Interesting Sensor issue
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Senior User
Joined: Apr 2010
Posts: 368
Likes: 3
From: Loganville, Ga
Interesting Sensor issue
Ok, I think it's interesting. Recently I finally fixed my electrical problems. Now the gauges work and the engine runs better, But. The temp gauge works now but not correctly. Now my engine is running fine and get's nice and hot like it should. The heater will run you out of the cab in 5mins. But the needle barely gets to the normal range. So this is what I'm thinking, tell me if you have seen this.
I have a temp gauge. I have seen that there are 2 different engine temp sensors, one for a gauge and one for an idiot light. IF, I installed the sensor for the idiot light by mistake how would my gauge read? What would the gauge do with the wrong sensor?
I have a temp gauge. I have seen that there are 2 different engine temp sensors, one for a gauge and one for an idiot light. IF, I installed the sensor for the idiot light by mistake how would my gauge read? What would the gauge do with the wrong sensor?
Senior User
Joined: Dec 2013
Posts: 143
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From: Lyman, SC
One for a light is usually called a temperature switch. It closes when temperature is reached so if it activated (which it should not if engine isn't overheating) the gauge should do a full swing to the hot side....high resistance = cold; low resistance = hot. If the gauge is moving at all then it's likely not a switch installed. One thing that can affect the reading from a sending unit is if thread tape or other sealant is used as it can prevent proper contact of the gauge body to ground although this can be checked by clipping a test lead or wire to the sensor body and grounding it to see if the gauge moves from previous.
On the EFI 5.0L I fixed up for dad, the gauge barely moved and the heat would work (eventually). Found that the thermostat in it had broken apart so it was running like having no thermostat at all. A temp gun showed about 140 degrees. It was also using gas pretty good since the ECM adds more fuel for a cooler engine. Installed a proper 195 degree thermostat and all is good now, and the gauge only gets to I'd say about 4mm past the cold side of normal range hash mark.
Beautiful truck by the way.
On the EFI 5.0L I fixed up for dad, the gauge barely moved and the heat would work (eventually). Found that the thermostat in it had broken apart so it was running like having no thermostat at all. A temp gun showed about 140 degrees. It was also using gas pretty good since the ECM adds more fuel for a cooler engine. Installed a proper 195 degree thermostat and all is good now, and the gauge only gets to I'd say about 4mm past the cold side of normal range hash mark.
Beautiful truck by the way.
Elder User
Joined: Jul 2012
Posts: 553
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From: Raleigh, NC
I believe the sender/sensor for gauges has two terminals while the one for the warning light has one. Wouldn't be easy to make that mistake. RockAuto.com has pictures of several senders with various connections and for some different applications. .
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Senior User
Joined: Apr 2010
Posts: 368
Likes: 3
From: Loganville, Ga
Yeah that's what the pics on RockAuto look like. I installed this 3 years ago but I have been fighting an electrical problem all the time. So It's never read right. I think it's time to invest in one of those IR temp guns and make sure I'm running at the right temp.
Elder User
Joined: Jul 2012
Posts: 553
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From: Raleigh, NC
Thanks for correcting that.
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Senior User
Joined: Dec 2013
Posts: 143
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From: Lyman, SC
I'm thinking the two wire sending unit would be for EFI, or ECM controlled gauge (not applicable I think). I would assume these trucks when equipped with EFI have two sending units installed: one unit (one wire) for the gauge, and another unit (two wire) for the EFI.
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Thread Starter
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Senior User
Joined: Apr 2010
Posts: 368
Likes: 3
From: Loganville, Ga
Yeah this is the fun part. In 86 the base models, like mine, were carb. The EFI was put in the XLT. EFI was an option for the base. I do have 2 sensors. One is in the block on the passenger side, back. The other is in the thermostat housing. That one is for the computer to control emissions. When I rebuilt this I took the cracked intake and exhaust manifolds off and installed the later model EFI exhaust manifolds and an Offenhauser intake, I have the 300cid. So I don't use the front sensor in the thermostat housing.
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