.

4) Replace the thermostat

5) Clean the condenser and radiator fins with simple green. I want to see how much of an effect this has on what should be a healthy system.

At this point if the system meets my goal I will stop. However I have a few more ideas in case the system can't cope with the extreme heat. It gets more expensive from here.

6) Replace the radiator with either a new factory unit or upgrade it with a more modern aluminum unit.

7) If the car still has trouble at idle I will either install a new fan clutch or switch to electric puller fans. This brings up concerns about the alternator, but I want the cooling system to be bullet proof.

8) Replace the factory pusher fan with something larger.

9) Address any final A/C issues - replace the dryer, try and clean the evaporator etc.

[rkroel]

1) Install the correct radiator cap. The car currently has a 13lb non-factory cap. EPC says the car needs a 1.2 BAR cap.


One bar is roughly 14.5 PSI. 1.2 bar is roughly 17.4 PSI. Obviously, at roughly 2/3 of the factory specified pressure, the existing 13lb cap will not cut it.


I ordered both a 1.2 bar and a 1.4 bar cap. After replacing the existing auto-zone special cap I have noticed decent improvement in the performance of the cooling system.


The car is now drivable around town with the A/C on. While moving the needle is locked onto the 3/4 mark. It also approaches the 3/4 mark much more slowly.


However the needle still climbs when sitting at a traffic light, just not as quickly.


I'm tempted to try the 1.4 bar (20.3 PSI) cap to see if I can get further improvement. I am concerned about straining the existing seals and rubber hoses, and this may not be a case of bigger is better. So I think I will hold off for now.


Next up, addressing the condenser fan.

[mbbuff]

I just went through something similar with my '73 280C. I did, however, need a new radiator and fan clutch unit. I figured it was time to modernize, as the price of the fan clutch unit was outrageous. The radiator tubes on the new one are closer together, hence more of them and more exposure to cooler air. I had them install a small bung in the top tank and installed a 14 in. puller fan controlled by an adjustable thermostat. I have it set to 185 degrees, the rating of the thermostat. I then chucked the stock A/C fan in front of the radiator and installed a 16 in. pusher fan, hooked it to a relay which is activated when the A/C compressor is on (mine didn't do this stock), and also by the stock 100 C switch. I've been running this setup for only about a month, but it has been 100+ for the past two days here, and the hottest the engine has gotten is about 200 F with the A/C on. Just for giggles I let it sit at idle, in gear, parking brake on, and the A/C on for about 10 minutes during the hottest part of the day. Again, the hottest it got was about 200 F. So far, no alternator problems, and I don't anticipate any. IMO, it is soooo much better than the stock setup, plus it only cost about $100 for the fans, relay and themostat setup. Also, saved a little weight as the two electric fans don't weigh what the stock one did with all those brackets, etc. Oh, by the way, mine has the 13 lb. cap that the radiator shop put on.

[230/8]

Actually, raising the cap rating and the internal system pressure does not lower the temperature. It raises the boiling point, but does not lower the temperature. What it does do is allow the system to operate at higher temperatures before boiling occurs.

You have a basic heat transfer problem. The engine heat in the coolant mix is not being transferred to the air flowing through the radiator. Either there is not enough air flowing through the radiator or there is not enough radiator area for proper heat transfer to take place or there is a restriction in the system that prevents the water from circulating at a rate that allows heat transfer to the radiator and then to the air.

In my experience, most MBs of early vintage with AC do not have radiators large enough to cool reliably in hot climates when the AC is used. Leave the AC off and you probably do not have a serious problem. (My '69 is like this in hot weather...just leave the AC off and the car does not overheat, just me.) They tend to run hot and so long as the oil is stout enough they can run hot without serious problems. I believe the solution to your problem, which I assume is chronic being in AZ, is to locate a good radiator shop that can install a higher capacity modern radiator core using the existing tanks off your radiator. Good luck with this, because most shops only want to deal with simple things. You might consider a call to a local hot rod shop for their recommendation of a shop that likes this kind on challenge. A custom radiator is very doable, but not terribly cheap. You have limited space to work with, (in every dimension) which also adds to the challenge.

In the interim, your plan to clean and flush the radiator and refill is a good one. Try blowing air back to front through the core to remove crud that could block air flow. Also, apply a very thin coat of black paint to the core after it is clean, use cheap paint and do not lay it on in multiple coats (it can insulate if too thick). I would also suggest that you use the minimum amount of antifreeze/coolant and add Redline Water Wetter (or something similar, there are several similar products out there) to increase heat transfer efficiency. Plain water is actually more efficient at transferring heat than the water/coolant mix. Living in AZ you probably do not need to worry about freeze protection to -35F.

[rkroel]

Originally Posted by mbbuff

I just went through something similar with my '73 280C. I did, however, need a new radiator and fan clutch unit. I figured it was time to modernize, as the price of the fan clutch unit was outrageous. The radiator tubes on the new one are closer together, hence more of them and more exposure to cooler air. I had them install a small bung in the top tank and installed a 14 in. puller fan controlled by an adjustable thermostat. I have it set to 185 degrees, the rating of the thermostat. I then chucked the stock A/C fan in front of the radiator and installed a 16 in. pusher fan, hooked it to a relay which is activated when the A/C compressor is on (mine didn't do this stock), and also by the stock 100 C switch. I've been running this setup for only about a month, but it has been 100+ for the past two days here, and the hottest the engine has gotten is about 200 F with the A/C on. Just for giggles I let it sit at idle, in gear, parking brake on, and the A/C on for about 10 minutes during the hottest part of the day. Again, the hottest it got was about 200 F. So far, no alternator problems, and I don't anticipate any. IMO, it is soooo much better than the stock setup, plus it only cost about $100 for the fans, relay and themostat setup. Also, saved a little weight as the two electric fans don't weigh what the stock one did with all those brackets, etc. Oh, by the way, mine has the 13 lb. cap that the radiator shop put on.

I'm not sure what the current rating on the alternator in your car is, but I believe mine was 55 amps from the factory. That's pretty low.

You can double check the ratings on your fan for current consumption or measure it yourself. It is pretty simple to measure how much current your fans are pulling.

How does the car do at highway speeds now? I've seen some concern a bigger pusher and an electric puller will impede flow on the highway. It would take some serious testing to determine that if the car still cools fine, which is obviously the main concern.

Thanks.

[rkroel]

Originally Posted by 230/8

Actually, raising the cap rating and the internal system pressure does not lower the temperature. It raises the boiling point, but does not lower the temperature. What it does do is allow the system to operate at higher temperatures before boiling occurs.

Increasing the pressure in the cooling system can raise the temperature at which localized boiling occurs and also increases the efficiency of heat transfer. Both of these things help improve the cooling capacity of a system.

Running a cap that has a lower rating than factory would result in a system that boils completely at a lower temperature.

Increasing the cap rating above the factory spec would only help if the heat in the system exceeds the original design specifications. However it may allow you to run the system hotter than the components can sustain, essentially masking a problem.

Originally Posted by 230/8

You have a basic heat transfer problem. The engine heat in the coolant mix is not being transferred to the air flowing through the radiator. Either there is not enough air flowing through the radiator or there is not enough radiator area for proper heat transfer to take place or there is a restriction in the system that prevents the water from circulating at a rate that allows heat transfer to the radiator and then to the air.

I agree. Returning the system to normal pressure helps this some. I fully expect that repairing the condenser fan and cleaning the radiator/condenser will provide the most significant improvement of my first 5 items.

Originally Posted by 230/8

In my experience, most MBs of early vintage with AC do not have radiators large enough to cool reliably in hot climates when the AC is used.

Not what I wanted to hear, this will make my cooling efforts more expensive. Your idea for a new core with the factory end tanks is a good one, I like it! Now to figure out whether or not the radiator in the car is factory. I know it was replaced within the last 10 years or so.

Originally Posted by 230/8

I would also suggest that you use the minimum amount of antifreeze/coolant and add Redline Water Wetter (or something similar, there are several similar products out there) to increase heat transfer efficiency. Plain water is actually more efficient at transferring heat than the water/coolant mix. Living in AZ you probably do not need to worry about freeze protection to -35F.

I have seen concerns about using Water Wetter with certain types of coolant. Do you know what the Mercedes Benz stuff is made with?

Thanks for your thoughts!

[rkroel]

2) Trouble shoot the pusher fan mounted in front of the condenser. I can't get this fan to come on with the AC running and the car hot.

So, currently pusher fan does not function. I have begun to trouble shoot the fan, here is what I have found so far:

A) The fan works when the positive lead is connected to +12v. So the problem must lie in the relaying, power source or the harness side of the plug.

B) I measured that the fan pulls 4.5 Amps at full tilt. I'll have to check the voltage of my battery tomorrow. It seems to be in good shape, so I'm going to say 12-13 volts. A whopping 54 watt fan. Good data point for use later if I decide to upgrade it.

C) According to the manual I have, the fan is wired to the normally open side of it's relay. Power comes through the "Auxiliary Fuse Box". The relay's coil is connected to +12v from the auxiliary fuse box. The other side of the coil is wired to both the optional "Thermostatic switch for dehydrator" and the "Thermostatic switch 212 deg F".

So the fan should run whenever either the dryer's thermostatic switch or the 212 deg F thermostatic switch go high, thus grounding the other side of the coil in the fan relay.

Since the AC functions and the car seems to run fine I'm going to assume, for now, that both relays are not out of commission.

So I need to check the auxiliary fuse and the #2 fuse in the main fuse box. I found the main fuse box and saw no physical damage to the #2 fuse.

Tomorrow I'll go hunting for the auxiliary fuse box. I'm guessing it is near the relays by the firewall.

Can someone give me the low down on these goofy fuses?

[Mike D]

The aux. fan engages when the temp unit CLOSES. Ground the wire from the unit itself, usually mounted in the thermostat housing, to see if the relay kicks on the fan. The relay is activated by grounding.

If you have the old relay it is not uncommon for them to stick open because they are mechanical and after not being used for a long time they will collect corrosion. Hmm, metal canister, semi-enclosed, electric coil creating heat, mounted in a compartment subject to constant hot-cold-wet-dry cycles. Not the best thought out design in the world. I re-wire the relays to use the newer plastic cased relays. Still not fool proof but they are an improvement over the metal cans.

Do NOT use a higher pressure cap unless you want to learn how to replace the heater core. Good advice about the "water wetter" and the reduced antifreeze. Try to use filtered water instead of the crap we get out of the taps here in AZ. Pick yourself up one of the 2.5 gallon water jugs from the store and keep it full in your garage. Pop by any of the vending machines to refill the jug.

[Arthur Dalton]

The a/c uses the aux fan when the drier sw trips closed OR the coolant sensors grounds.

So, jumper the drier sw and see if you have fan. If NO , then go to the relay and jumper pins 30 and 87..if still NO, you have a fuse feed problem
The trick to a/c aux fan activation can often be the drier switch is not activating b/c the a/c unit is slight low on refrigerant. The results of NO fan W a/c is the cars coolant temp will rise b/c the radiator is being passively burdened by the high thermal load of the hot condenser ..so you think the coolant system is at fault , when in reality, the actual fault is the a/c system is not triggering the drier sw., so no air-flow across condenser.
Many guys wire a relay to the fan using the compressor signal so that when you have comp , you auto have fan.
Others just jumper the drier sw and bring those two leads into the cabin and mount a manual toggle sw...that gives you a manual fan control over-ride anytime you want it [ like towing/traffic/hot days/ etc] without changing any of the stock system
On relays , as Mike suggest, the $6 RS relay is a good choice b/c the are transistor/potted relay, vs open coil.contact ones.
On the fuses , remember that a relay circuit has TWO fuses ..one fuse protects the coil side of the relay [ primary side/ smaller rated fuse] and the LOADS side [ higher fuse for fan motor ]...you can have a good load fuse and still not have a fan b/c the relays coil side has no feed... the coil side is the sensor circuit, the load side is the fan motor circuit.
So , when checking a relay you want V to BOTH the load and coil [85/86] terminals
If you jumper 30/87, you are jumping the load contacts...if no V to the coil side , you have a sensor or fuse problem.
A trick we use for relay coil V and activation is to jumper the sensor/s and listen real close to the relay ..if it CLICKS , the sensors side of the relay is Good, so you go look at the load side for your problem..if no click, your prob is in the sensor circuit side. [ 85/86]
*
Note ..this "Listen for relay click" does not apply if relays have been changed to the new transisitor type, simply b/c they are not mechanical switching..ie, no moving parts ]

[berfinroy]

See page 83-12/4 of the service manual for location of the auxiliary fan relay for the 4.5. It's on the ECU bracket next to the radiator. These are still available @ around $50, but they are easy to fix by simply prying off the aluminum casing and running 1000 grit paper between the points.

[Graham]

I have that same 4.5 engine on my '72 350SL. I used to have an overheating problem and this was likely cause of having two exhaust manifolds crack.

I have not changed any parts of the cooling system except the cap which was the incorrect one. Only thing I have done, is adjust the timing to the specs given for Euro cars in the manual - From memory, I think it is 30deg BTDC at 1500rpm with vac line plugged.

The temp gauge is hard to read on these cars, but mine is always just below the 9 o'clock position.

[Arthur Dalton]

>>The temp gauge is hard to read on these cars, but mine is always just below the 9 o'clock position.>

The lines are in 20 degree C increments.

[Graham]

Originally Posted by Arthur Dalton

>>The temp gauge is hard to read on these cars, but mine is always just below the 9 o'clock position.>

The lines are in 20 degree C increments.

Just checked - my gauge has just 3 numbers - 100, 175 and 250F. There are hatch marks in between, so I guess they would be at 137.5 and 212.5.

This would mean the mark between 175 and 250 is 212.5F (i.e. 100C or boiling point of water at atm pressure)

I would guess my car runs at about 195-200F.

One thing about the radiator caps. The manual for my '72 M117 engine specifies that the cap should be a '100' (Set to blow off at 1bar GAUGE pressure. Coolant would have to get to about 265F (129C) before cap blows off.) . The one I installed on my car is a 120 (1.2bar) - I think I read somewhere that the 100 was no longer recommended - Is that correct?

[Arthur Dalton]

<

Just checked - my gauge has just 3 numbers - 100, 175 and 250F>

US Version.

The 20C increments are for Euro Model gauges and all later models.

[rkroel]

Originally Posted by Mike D

The aux. fan engages when the temp unit CLOSES. Ground the wire from the unit itself, usually mounted in the thermostat housing, to see if the relay kicks on the fan. The relay is activated by grounding.

You are correct. I realized last night after I logged off that "high" wasn't the best choice of words. I meant "high" in the logical sense, rather than electrical. In my nomenclature the switch goes "high" (or logical truth) at a certain temperature and the switch closes the circuit to ground. Which in turn grounds the fan coil.

Originally Posted by Mike D

If you have the old relay it is not uncommon for them to stick open because they are mechanical and after not being used for a long time they will collect corrosion. Hmm, metal canister, semi-enclosed, electric coil creating heat, mounted in a compartment subject to constant hot-cold-wet-dry cycles. Not the best thought out design in the world. I re-wire the relays to use the newer plastic cased relays. Still not fool proof but they are an improvement over the metal cans.

Good idea. Some of those relays look crusty at best.

Originally Posted by Mike D

Do NOT use a higher pressure cap unless you want to learn how to replace the heater core. Good advice about the "water wetter" and the reduced antifreeze. Try to use filtered water instead of the crap we get out of the taps here in AZ. Pick yourself up one of the 2.5 gallon water jugs from the store and keep it full in your garage. Pop by any of the vending machines to refill the jug.

I use distilled water. I never thought about using the filtered stuff from a vending machines. Might be worth a shot.

I don't think I want to learn to replace the heater core anytime soon.

Thanks.