Busted coolant temp sensor - workaround - PeachParts Mercedes-Benz Forum

tinypanzer · #1 12-02-2008, 06:15 PM

So, I was replacing the coolant temp sensor that goes to the temp gauge. While I was in there I decided to check out the main dual-element coolant temp sensor. Well, it crumbled into pieces and the wires pulled out of it, making it totally dead. Sure enough, the car wouldn't run at all without it.

Did I mention that I did this in the parking lot at work, totally stranding myself?

After frantically calling parts stores (none had the part), I was running out of options, so here's what I did...

1) I took the half of the sensor that has the 4 pins on it and removed it from the plug
2) I cleaned up the 4 pins (on the side that used to be inside the sensor) and removed the old wires from the posts.
3) I consulted WIS and looked up the temperature coefficient table for the temp sensor
4) I took two 470 ohm resistors, and soldered them to the posts in the criss cross pattern specified in WIS.
5) Insulated with shrink wrap and electrical tape.
6) Plugged my new "sensor" back into the sensor plug.

Now the car starts, but runs rough until it's warmed up. This is not surprising, because I have tricked the computer into thinking that the coolant temp is around 60 celsius.

Once the motor gets warm, the engine will run well enough to get me home tonight. I fully expect soot in my tailpipe.

Just thought I'd post this in case any of you wind up in a similar pickle.

Cheers!

-tinypanzer

LUVMBDiesels · #2 12-02-2008, 08:42 PM

smart thinking

tinypanzer · #3 12-03-2008, 02:47 PM

Well, it worked..... Sort of.... The car ran fine and got me home, but starting it when it was 30 degrees outside was difficult. I had to hold down the accelerator and rev it to 2K for a couple minutes before it would hold an idle and not stall. After that, it ran fine..... Plenty of power, smooth idle.

Still not gonna drive it until I get the new sensor.... Time to call Phil!

BTW- Phil, you were right, the single pole temp sensor was the culprit for my incorrect readings on my temp gauge.... Should have just ordered both of the temp sensors at the same time.

-tp

Gerard · #4 12-09-2008, 08:43 AM

You could get a variable resistor, have it sticking out of the dash, nice brass knob on it, put a few marks on it for various temps, just like a real vintage car!

G

Quote:

Originally Posted by tinypanzer

Well, it worked..... Sort of.... The car ran fine and got me home, but starting it when it was 30 degrees outside was difficult. I had to hold down the accelerator and rev it to 2K for a couple minutes before it would hold an idle and not stall. After that, it ran fine..... Plenty of power, smooth idle.

Still not gonna drive it until I get the new sensor.... Time to call Phil!

BTW- Phil, you were right, the single pole temp sensor was the culprit for my incorrect readings on my temp gauge.... Should have just ordered both of the temp sensors at the same time.

-tp

tinypanzer · #5 12-09-2008, 04:49 PM

That's funny, my colleague suggested the same thing. Y'know, that would be a real easy way messing with rich/lean. Just drop the "sensor reading" down a few degrees, and that should richen the mixture.

Incidentally, I was correct when I stated that I fully expected soot in my tailpipe. I got plenty of soot.

-tp

slk230red · #6 12-10-2008, 09:06 AM

Quote:

Originally Posted by tinypanzer

That's funny, my colleague suggested the same thing. Y'know, that would be a real easy way messing with rich/lean. Just drop the "sensor reading" down a few degrees, and that should richen the mixture.

Incidentally, I was correct when I stated that I fully expected soot in my tailpipe. I got plenty of soot.

-tp

Hi tinypanzer,

Just curious, what year/model/engine do you have?

tinypanzer · #7 12-10-2008, 04:21 PM

It's a '90 300SE (M103 motor).....

-tp

pawoSD · #8 12-10-2008, 04:45 PM

Unplugging the idle valve will allow for good cold starts, and runs ok when warm. Thats what I did...I have a new temp sensor coming friday....hopefully that fixes my problem.