Help Replacing Water Temp Sensor 300E - PeachParts Mercedes-Benz Forum

sptt · #1 10-17-2009, 12:06 AM

Help once again! I ordered a new Water Temp Sensor for my '91 300E in hopes that it might help solve my "warm idling" issue but I can't figure out what tool to use to get that little sucker outta there. It takes a 7/8 socket, but it won't fit over the head and there's no room towards the back side to slide the socket over if it could go on. Tried to get a wrench in there but also no room. Is there a special tool for this that I need to get?

pawoSD · #2 10-17-2009, 12:41 AM

Its not 7/8. Its a 3/8 drive 22mm deep socket. Available at your local autozone, Napa, or sears.

sptt · #3 10-17-2009, 12:55 AM

I thought I tried the 3/8th but it didn't fit. The 7/8th did fit. Maybe I'm remembering incorrectly. I'll take the replacement part with me to be sure. Thanks so much for quick response! I take it the socket will fit down over it with enough clearance in the back.

Ivanerrol · #4 10-17-2009, 05:56 AM

The 22mm fits fine when the sensor is not on the car - otherwise it's hard to fit. The valve cover gets in the way.

I ended up taking the valve cover off to to this job - with a 22mm ring spanner. The valve cover had been on for a looooong time - it was hard to prise off.

Don't over tighten the sensor. Otherwise this might happen.
(Broke the air tube when taking the valve cover off)

865sp300e · #5 10-17-2009, 06:57 AM

I removed the valve cover and used a 22mm open end wrench. The task was rather simple, take care not to over torque the valve cover bolts during reinstallation.

sptt · #6 10-18-2009, 01:08 AM

Wow. Thanks guys! Is it hard to remove the valve cover? Never done that before. Also, after I took off my sensor wire and reconnected it (wasn't exactly sure how it came off with the 1,2,3,4 numbers positioning), the car is not idling at 700 RPM when cool and 500 RPM when warm. It's just idling at 500 RPM no matter what. Did I stick the wire back on in the wrong direction? Does it matter? Would this possibly indicate another problem?

pawoSD · #7 10-18-2009, 01:59 AM

500rpm no matter what means it is running on mechanical injection alone and is not using the computer to control idle speed. This indicates a sensor issue most likely. Mine was doing that too before I replaced the temp sensors.

Odd that the socket won't fit....I was able to put it right in place no problem with the 22mm socket I bought....no valve cover removal needed.

Cal Learner · #8 10-18-2009, 06:19 AM

Quote:

Originally Posted by sptt

... Did I stick the wire back on in the wrong direction? Does it matter? Would this possibly indicate another problem?

You could always try plugging it in the opposite way and see if your original condition returns. But it shouldn't matter which way it's connected, both sensors are identical, they just supply signals to different control units. If it does make a difference, that might indicate that one of the two sensors is kaput. Did you test the sensors (1-3 and 2-4) for resistance values at various engine temps to see if they're both giving identical readings? The test procedure and resistance specs are listed in the FSM.

sptt · #9 10-18-2009, 06:48 PM

OK guys. Got the new sensor in with the help of the 22mm socket and a hammer to tap the engine lift handle out of the way. Bad news...same result. I also turned the wire in different directions and you guys were correct, no difference. Engine gets to 80c and drops to 500 RPM idle

I don't think I have much else left to replace in this thing. When I disconnect the wire from the sensor, idle surges and then returns to normal as if it is on. I attach it back and no change. So, there is signal getting through to wherever it goes, right? Thoughts?