On my 87 300SDL, the temp runs right around 85-90C when driving, possibly a tad higher with the AC on.

That sounds about right. But...when the outside temp is above 85 degrees with the AC on, the temp gauge rises to over 100, almost to 120 at times when the car idles for longer than 3-5 minutes, like when sitting in heavy traffic. Once moving again, the temp falls back to "normal" range around 85 or 90C.

As I indicated above, this only occurs when it's relatively hot outside and when the AC is running. This morning, I sat outside the office idling without the AC on for about 15 min. It was about 65 degrees outside and the temp gauge stayed around 90C.

The owner's manual states when the car sits idle, the temperature gauge may climb and be close to the max range indicated on the gauge and this is normal. I'm concerned with this. Just what exactly is normal? Given the aluminum head is sensitive to high temperatures, I really don't want have to replace it anytime soon.

Also, which is the auxiliary fan? I'm assuming it's the one that is visible when the hood is raised. Is this correct? I don't believe I've heard it run...I've got the W126 MB CD, but haven't had the chance to look through it yet.

Thanks in advance for help.

__________________
Lance Allison
Lance@LanceAllison.com

Current:
11 MB S550 4Matic, 55k miles, Designo Black/Black
14 Ford F150 XLT Lariat Crew, 73k miles, 5.0
Coyote V8 4x4. Black/tan.
09 GMC Envoy Denali, 5.3 V8, 4x4 SWB. 38k miles,
Jewel Red/Med Gray.

Gone:
87 MB 300SDL, 320k miles, Astral Silver/Blue.
98 VW Jetta TDI, 488k miles, Classic Green/Gray.
85 Olds 98 Brougham FWD, 4.3 DIESEL V6, 80k
miles, 3x Gray.

MBCA Member, Chicago Region

Lance, aux fan is out in front of the condenser and radiator. If you haven't heard it come on in the temp ranges you describe, check the fuse first. This is usually the culprit.

__________________
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

My '90 300SE does the exact same thing! The auxillary fan does come on at around 110 degrees. I'm not sure if this is normal either.

__________________
Mr. BILL

91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)

The auxiliary fan has 2 speeds, depending on coolant temperature and refrigerant pressure. There is also the clutch fan, which should operate at about 110 degrees. If fuses are OK, check the operation of these fans with and without the A/C running. If they dont run, (volt)meter them with the engine temp. high. Temperature and pressure switches are the probable culprits if the fans are electrically sound. You can check the electric fan with a battery charger (make sure it can supply the amps), or with jumper cables from the battery. Careful here to disconnect the fan from its normal supply first and dont turn the ignition on. Spin it by hand prior to hooking up a power supply to check for freedom of motion. The clutch fan can be checked by simply supplying a known good power source to the clutch with the engine running, either cold or warm. You need to only check to see that it is capable of locking up to run.

The clutch fan should lock-up at 94C & the aux fan should come on at 107C(hi-speed) on most cars after 1988. The a/c pressure switch should turn the low speed fan speed at 16 to 20 BAR of pressure.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

Thanks for posting the specifics, M.B.Doc. I probably shouldn't rely on my memory as much as I do.
Something I found on my old 190e was that the connection from the refrigerant pressure switch was bad, preventing the electric fan from running with the A/C on. Indeed, all the connectors associated with the electric fan should be cleaned. I had the exact same problem years ago with a 7 series BMW too.

I took the '95 300D to the drag races this weekend and noticed that when I got into traffic the aux fans did not come on. I got out and "spun one of the fans by hand and then they both came on like they should. The next day I got the car up to operating temp, turned the a/c on and the fans did not come on. I got out again and "spun" by hand (by turing the round housing in the middle to keep it safe). Again both fans came on. Sounds like I need to start checking/cleaning the connections. My question is which system provides "grounding" for this circuit.
thanks,
Jim
'85 300D
'95 E320
'97 CRV

My two cents worth.

Remember - If driving 60 mph or higher the main fan and aux fans do nothing. And if the connector is pulled from the sensor in the block the aux fan comes on high speed.

__________________
1991 350 SDL
183,000 miles

1982 240D
130,000 miles

the 1995 E320. Has anyone got a good schematic showing the wiring associated with the aux fans. To follow up a little, once I get the fans going by hand there is no more problem. If anyone can fax me a schematic email and I will email my fax.
Thanks,
Jim

Jim,
It is in your CD, but it is under the A/C -104
wiring section.
D:\Program\ETM\24832001.pdf

Go to Wire Diag, then A/C , then 104/hfm,
diagram #1.

As they are wired in parellel, it is hard to see where one would affect the other, so the ground would be suspect.
Arthur

If you van not find the sch. , I will send you one.

Thanks Arthur,
I figured it was in there somewhere but got back so late that I did not do too much research on it last night. I also tried by best not to get in stop and go traffic with it.
Jim

Lance,

I believe it is not normal for the temp to go 120C in traffic w/the ac. At 120C, it is possible, that your coolant could be boiling, reducing heat transfer.

First your aux fan should activate as previously said. Second, you may want to ensure that you have a 50/50 mix of coolant to water, also ensure that you have a good thermostat (you may to replace it ,it is cheap).