1998 Jimmy coolant temp sensor problem

[Chris Fields]

My check engine light came on last week. I had it checked and the code was a coolant temp sensor. My temp gauge drops off while its running , it bounces down. It never goes up it only goes down in temp. While its bouncing the truck surges really bad. I replace the sensor at the intake and it still does it and shows the same code. Could it be a wiring problem?
For the time being I disconnected the wire to t he sensor. Problem with that is I loose my gauge all togher. I was told it was messing with the computer when the sensor is bad. Any suggestion. Oh yea. Took it to a dealer says nothing is wrong with it. lol
Thanks for any advise!

[old_master]

Is the cooling system full? Do you get sufficient heat from the heater? (possibly a faulty thermostat if not.) When they checked the codes, did they happen to check coolant temperature on the scan tool? If not, have them check it. The fact that the engine surges with the guage fluctuations, may indicate a poor connection, possibly for the PCM temperature sensor. Check both the sensor wire connector and the ground connection, make sure they are clean and tight.

[scholzee]

The dash gauge sensor is on the drivers side of the block down near the spark plug area. It has a single wire going to it and the wire has a connector that unhooks up near the top of the intake manifold. Coolant will come out of the block when you unscrew it. As Old Master said the dash gauge is just for the drivers view and the sensor near the thermostat is for the computer, they should not interact. You may have a ground or someother wiring problem going on.

[Chris Fields]

Thanks for the information!!! I see now I have a real problem. I have no wire running to the engine block on the left side at all. I have a connector at the intake manafold that has a two wire plug. Acording to what you were saying my gauge should not work at all with no wire running to the sensor on the block, right? Well, lol, it does work. More information for ya, the jimmy is a 98 with a 99 motor in it. Could they have messed up the wiring harness when they installed it? It appears that the gauge and the computer is working off the same sensor that is in the intake. Im not sure how this is possible. Got any ideas?

Thanks,
chris

[old_master]

On the 1998 4.3L The ECT (engine coolant temperature) sensor is located in the left cylinder head between #3 & #5 spark plugs, and uses a black and a yellow wire. The PCM uses class 2 communication, (serial data communications) to send engine coolant temperature to the dash guage. There is only one coolant sensor used on this engine.

[muddog321]

My 98,99, and 01 all only have 1 sensor and its on the drivers side head between plugs 3 and 5 so confused me here on this one. Must be earlier model years - 95 earlier??

[blazee]

Originally Posted by muddog321

My 98,99, and 01 all only have 1 sensor and its on the drivers side head between plugs 3 and 5 so confused me here on this one. Must be earlier model years - 95 earlier??

That's correct. 1998 was the year that things changed. 1998+ have a single ECT located between plugs 3 and 5. The sensor is for the gauge and the PCM. 1997 and before had 2 sensors, one near the plugs for the gauge, and one near the thermostat for the computer.

[scholzee]

My Bad I was refering to my 1996 with 2 sensors, I did not know things changed in 1998.

[old_master]

Thanks Blazee, good catch, I stand corrected! Both of my previous posts have been edited to indicate the correct information, straight from the GM shop manual.