john2000

My 2000 Express 3500 temp gauge reads low and shows about 160F when the actual coolant temp is 188F. Is there a way to adjust the temp gauge calibration from behind the gauge? Is there a mini potentiometer on the back of the gauge? The coolant temp is confirmed from OBDII data and matches exactly with the recently replaced 195F thermostat. In real time I can watch the thermostat cycle once the temp hits 195F. I replaced the temp gauge sender and that raised the indicated gauge reading from 110F to 160F. I have checked the resistance of the sender and it seems to match online charts I've found. About 5K ohms at 48F, 255 ohms At 189F. I've thought about adding a resistor in parallel (not series) with the temp gauge. I have confirmed that the temp gauge is able to deflect full scale. Would be nice to have gauge show actual temp. Another data point is that the resistance through the gauge itself is 47 ohm which seems high but don't know the spec on this.

mountainmanjoe

where exactly did you measure this?

Did you open up the instrument cluster to look for damage, corrosion etc?

john2000

Measured from the connector on top of the engine to a ground near the instrument panel. Not sue if there's a published spec on what the resistance through the gauge itself should be measured from the connector on the back of the instrument cluster.

mountainmanjoe

I would guess the gauge itself shouldn't have more than an ohm.

You need to measure at the IPC connector, or possibly even inside the cluster.

Little Bear

I would think about the only cure would be to remove the pointer from the gauge and put it back on so that it better indicates the true temp.

john2000

After watching some YouTube videos I pulled out instrument cluster last night. I agree with Little Bear's idea to reposition the temp pointer. I'm looking for it to point correctly at 195F (it's a 195F thermostat) so I can quickly detect a situation when temps start to go higher. I got ambitious and pulled out jittery oil pressure and fuel stepper motors and have some refurbished replacements on the way. Btw I measured 87 ohms across the resistor pack on the back of the temp stepper motor and 167 ohms between terminal 1 and 3. Not sure if this is in spec. Also found that three 100 ohm J resistors on the board are actually measuring around 34 ohms.

mountainmanjoe

3x 100 ohm resistors in parallel should measure 33 ohms.

john2000

Installed 2 refurbished stepper motors (oil and fuel) and they are working great nice smooth movement no jitters. Re-positioned the temp needle to indicate correctly at operating temp and now happy with that. A plastic fork works well to lift off the needles. @mountainmanjoe is correct, those 3 resistors are in fact in parallel so all is good there.