W126 Temperature - PeachParts Mercedes-Benz Forum

drosen · #1 03-13-2010, 10:47 AM

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.

mak · #2 03-13-2010, 10:54 AM

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

280EZRider · #3 03-13-2010, 04:23 PM

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.

drosen · #4 03-13-2010, 11:19 PM

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.

Hit Man X · #5 03-13-2010, 11:42 PM

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.

drosen · #6 03-14-2010, 10:33 AM

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.

Cal Learner · #7 03-15-2010, 07:20 AM

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.

86560SEL · #8 03-15-2010, 10:06 PM

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.

drosen · #9 03-15-2010, 11:04 PM

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.

drosen · #10 03-24-2010, 06:05 PM

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?

Hit Man X · #11 03-25-2010, 01:34 AM

Check fuses, check relays.

Cal Learner · #12 03-25-2010, 06:47 AM

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.

drosen · #13 03-25-2010, 11:12 PM

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.

Cal Learner · #14 03-26-2010, 06:20 AM

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.

Hit Man X · #15 03-26-2010, 11:50 AM

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.

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.