Cooling Temperature Sensor system problem

Asked by Oct 01, 2014 at 06:59 PM about the 1989 Dodge RAM 150

Question type: Maintenance & Repair

1989 Dodge Ram 100 pickup truck 3.9L V-6 EFI mileage 66,586   I purchased this used vehicle & have driven it 460 miles for short trips up to 20 minutes in duration without any cooling system problems. COOLING SYSTEM CHECKS: No external coolant leaks, all heater & radiator hoses OK, belts & belt tension OK, no change in coolant level in radiator; plan to replace bypass hose (no leaks) for prevention & change the coolant.  ROUGH CHECK OF THERMOSTAT: after 10-15 minute drive I checked the upper radiator hose (URH) , LRH, & bypass hose (BH);  URH-too hot to keep fingers on it,  LRH-hot, but can just keep fingers on it, BH-warm    COOLING TEMPERATURE SENSOR  (CTS) PROBLEM: Dash temperature gauge doesn't move when the engine is hot.  1)checked cold resistance of CTS - 7,390 ohms, supposed to be 10800-17,900 range, replaced CTS, did NOT correct the problem  2) checked the reference voltage at the CTS, 4.97v, close to correct 5.0v   3)Possibility: the pin for the dash gauge arm is dirty & despite current at the dash gauge, gauge arm fails to move. I had a VW Bug with a similar problem; bad CTS, replaced CTS, no change in dash gauge. Supporting evidence for this: Dodge dash oil gauge arm does NOT move despite changing the oil & having the correct oil level. The truck was unused for 2-3 years . Q. Any other possibility for the CTS system malfunction?    I plan a long trip & I want to be certain that the cooling system is OK for extended driving times (hi engine temperature) & I want to be able to monitor the engine temperature.  Apparently my driving time of 15-20 minutes is sufficient to get the engine temperature to intermediate temperature range where the coolant flows through BOTH the URH & BH. At high engine temperature the thermostat is supposed to block the coolant flow through the BH. Q Would you expect  at maximum engine temperature for the BH to be at ambient temperature or at least the temp. under the hood?   Q Is this  just a simple BH or a BH with a  internal restrictor?  THERMOSTAT REPLACEMENT: Presumably, from my rough check it is operating OK. Q  If my test is insufficient then is replacement necessary?  Q Why?  Hayes Repair Manual for Dodge pickup -  opening thermostat temp: 185-195F    Replacement thermostats are rated 180, 192, & 195F   Q Any suggestions on which one? The truck has an electrical heating element in the LRH for winter use & an electric heating element in the engine block.   Q Any suggestions on brand? Motorad, Stant, Gates?      Thank you for your insight.

1 Answer


Don't forget you have a coolant temp sensor for the ECM and a coolant temp sender for the gauge. I would use the 195 thermostat as it appears that you have the cold weather equipment. I use Stant and Gates thermostats, both are good units. If you want to really watch the temp your could put on a manual temp gauge temporarily just to put your me at ease. HTH

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