Temperature gauge sending unit issue

[andyblack]

Hello everyone. I used to be on this forum a few years ago but then they blocked this site on my computer at work, I couldn't add it to Tapatalk on my phone... yack yack blah blah.

Anyway I'm back to annoy you all with a question. I have an '85 truck. I took out the original 305 a few years back and put in a 350 from a '72 El Camino. When I did the swap, I used the sensor or sending unit or whatever from the '72 350 because the one from the '85 was about twice as big in diameter and wouldn't fit in the block. Anyway, now the temp gauge only registers the coolant temp when the ignition is switched off. That's right. The gauge reads 0 degrees until I stop the truck and turn the key to off; then the needle will go up to 195 or whatever. This has caused me to overheat once and I don't want it to happen again. So my question is, is there an adapter or something that will make the '85 gauge work with the '72 sending unit? The gauge worked perfectly before the swap. Or do I need a gauge that will work with the '72 sending unit? The electrical plug was the same btw.

 

[bp71k5]

Hello everyone. I used to be on this forum a few years ago but then they blocked this site on my computer at work, I couldn't add it to Tapatalk on my phone... yack yack blah blah.

Anyway I'm back to annoy you all with a question. I have an '85 truck. I took out the original 305 a few years back and put in a 350 from a '72 El Camino. When I did the swap, I used the sensor or sending unit or whatever from the '72 350 because the one from the '85 was about twice as big in diameter and wouldn't fit in the block. Anyway, now the temp gauge only registers the coolant temp when the ignition is switched off. That's right. The gauge reads 0 degrees until I stop the truck and turn the key to off; then the needle will go up to 195 or whatever. This has caused me to overheat once and I don't want it to happen again. So my question is, is there an adapter or something that will make the '85 gauge work with the '72 sending unit? The gauge worked perfectly before the swap. Or do I need a gauge that will work with the '72 sending unit? The electrical plug was the same btw.

Probably not. The 1972 is probably just a dummy light. It's probably not for a gauge.

 

[Fordum]

Does sound a little strange. But might be an easy fix. Got an ohmmeter? You need to take the wire off the sending unit with the engine cold. Then measure the resistance from the connector on the sending unit to the block.
Ok, hold that thought. First, does it have one round "top hat" looking connector or two spade connectors.
If it has the single connector, then check the ohms. If it has the two spade connectors, don't bother.
The single connector is a variable resistor for a gauge. The two connector one is a switch for a idiot light. Just going by the pictures on RockAuto, the gauge units look bigger than the switch, so that is probably why your old unit would not fit.

That motor used either a gauge or a light depending on the trim version of the vehicle it was in. So there has to be a way to use either unit. Folks here who are much more knowledgeable about GM engines than I am should know, but I'm pretty sure there is a plug real close to that unit that is the hole for the other type. Probably a hex head socket plug.
Grab a hex head adapter for a socket wrench and see if you can loosen it. May take penetrating oil and/or heat. Once you loosen it, stop and go get a sending unit for a gauge. Don't buy the sender until you are sure you have a place to put it, and don't take the plug out until you have something to put in there so you don't lose too much coolant.

 

[andyblack]

Wow thanks guys. Looks like I must have the sending unit for the idiot light only model. I never would have even though if that. I'll go get the proper one for my gauge and I should be good to go.

 

[dyeager535]

Wow thanks guys. Looks like I must have the sending unit for the idiot light only model. I never would have even though if that. I'll go get the proper one for my gauge and I should be good to go.

Obviously sounds like you found the likely culprit, but you can always test gauge function with the key in "run". Disconnect the wire, watch gauge. Should peg out one direction. Ground the same wire, gauge should peg out other direction. If it doesn't, it's not just the sending unit that has issues.

 

[diesel4me]

The temp sender for the gauge on older engines was located on the drivers side cylinder head,near the #1 cylinder exhaust port..

Sounds like you have a idiot light type sender hooked to a gauge,as suggested above...your gauge "seems" to read with the key off,but in reality its not showing the actual temperature,all GM gauges act that way --with the key off the gauge may retain the last reading it showed,go higher or lower,any where in between..

You need to get a sending unit for the vehicle application "with gauge"...

 

[andyblack]

Yeah you're right. That's exactly where the sending unit is. I never knew the difference before. Thanks again fellas.