Truck Repair: Temp gauge pegs high, coolant temperature sensor, coolant temp

Question
93 Chevy Silverado,ext cab, 350 cid, 2wd with ac.

Temp gauge pegs to the right and "Check Engine" light stays lit.

I changed the coolant temp sending unit (top of engine, next to t-stat) with two new units.  Same results each time.

There is a similar unit on drivers side of engine by exhaust manifold. Is it cooling related?  Am I missing something else?

Thanks.


Answer
Hello William,
There should be a coolant temperature SENSOR for the computer, and if you have a gauge, there should be a coolant temperature SENDER.
The sender for the gauge will probably only have one wire, and although my book is older than your truck, the wire should be dark green.
The sensor wires for the computer temp sensor are probably a yellow and a black wire.
Go to the green wire, remove it from the sender, and measure the resistance of the sender with an ohm meter.
If the coolant is cold, less than 100 degrees, the resistance should be 1,365 ohms.
When the coolant is hot,the resistance should drop to about 55 ohms.
Thus, if you unhook the wire, the gauge should drop. If you ground the wire, the gauge should peg hot.
Without a later book, I can't swear that the check engine light is associated with the gauge sender, or temp sensor, but if you think the gauge is incorrect, do the green wire check.
And yes, I believe it is by the left ex manifold.
Good luck,
Van