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Old 04-11-2000, 03:13 AM
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Join Date: Feb 2000
Location: Santa Clarita, CA, USA
Posts: 133

I have a 1984 300SD. I was wondering what the 2 temperature sensors (thermistors)on the cylinder head are for. They are on the left side (injection pump side). I know the one towards the rear is for the temp guage on the dash. I have some questions regarding them and the sensor on the thermostat housing.

1) I want to know what the sensor towards the front of the cylinder head is for. It's not obvious to me what it operates. I thought it might be associated the glow plug timer, but the pre-glow light on the dash comes on for a consistent 3 sec. no matter what engine temp or with the plug on or off. The car starts easily by the way with only those 3 sec. What does this sensor control?

2)Does the auxiliary fan have a high coolant temp switch or is it solely operated by the a/c rec./drier switch?

3) The sensor on the thermostat housing is for the a/c cutout I assume. I observed with it disconnected, the climate control will not run. Does this sensor control anything else?

Any input would be much appreciated. I plan on modifying the cooling system wiring so that the aux. fan comes on with high coolant temp. I hope to put in a thermal switch on the t-stat housing. Thanks in advance


1984 300SD 268,000 miles
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Old 04-11-2000, 09:48 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
In my experience, it is very strange for these cars to EVER run warm. I do not see them run over 100 even in the summer with the A/C on... I'd make sure the cooling system is in good shape and run it.

Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
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Old 04-12-2000, 11:20 AM
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Join Date: Jul 1999
Location: Tampa, Florida, USA
Posts: 214
I didn't notice any mention of a thermostat replacement in this post, TXBill. I assume you replaced that as well? That's probably the first thing to check if you're running hot.

I had some engine cooling issues - running betwen 100 (C) and 105 on the interstate, sometimes approaching 110 - so I replaced the thermostat and now it hovers between about 88 and 92. This is in the Miami area, so it was pretty hot even in January and February.

- Nathan
'83 240D, 250k miles
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Old 04-12-2000, 01:35 PM
Registered User
Join Date: Feb 2000
Location: Santa Clarita, CA, USA
Posts: 133
In the summer time here in Santa Clarita it can get up to 110F on a regular basis. My cooling system hovers around 87-92C normal driving and can get up to 100C in traffic with the A/C on. Those temps don't bother me too much, I'm just curious about the wiring of all the sensors. I want to know what drives the aux fan. If it indeed has just the a/c recv/drier pressure switch or if it also has the 100C switch for engine coolant temp. It's a 1984 300SD. I've gone through the archives and can't find any specifics about that model. I've read other newer models have 2 stage or 3 stage operating modes for the aux. fan. I did come across a post that suggests that the early 80s models don't have a coolant temp switch. I just want to confirm this. After all, if the aux fan should come on with high coolant temp also, I want to fix that. Thanks for all the input!

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