M104 Coolant Temp Resistor Mod
 

Hello all! I have a 1992 300CE (M104).

The car is a weekend cruiser, but with the territory of living in South Florida, it sees traffic. Just like a normal M104, it runs a bit on the warm side. I have yet to make the aux fans kick on, but I don't feel comfortable watching the temp creep up over 100C on the gauge in traffic. As soon as I start moving 30-40 mph, the gauge quickly falls back down to 85C-90C.

I've tested the fans and they work. A/C on high, they both spin, A/C on low one spins. Everything seems normal.

I just performed a cooling system maintenance too. Zerex G-05, 79C Behr thermostat, Red Line Water Wetter, and a new reservoir cap. I'm at about 70% coolant 30% distilled water. There aren't overheating issues, I just want some insurance. The cool harness looks great, but I have a hard time justifying the $90.

I picked up a pack of resistors and went to town. 2.2k 1/2w Radio Shack flavored resistors. I spliced into my CTS wires and soldered them up (see pic 1). Followed it with heat shrink tubing. Is it okay that I spliced the resistor this way?

Now the problem, the mod didn't work. From what I've read in other threads on here and other forums; this resistor should "trick" the fans into turning on at about 98C.

I did the mod, buttoned things back up, drove the car around for about 30 minutes, let it sit idling for 30 minutes (keeping a close eye on it), the gauge crept up over 100C (see pic 2), nothing. Didn't quite get hot enough for the fans to kick on, so I just shut her down. Let it heat soak for 10 minutes, started it back up, fans kicked on right away and brought the car down to 85C as they should.

Looking for some guidance here. Don't think I could've plugged the CTS plug in backwards.. Any help is greatly appreciated.

Thanks
Nicholas

http://i143.photobucket.com/albums/r...psdkc5tdwq.jpg

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Is the Clutch for the fan working? Sounds dead to me.

Do you know how to check it?

get rid of the rear wheel tyre combo

I second fan clutch issue. See how easy it is to spin when it is at that temp, right after you shut the engine down.

At that temperature with the engine off I should I be able to spin the fan?

Yes, but it will be tough at first

when you check your fan check if it has play in it.Could be time for a fan mount with new bearing on your M104.I have to replace mine at 190,000 miles
going ahead and replace water pump too.your staggered tires look good.

Hi,
have you tried to reverse your coolant ratio e.g. 70% Water and 30% Coolant?
You might be surprised on the effects!
Being in South Florida the only concern is to lubricate the the parts involved with the cooling system, but absolutely not about freezing.
Just an other thought.

Deleted misleading paragraph completely.

I took the link off, since it wasn't representing the real thing.

EDIT:
I am sorry I didn't see it clearly in the photo. The resistor is in parallel to the switch (NTC). A much lower value might trigger the FANS earlier, however the fan(s) do draw quite some current and the 1/2 watt resistor could burn away at lower resistance.

I would have to check with the wiring diagram, since there might be a relay involved!

Hope I didn't get too technical with my reply.

OK, just had a quick look at the control circuit.
As it looks the Temp Sender unit is communicating with the ECM (Engine Control Module)
In return it switches the FAN via relay (in Relay Box) at a elevated temperature on.
If the temperature rises further, it switches the second FAN via relay on.

To fake the hot condition, you would have to unplug the connector and put a, probably 1K resistor between the pins. The Value, I only guess if the cold resistance is 10K.
The 1K may be less, possibly in the 100 Ohm range. The ECU is probably reading a 0-5V DC so 5V/100 Ohm = 50 mA of current. 5VX50mA = 0.25 Watt So a quarter Watt resistor will do.
You can test the auxiliary FANs just by idling the engine and put that 100 Ohm or the 1K resistor across the pins. If they come on, I would do this permanently, but your fans would run at all times and I don't know if the ECU would catch this and throw an DTC at you. Remember, the higher the resistance the less power is dissipated across the resistor, so a quarter watt or higher does the job.
MAKE SURE THEY STOP ONCE THE ENGINE IS OFF!

Putting a different value across the Temp Sender Unit, would work too, but the ECU might throw an over temp warning at you.

I had to do the same thing for my diesel when they sent me the wrong climate controller, (was for a gas) My fans would start at 90 but being a diesel that was too hot needed to be 80 degrees. I spliced in a 950 ohm resistor now working great.

I had a short conversation with Arthur, he did set me straight.
A lot of the control circuits are all across my Manual and it takes hours to track back, also I have to be very careful not to look at a different model.

In fact the parallel resistor will lower the trigger point of the FAN circuit which is not controlled by the ECU, as Art told me.

My apologies, if I went in a little too fast but on the other side, it is really based on the OHM's law, hi, hi.

I also have the m104 (1995 e320), and I'm looking for the correct P/N for the 79C thermostat. Can someone recommend. Thanks

Also, I don't have the thermostat housing with the bleed bolt. Should I replace?

I think I found the answer to my own question.

79°C Thermostat (MB p/n 110-200-06-15)
https://www.*******.com/products/mercedes-engine-coolant-thermostat-1102000615#fitment