Engine temp gauge not working ... new ECT/thermostat already

[98t5M]

98 s70 t5

 

Soo early last week my engine temp gauge was fine, got in the car Thursday and noticed the needle was not moving past the bottom line on the gauge. It does however fall below the bottom line when the key is off and immediately jumps to the bottom line when I turn the key. I checked the coolant temp with my scanner and it was reading around 196F once the car had warmed up, which is normal. Today I threw a coolant temp sensor at it and still have the same problem. Any ideas?

Thanks

[gdog]

1 hour ago, 98t5M said:

98 s70 t5

 

Soo early last week my engine temp gauge was fine, got in the car Thursday and noticed the needle was not moving past the bottom line on the gauge. It does however fall below the bottom line when the key is off and immediately jumps to the bottom line when I turn the key. I checked the coolant temp with my scanner and it was reading around 196F once the car had warmed up, which is normal. Today I threw a coolant temp sensor at it and still have the same problem. Any ideas?

Thanks

Sounds like an issue with either the ECT connector and/or the wiring to it.  

Disconnect the ECT sensor and ohm the brown wire (on harness side) to ground; should be zero ohms.  Open or significantly higher than 0 ohms is an issue.

If that's fine you can check to see if you have 5v with key in run on the other wire.

Could also try shorting the ground and sensor wires on the harness side with a jumper wire.  Then when key in run, gauge should go all the way up, but may take a few seconds.  If it doesn't, you got an issue with either the ground or sense wire.  If it works as expected, then maybe you have poor connection at the connector.

Here's a link that helps explain:

https://www.freeasestudyguides.com/diagram-ect-a8.html

[98t5M]

15 hours ago, gdog said:

Sounds like an issue with either the ECT connector and/or the wiring to it.  

Disconnect the ECT sensor and ohm the brown wire (on harness side) to ground; should be zero ohms.  Open or significantly higher than 0 ohms is an issue.

If that's fine you can check to see if you have 5v with key in run on the other wire.

Could also try shorting the ground and sensor wires on the harness side with a jumper wire.  Then when key in run, gauge should go all the way up, but may take a few seconds.  If it doesn't, you got an issue with either the ground or sense wire.  If it works as expected, then maybe you have poor connection at the connector.

Here's a link that helps explain:

https://www.freeasestudyguides.com/diagram-ect-a8.html

Thanks for the reply. I shorted the ECT sensor connector and it immediately reads 244f on my scanner. Uplugged the sensor and it drops to -40f. With the sensor connected I get accurate temp readings (on the OBD scanner) so I know the sensor and the wires to it are good. The temp needle still doesn't move from the bottom line on the gauge with the sensor harness open or shorted. Everything else in the cluster is working a-ok so either the gauge itself just went bad, or maybe a wire to the cluster is corroded/damaged?

 

Guess it's time to dig out the wiring diagrams :(

 

[Randy850]

could be failing thermostat that is what caused my issue

[gdog]

I don't see how a thermostat could cause those symptoms.

Sounds like the sensor and wiring to the ECU is working properly which means either the wiring between the ECU and instrument cluster or an issue inside the cluster itself. 

[erikv11]

I think I would swap in a junkyard cluster at this point.  That's less work than chasing bad wires, and if the cluster swap doesn't fix it then you have nailed it to somewhere between the ECU and the cluster (eh).