jhaase

My 89 RS, with 305 TBI has a intermittent water temp gauge, most times it wont work accurately. I have replaced the block sender & still have the same problem. If you short the block sender wire to ground should the gauge read hot?? Are these gauges famous for problems?
Thank You,
John H

Injuneer

I believe the gauge snesor is in the driver's side head.

Short to ground = 0 resistance. Low resistance causes the gauge to indicate high temperature. Should peg at the max reading.

Wire off the sensor, not touching a ground = infinite resistance. High resistance causes the gauge to indicate low temperature. Should put the needle at the bottom of the temperature range.

Check the wire from the gauge to the sensor. With the connector off the sensor, should read 12V when measured from the single wire to ground. Jiggle the wire while making the measurement, to see if there is an intermittent open circuit.

jhaase

I ran all the tests, & some others from GM. All seems to point to the temp sender in the head. I installed a new sensor a while ago & still had the same temp gauge readings. What should the ohms be on a good temp sensor?
Thank You,
John H

Injuneer

This is a standard GM temp sensor ohms vs *F/*C, at least used in the 4th Gens. I've seen slightly different numbers on supposed 3rd Gen charts. But this will get you close enough - read the entire procedure:

4th Gen LT1 F-body Tech Articles

Injuneer

Just noticed.... this belongs in 3rd Gen, not "Classic". Moving......

jhaase

I ohmed out the head sensor, it read 2840 @ 78 deg, by the chart that is normal.
Any other ideas??

Injuneer

Wire off the sensor, key on, do you have 12V on the wire to the sensor, measured to the block (ground). That was covered in the procedure I linked.

jhaase

Yes, I have 11.9 v

Injuneer

Did you jiggle the harness wire while measuring the voltage? Something is intermittent. It isn't likely the sensor. Wire is a more likely problem. If it's not the wire, then there may be a loose connection in the dash cluster at the coolant temp gauge.

jhaase

I will start tracing the wiring back to the dash.
Thank You,

tritonium

Hello

I'm in the same boat - I've had a bad dash circuit board in the past - perhaps that could be the issue?

My next test is going to be to apply resistance to the circuit board contacts and see if I can get the dial to move. (of course ill try this on a spare gauge cluster first)

Injuneer

The gauge won't move unless is has a 12V power supply. The sensor resistance sits between the positive and ground leads from the gauge, and the variable resistance results in a variable voltage, and it's the voltage that causes the deflection.

tritonium

Hello,

Thanks so much for the information ... Great news in that my gauge does move with voltage 😀 Now I need to figure out why it does not move. I've run a continuity test between the sensor pigtail and the gauge cluster and find many points of continuity . I thought it would be connected to dk green pin three but it seems more items are at play than a single run . I've determined the line isn't broken and I've tried two sensors .. I did look under the dash and found two interesting things that may or may not be relevant .. There is a small plug under the dash by my feet marked ,PED with a grey and striped black wire not connected to anything . I'm assuming this means pedal.. This is a manual car and everything seems to be working fine with the pedals . I also noticed one open plug connector on the brake pedal but this plug does not fit here .. Iwaswondering if maybe the temp gauge used this as a groud. This car is a auto to manual and v6 to v8 convert . Everything works except the water temp gauge . Any other ideas please let us know . We also checked the circuit board in the gauge cluster and all seems okay. I'm hoping we are missing something small.

Very excited as I'm having the interior re done soon as soon This gauge is happy again.

Thanks again guys !

tritonium

Update :

We are able to make the gauge go up and down by applying voltage from the engine bay so we know all wires are good . The problem is that when we do our test with a new water temp sensor for the head , with a ground wrapped around the brass we get no reading on the gauge .. Just an initial needle jerk toward zero upon ignition on.

We are using a very hot cup of water to simulate hot water ... Also we wanted to make sure the car doesn't need to be running for water temp to work ? We have just been using key on to monitor the voltage

tritonium

Okay looks like my issue is a bad connector - I ordered one - will update thread if this works.