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Old 06-06-2012, 01:49 PM
retmil46's Avatar
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Join Date: Jun 2006
Location: Mooresville, NC
Posts: 344
Coolant Temp Sensor B 13

For a 1987 300D, build date 7/86, this is the coolant temp sensor that is located in the side of the cylinder head facing the IP, between glow plugs 2 and 3. Used for coolant temp input to the climate control unit to limit blower speed at low engine temps, and also feeds the dash coolant temp gauge.

I found a table of resistance values at different temps for it, in the climate control section of the W124 service library CD, for doing testing of all the electrical components of the climate control system.

But here's the kicker - the values listed in this table are measured at the connector that plugs in to the back of the CCU - IOW, downstream of where the dash temp gauge piggybacks off the same sensor - and for approx 20 C, the value is listed as something like 120 to 150 ohms.

At the engine, pulling off the sensor wire and measuring the resistance of the sensor itself, ambient temp somewhere between 20 and 25 C, I measured 0.650 kohms, or 650 ohms.

Could the dash temp gauge circuit account for the difference in resistance readings, between measuring them at the CCU connector, and measuring the resistance of just the sensor itself?

IOW, is this a normal resistance reading for just the sensor by itself, or is it fubar and in need of replacement (appears to be the original sensor)?

Haven't seen any inidication of malfunction with the dash coolant temp gauge, but the CCU does look to operate differently than my first 87 - even when at full coolant temp, in auto and set for heat, at idle it will keep the blower speed on very low, or sometimes take a couple minutes to even start running - then when starting down the road and getting up to around 30 mph, the blower will drastically increase speed and output.
Just say "NO" to Ethanol - Drive Diesel

Mitchell Oates
Mooresville, NC
'87 300D 212K miles
'87 300D 151K miles - R.I.P. 12/08
'05 Jeep Liberty CRD 67K miles
Grumpy Old Diesel Owners Club
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