W126 Temperature
 

Regarding a 1990 300SEL

When I first got this car, the water temperature would usually stay at about 80 (C). In very heavy traffic it would sometimes go up to just under 90, after which it would suddenly drop back to about 85 - I assume that was because the fans came on.

Recently I had the dealer flush and refill the radiator, and install a new thermostat (don't ask - $250!!!!). Now on vacation I see it runs at about 85 and in traffic it went up to a little over 90 and didn't come down until traffic started moving and I was driving at 65mph for a while. Scared me, I was just waiting for a hose to burst or something else bad to happen from all that heat.

Makes me wonder what the dealer did and if the fans are still working.

Any thoughts from anyone, and how can I test the fans coming on at the right temperature, whatever that is....
Thank you.

The temp is normal even if it goes above 90 as stated in the manual.
to test the fans simply pull the plug on the sending sensor and fans will come on, insert and they will go off if the temp is low.
mak
300se

There may a little air trapped in the system. After the engine reaches operating temp, park the car on an incline - nose up - and release the radiator cap just enough to allow air to escape.

But mak is correct. In fact, the thermostat doesn't even open until the coolant reaches 87c.

Thank you both so very much.
Turns out I misstated the temperature in my first post. The gauge is marked with an "80" and a "120", so the unmarked line in between I assume is 100.

It used to run at 80 and go to 90 if stuck in traffic, and then drop fast to 85 (which I'm guessing is because the fans came on).

After the super expensive new thermostat and flushing by the dealer, it now runs normally at 85 and when stuck in traffic stays at 100 (the unmarked line) until I get really moving again. That worries me because it is much higher than before the work was done.

M103s have two different thermostat options, at least here in the US.

79°C and 87°C remember, that is when they begin to open

You do realize most domestic come with a 195°F or ~91°C... and that is when they begin to open.

Temps you are seeing are normal and fine. I would begin to be concerned if I was over 100°C into the 110°C area

Verify your aux fan switch works on the cyl head and motor on as well as the fan switch for the A/C system.

FWIW I installed the lower temp one in my 300SEL a while back as I became tired of the fan clutch locking and basically being locked in city traffic (really saps power from an already underpowered car). No issues with that now and no cooling problems after two summers.

Thank you very much for your help.
Could you please tell me exactly how to:

"Verify your aux fan switch works on the cyl head and motor on as well as the fan switch for the A/C system"

Thank you again.
D.

drosen, here are info and photos from my 103 motor (W124 chassis). Two engine components control the aux fan:

1) a coolant temperature sensor located on the top left side of the engine block, back at about cylinder 4. It's a 2-wire thermistor with a blue base, which M-B refers to as component B10/8. That one is designed to turn the aux fan on to high speed when coolant temp reaches around 100 degrees C. Test it like this: key on, engine off. Pull off the connector to B10/8 and the fan comes on high speed. Replace connector and it goes off.

2) a red-colored high pressure switch located on the A/C receiver/drier, designed to turn the aux fan on to low speed when the A/C condenser pressure reaches about 15 bar, as I recall. Test it like this: key on, engine off. jumper the 2 wires at the pigtail connection and the fan comes on low speed. Just slide the plastic sleeves back a bit, and jump those wires. Easy and fast.

If either of those tests is unsuccesful, post back, and get advice on the next tests to perform.

My 300SEL stays around 80°C in the winter and slightly above in the summer. Never have seen it more than 1/2 way up.

Mr. Learner:
I cannot thank you enough or the most PERFECT response telling me exactly what to do, and even including pictures. Than you so much!

On the road, home in 2 days, will post back as soon as I get home.

Finally got back from out of town, sorry for the delay.

Key on, engine off, pulled off connector from blue base, fans do NOT run.

Key on, engine off, pulled the pigtail connectors apart, jumpered the two male spade lugs, fans do NOT run.

Fuse #1 is labeled auxiliary fan, checked and fuse is good.

What now?

Check fuses, check relays.

drosen, if both tests fail, the most likely suspects are fuses and fan motor or wiring to the fan motor. There are 2 fuses and two relays involved with the aux fan. You said that fuse #1 is indicated as aux fan, but that doesn't agree with the electrical schematic, which shows fuses 7 and D. One is for the coil side of the relays (shown in the attached photos as B and C) and the other fuse is for the load side, I don't recall which at the moment, but both fuses should be checked before you do anything else. Best to remove both fuses from their holders, inspect carefully, gently clean up the metal end caps and re-fit them into their holders. Then redo your jump test at the red high pressure switch. Do you hear a relay clicking? If yes, the relay coil is working but power isn't getting out of the relay. Replace relay in position C. If that doesn't do it, suggest you check voltage (key on, engine off) at both sides of the series resistor located down low behind the driver side headlamp assembly. If there is battery voltage on both sides, it points to bad fan motor or the wiring to the fan. But before replacing motor, tap gently on the fan hubs (you have 2 aux fans) to be sure the fans aren't just stuck, but still functional.

http://www.peachparts.com/shopforum/...1&d=1243511924

Another fabulously helpful response that even my tired little brain can follow.
Thank you so much, again.

I got the fuse info from the paper glued to the underside of the fuse cover.
I would still wonder if it is normal that the aux fans are supposed to routinely come on, when I thought they would only come on in unusual circumstances of overheating.

It is raining like dickens here and I hope you won't desert me if it takes another 2 days for me to do what you suggested and then repost. It's just too messy working in the rain.

It isn't normal for the aux fans to routinely come on. They're called "auxiliary" for a reason, because they only come on to provide additional airflow across the condenser/radiator under conditions of high thermal loads. As for the fuse designation you say is on the printed card under the fusebox lid, I would hope that would be correct, but it doesn't agree with what the FSM electrical troubleshooting guide shows for your year M103 motor. If you have access to that document, look in Supplement 14 at pages 102-104. Someone else is going to have to chime in on that discrepancy.

Someone could have wired a resistor in the wiring to 'fool' the fans into coming on. My PO had a relay rigged to run mine all the time instead of replacing the fan clutch. :rolleyes:

Be sure air is able to come through the radiator (i.e. condenser is clean) and you have the late style plastic fan. I think it is white.

If you have never removed the fans for cleaning around them, go ahead and do this. Be prepared for lots of **** to come out.

Attached please find two pictures - one is the open fuse box which looks different from your picture, and the other is the fuse description card which indeed shows the auxiliary fan fuse as #1.

What do I do from here?

Nobody has mentioned the infamous R15, auxiliary fan resistor.
This connection fails fairly often. It's a big ceramic resistor waaaaaay down in the fender well, sorta under the front right side of the fuse box, under the AC hoses and other wires. It's sometimes hidden by the leaves that collect down there. Usually the wires get corroded by the heat it generates and they fall off. Can be a fire hazard also.
R15 only comes into play on the fan low speed, which is turned on by the AC pressure switch, then routed through relay K10, to the input side of R15. But, the wire to the fans coming from relay K9 for high speed is also hooked to the output side of R15, and it could be corroded off.
I had to replace one last year.
Do a search under Auxiliary fan resistor for the exact location and wiring details.

Also, to check the actual fans, just find the wire plug right near the AC accumulator, and unplug the connector; then put 12V on the wire and both fans should run on high.

DG

Today I finally could get back out there to pursue this temperature problem, and checked the fans.

As suggested in a post above, I separated the two fan wire connectors. Coming from the fans to the male spade connectors are one brown wire and one brown with white. When I connect 12v to these male lugs, nothing happens whether the ignition is on or off (motor not running).

Those wires connect to the female connector, one wire brown and the other brown with blue. When I connected the 12v to these female connectors, nothing. HOWEVER when I connected the negative ONLY to the brown/blue, with the ignition OFF, both fans came on.

Sounds crazy to me, but does that mean the fans are actually working? I still don't like the temp being 85C as the normal temp, going up to almost 100C when stuck in traffic, and then (maybe the fans are actually working) dropping to a little over 90C.

Before I got the radiator flushed and the antifreeze changed by the dealer (at a high price) the car ran normally at a touch over 80C and at worst in traffic moved up to about 85C.

Thoughts?

Fans are working if they turned on. My suggestion is look online for "cool harness" to trick your fans to come on earlier but the right fix is probably your tubes in the radiator have scale and isnt as efficient and the thermostat is failing and your water pump is old so between the 3 you get higher than your used to temps. Once 1 part goes bad the rest start to follow suit as they help each other and are not redundant features.

When the engine is operating normally at ambient 85 - 90 degrees no fans are in operation. If the car is going over the speed of 30 kilometers an hour enough air should be drawn through the condenser/radiator to provide enough cooling for the engine system.

If in slow traffic or very hot conditions if the temperature passing the viscous fan reaches above 95 degrees then this fan will "lock up" and cause extra air to be pulled through the condenser/radiator. You can hear this when it happens, there is a fan roar when revving the engine.

The AC controls will decide if there is extra cooling required across the condenser and will activate the electric fan at low speed.

If the engine temperature exceeds 105 - 110 the sensor on the head will activate high speed electric fans.

There are two relays - one each for high and low speed electric fan operation. You can remove the relays and bridge the active contacts to ensure the circuitry to the fans is O.K. and to test the resistor that controls fan speed. You need a little bot of technical nouse to be able to do this.

Cooling also depends on the condition of the radiator, coolant level, leak free coolant system, proper operation of the thermostat and proper circulation operation of the water pump. The serpentine fan belt must be at the correct tension otherwise slippage of the pulley driving the viscous fan and water pump can occur.

Temperature rise and fall in traffic or hot days is normal. Sometimes I speculate wether or not MB should have put an idiot light in the instrument cluster rather than a gauge. - Keeps people from worrying about the rise and fall.

Except in extreme cases, if the temperature rises constantly above 105 - 110 degrees there is some case to worry. Last summer in my city there were three days in a row over 45 degrees Celsius (115 degrees Fahrenheit). The temperature on my M103 got to a constant 110 degrees driving on a freeway with both the viscous and high speed electric fans on.

Thank you for helpful thoughts.
I'm paranoid because it is an older car and especially it DIDN'T RUN THIS HOT BEFORE THE DEALER SERVICED IT!!!

Check the coolant mix and make sure it is not too much glycol!! That will make things get hotter for sure. I only say this because you point out that you did not have this problem until the dealer service. Perhaps they forgot to dilute the coolant, and glycol by itself doesn't cool nearly as well as when mixed with water. A little $2 eye dropper coolant tester should tell you, though I've never used one with MB antifreeze. Anyone know for sure if they work with MB antifreeze?

For what they charged, they better not have goofed that up!
Will check tomorrow morning and post back if that is the problem.
Thank you.

I mean it's totally unlikely of course, but you were wondering what they could have possibly done to make such a difference, and there's not much more than that (unless they installed a hotter T-stat).

I guess it's also possible that the flush knocked some crud loose that is now plugging passages in the radiator or something.....

This is why I miss radiator caps that let you look down into the radiator!

Very hard to say without being there!

I took the car back and told them to test the thermostat to make sure it was not defective and to reflush the radiator in case they dislodged some gunk and didn't get it out.

Naturally they weren't going to do any extra work, so they told me they did "the research" and found that Mercedes now specifies an 87C thermostat (instead of 80) because if the engine runs hotter it is better for emissions.

Sounds like baloney to me, or at least that I should have been informed to make my own choice if I wanted a higher temp thermostat or not.

What would be the issue if the car failed the jumper test for the pressure switch? I hear the buried fan relay clicking but that's about it.