Coolant temp switch, or gauge?
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New User
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Coolant temp switch, or gauge?
Hello everyone. First post here. Excellent board!
I'm going through stuff on my '86 F-150 this week. Had this truck since new. 5.0L FI, 4 speed. Retired contractor. This was the best of three that I had, so I kept it. Now up to 140K, with lots of life left.
So, I have a dead temp gauge, maybe. I assume that if I put 12V on it directly, off the truck, the needle should pin. What happens, is that it slowly moves to the upper end of normal range, and stops. Is there some kind of buffer on the gauge, or is it bad? It is dead on the truck, so the sensor may be fried as well. The wiring is good.
TIA for the help.
I'm going through stuff on my '86 F-150 this week. Had this truck since new. 5.0L FI, 4 speed. Retired contractor. This was the best of three that I had, so I kept it. Now up to 140K, with lots of life left.
So, I have a dead temp gauge, maybe. I assume that if I put 12V on it directly, off the truck, the needle should pin. What happens, is that it slowly moves to the upper end of normal range, and stops. Is there some kind of buffer on the gauge, or is it bad? It is dead on the truck, so the sensor may be fried as well. The wiring is good.
TIA for the help.
Postmaster
Joined: Oct 2003
Posts: 4,516
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From: SoCal
Quick, simple test. Remove the wire from the sending unit & ground it to the engine, have someone turn the key to run, if the gauge pegs at full hot, replace the sending unit.
Theres 2 temp sending units the temp gauge sending unit has a single wire.
Theres 2 temp sending units the temp gauge sending unit has a single wire.
Fleet Owner
Joined: Aug 2005
Posts: 23,667
Likes: 301
From: Easton,Ks
I do not know how you put 12V on it. It is mounted on a PC board and feed is from the Instrument Voltage Regulator (IVR) on one side and the other side has a variable resistor to ground. If you somehow put 12VDC on the IVR side it would read higher and if you put it on the other side it would read zero. Pull the wire ( R/W 39) off the Temp Sender, turn the key on and see what it says. Then short the wire ( R/W 39) to ground and see what the Gage says. If the Gage moves peg to peg in this test the gage is good.
I your Gas and Oil Pressure Gage work OK then your IVR is OK and since your Gage moved I would think it to be OK too. Make sure you coolant is full and if it still reads low replace the Temp. Sender.
I your Gas and Oil Pressure Gage work OK then your IVR is OK and since your Gage moved I would think it to be OK too. Make sure you coolant is full and if it still reads low replace the Temp. Sender.
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I know the difference between the temp sender on the block (single wire), and the one on the t-stat housing (2-wire). The gauge failed the "short to ground test". That was the first thing I tried. Not convinced that the wiring was good, I pulled out the instrument cluster. (Needed to replace a bunch of lamps anyway). Temp gauge or sensors have failed on every Ford truck I've ever had, and that's more trucks than I can remember! Usually the sensor, but frequently the wire shorts out or breaks. I checked continuity of the lead. Good. So, I bench tested the cluster, applying 12 V to the lead on the circuit board. That gave me gauge motion, but didn't peg it. At this point, I'll probably change the temp sensor on principle, but I'd rather not put the cluster back in and find the gauge still doesn't work. Everything else on the dash works fine.
So, shouldn't 12v be enough to peg the gauge?
So, shouldn't 12v be enough to peg the gauge?
FTE Leadership Emeritus
Joined: Jan 2003
Posts: 161,999
Likes: 75
From: 3rd Rock
Originally Posted by Buds_4
Hello everyone. First post here. Excellent board!
I'm going through stuff on my '86 F-150 this week. Had this truck since new. 5.0L FI, 4 speed. Retired contractor. This was the best of three that I had, so I kept it. Now up to 140K, with lots of life left.
So, I have a dead temp gauge, maybe. I assume that if I put 12V on it directly, off the truck, the needle should pin. What happens, is that it slowly moves to the upper end of normal range, and stops. Is there some kind of buffer on the gauge, or is it bad? It is dead on the truck, so the sensor may be fried as well. The wiring is good.
TIA for the help.
I'm going through stuff on my '86 F-150 this week. Had this truck since new. 5.0L FI, 4 speed. Retired contractor. This was the best of three that I had, so I kept it. Now up to 140K, with lots of life left.
So, I have a dead temp gauge, maybe. I assume that if I put 12V on it directly, off the truck, the needle should pin. What happens, is that it slowly moves to the upper end of normal range, and stops. Is there some kind of buffer on the gauge, or is it bad? It is dead on the truck, so the sensor may be fried as well. The wiring is good.
TIA for the help.
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Moderator
Joined: Jan 2001
Posts: 56,980
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From: Virginia
So, shouldn't 12v be enough to peg the gauge?
If you put a voltmeter on the sending unit wire, you should see the meter flicker up and down. This is the chopped up voltage coming from the IVR.
