[DeTomaso] Water temp gauge question

Fred Foreman fred4man at att.net
Fri Mar 20 14:20:11 EDT 2020


Steven,

"Divide and Conquer" - I just went thru this process.  Here is the Cliff Notes version.  BTW - If you don't know how to use a VOM, get someone to help you or you will be spinning your wheels!!!!

1) Check operation of only the gauge first.  Disconnect the wire at the sending unit and take a volume control from an old radio (also called a Pot - potentiometer) and put it inline from the disconnected wire and a line attached to chassis ground.   I bought one with 100Ω to 200Ω which is perfect for this job.  Most from a radio will have much higher resistances and therefore harder to adjust.   
Actually, before putting it inline adjust the pot to give you, let's say 100Ω.  This should be enough to drive the meter to the first mark on the gauge when the resistance is placed inline between the sending unit wire and a ground.  As you rotate the pot to lower values of resistance, the current will rise and the meter's needle will rise.  Adjust the pot to put the gauge at 190F, and depending on the gauge (they all vary to some degree - yes a, pun), the resistance measured across the pot would be around 27 to 35Ω.  Adjust the pot to put the gauge at 260F, and depending on the gauge, the resistance measured across the pot would be around 14 to 19Ω.  These are ball park figures and will let you know if your gauge is working anywhere near properly.

2) Check operation of the sending unit.  At room temp the sending unit should probably show about 100 to 400Ω.  If it shows open circuit, ie, infinite resistance, then it's not looking good for the sensor.  Take a hair dryer or heat gun and heat the sensor to 190F and the resistance across the sensor (one lead on the body and the other lead on the wire attachment stud) should be around the same  27 to 35Ω.  If it is way off, then the sensor is not for the gauge.  If the sensor shows that it is still open circuit or, you got it hot enough to show high engine temp,  a short circuit, then it is an idiot light switch not a sending unit.

Fred Foreman


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