I need help please. My 1989 GMC Sierra 1500 pickup’s 5.7L, 8 cylinder, extended cab, long bed, temp gauge pins and peg to high full hot, as soon as I turn the key, and start the engine, even when its cold. It has been this way as long as I had it. I bought it two years ago. I want to fix it, because I am worried about it overheating, in these California summers. Last year I replaced the coolant temp sensor hoping that would do the trick, but no luck. I pulled the connector off of the temp sensor, while it was running, and the engine revved and the RPMs increased. When i plugged the connector back into the temp sensor, the RPMs decreased, back to normal. I jammed multi-meter leads into the back of the temp sensor connector, and i was reading 1.2 -1.3 volts, while the engine was hot. Any help would be greatly appreciated. Thanks in advance
There are two different temp sensors for your vehicle.
One for the computer and one for the gauge.
If you unplug the one for the computer, the computer may think the engine is cold. and the RPM’s will increase.
I think you need to find the coolant temp sensor for the gauge.
Its a 5.7 8 cylinder extended cab long bed.
Thank you tester. Do you know where the other sensor is located?
Update, I found the (Temperature Sender) unit. It is between the 1st and 2nd spark plugs on the driver side closest to the front of the vehicle. The sensor on the top of the engine to the left of the hot coolant hose and thermostat is the Coolant Temperature sensor CTS or Engine Coolant Temperature sensor ECT. This sensor is connected to the ECM computer behind the glove box, and not connected to the gauge on the instrument panel. While the truck was running, I unplugged the single connector wire from the Temperature Sender, and the gauge dropped down to cold. I plugged it back in and the gauge spiked to full hot, again. I am going to O’Reilly’s today, and picking up a new Temperature Sender. This is the part: BWD 1 Terminal Engine Oil Temperature Sender Part #WT733 https://www.oreillyauto.com/detail/b/bwd-3123/cooling---heating-16773/temperature-sender---universal-19565/d0b61e40b4d3/bwd-1-terminal-engine-oil-temperature-sender/wt733/2618127/1989/gmc/c1500-pickup-rwd?pos=3
Okay new update again. I bought and installed the new temperature sender, still same problem. I thought the gauge might be bad. I have the old Moonie gauge cluster. I forgot to mention, the fuel gauge, oil gauge, odometer, and gear indicator did not work either, so I decided to head down to the local Pick N Pull, and pickup a new (to me) Moonie gauge cluster. I found one, I swapped out all of the broken gauges, and they worked. All of the broken gauges work now, except for the temp gauge. It is still having the same problem. Arrg. Well at least my other gauges work now. I am happy about that. Okay next I am going to see if there is a short somewhere in the temp sender wire. If you have any ideas, that would be great. Thank you
Remove the sender electrical connector. The gauge should move to “100”. Does it? If not, check sender for short. If no short found at the sender, and replacing the sender doesn’t fix it, and it reads “260” , given you’ve likely got a working gauge, most likely you’ve got a short in the wiring between the gauge and the sender.
Could the sender wire be shorted against the exhaust manifold.
Thank you very much for the advice. When I unplug the wire on the sender, the gauge drops to 100F, like you said. When I plug it back in, it shoots back up to 260F, and the check gauges light comes on. I used a laser thermometer to check the temperature at the sender, and it read 220F, but right below that, i got a reading from the exhaust manifold, and it read 300F. Is that normal? I looked for a short, but I could not find one under the hood. I might need to tear the dash apart. I think that is going to be my next step. I appreciate all of the help.
If unplugging the sender wire immediately drops the gauge reading to 110* and reconnecting it throws the gauge to 260* with the engine cold it’s somewhat certain that the sender is bad. If you have an ohm gauge ohm the sender…
I just replaced the sender 3 weeks ago. Do you think the sender I bought was bad?
Given you have new sender and gauge, my guess is you’ve got a short to chassis ground in the harness wires that go from the sender to the gauge. How many ohms do you measure between the two harness wires that go back to he gauge, with the sender disconnected? Or if there’s just one harness wire for the sender , between that and a chassis ground? Same measurement, with the harness connected to the sender? What if you just measure the sender, without it connected?
It doesn’t matter how new the sender is. If it tests bad replace it.
!SOLVED! ------- There were two different problems. The old temperature sender I replaced was actually good, and the new sender was bad. I checked the old sender with a multi-meter while it was cold and i was reading 1800 ohms. with the new sender I was reading 300 ohms cold. I thought the sender was the issue, so I replaced it, thinking that would be a cheap easy fix. But when I replaced it, I replaced a good sender with a bad one.
It turns out the original problem was a resistor on the temperature gauge was bad. I tested it with a multi-meter , and i was reading infinite resistance. The new temperature gauge I picked up from Pick N Pull had a good resistor at 90 ohms.
When I installed the good temp gauge I was still getting a bad reading, because I just installed a bad sender.
Finally I re-installed the old sender, and bam temperature gauge reading normal.
new temperature sender: bad
old temperature gauge: bad
old temperature sender: good
new temperature gauge good