Truck Repair: 1991 Chevy Silverado, coolant temp, chevy silverado

Question
Hi,
I have a 1991 Chevy Silverado 1500,4x4,extended cab w/5.7 Litre engine(350) and automatic transmision.

My problem is in the temperature gauge or sending unit not sure which.
When I start engine or turn key to on posistion the tempurature gage goes almost all the way to the right and in a split second it does go all the way to the right(Extreme Hot)but the truck runs normal.
I have repaired ELECTRONICS FOR 35 YEARS AND DECIDED TO CHECK FOR VOLTAGE AT THE PLUG ON THE TEMPERATURE SENDING UNIT,I READ 4.69 vOLTS W/ KEY ON,IS THIS ABOUT NORMAL FOR THIS TRUCK?

ALSO CAN YOU TELL ME WHAT VALUE IN OHMS I SHOULD READ AT THE CONTACTS OF THE TEMPERATURE SENDING UNIT WITH THE ENGINE COLD?
One last question please,is the anyway I can use r34 freon in my r12 system without great expense?If so please advise how.
Thank you
Bob J. Lathim


Answer
Hello Bob,
On the temp voltage, you were probably at the temp sensor for the ECM, rather than the sender for the guage.
The sensor recieves a refference voltage from the ECM which is typically around 5 volts, and then returns a value, depending on the coolant temp. Sounds like it is doing fine in that department, although be sure to use a good quality didgital ohm meter when measuring those sensors, as an analog volt meter could damage the ECM.
So find the other sender in the coolant jacket to check the guage.
If you would find a Motors manual for that year, it will have a test proceedure for the guage.

As for the 134, there is a retrofit kit, available at most parts houses, and at Wal Mart, just for that. Requires draining the R12, draining the oil from the compressor, and refilling both with the 134A compatible stuff. Kit costs about $30.00, and includes new fittings, oil, and 134A, and the hose to fill it.
Good luck,
Van