Overheating? Bad temp sensor reading?

[bigchip06]

I just finished replacing the entire cooling system on my ‘77 K20 (radiator, hoses, thermostat, water pump, temp sensor) and after putting 2 and 3/4 gallons of coolant back in the system it consistently reads 1/2 on my temp gauge when idling. Before replacing everything it would sit at 1/4 and creep up when idling.

Is there still air in the system that I need to get out? My rad hoses are decently firm when it’s running but I can hear sloshing like there’s air bubbles when I squeeze them.

Maybe the new temp sensor is reading wrong? My reason for believing this would be it doesn’t go past 1/2 way on the gauge.

What do you guys think? Does anyone know how much coolant this system is supposed to hold?

 

[Goldie Driver]

Let it cool and check the level in the radiator, and is there coolant in the overflow .

If so, and both are where they should be, then perhaps the new temp sensor is wonky.

I don't know offhand the total capacity but 2 - 3 gallons seems about right when I changed my radiator.

If you have a fan clutch , is it working?

When cold and 1st started the fan makes a good roar with the blade spinning.

Best of luck!

Britt

 

[Snoots]

Idle it up to temp with the radiator cap off.

 

[75gmck25]

If you take the lead off the temp sensor and ground it you should see the gauge max out. If that works, the basic wiring is okay, and apparently the sensor works if it goes to half scale when connected. However, the resistance profile of the sensors changed over the years and it can be difficult to find the right sensor to match the gauge. The parts books are not always correct.

My truck idles with the gauge at about 3/8 scale, and even when coming back to idle after a highway run it never goes more than about a needle-width above 1/2 scale. If the cooling system is working well you won’t see much variation in temps.

 

[StickyLifter]

I had to replace the temp sensor in my truck earlier this summer and had some issues because I have a replacement engine that took a different sized sensor. The one the parts store came up with had the wrong resonance range. I found this info in a post and it helped me find the correct one. Hope it helps you. Sorry I couldn't get the "Quote" feature to work with this. It had been shared by AuroraGirl.

C10MixMaster said:
COOLANT TEMPERATURE GAUGE PARAMETERS

For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω

For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω

as you can see there are three different ohm ranges depending on the year. there are also 1/2" and 3/8" pipe threads so you need to match the size.

 

[Buck69]

I had to replace the temp sensor in my truck earlier this summer and had some issues because I have a replacement engine that took a different sized sensor. The one the parts store came up with had the wrong resonance range. I found this info in a post and it helped me find the correct one. Hope it helps you. Sorry I couldn't get the "Quote" feature to work with this. It had been shared by AuroraGirl.

C10MixMaster said:
COOLANT TEMPERATURE GAUGE PARAMETERS

For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω

For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω

as you can see there are three different ohm ranges depending on the year. there are also 1/2" and 3/8" pipe threads so you need to match the size.

Exactly what I was looking for. Thanks!
Replaced the cluster in my truck with one from an unknown year. It was a bare cab at a salvage yard. The gauge will not noticeably climb with the '86 sender (@150 F) but will start to peg if momentarily grounded. The '74-'78 would make sense.
Time to pick through my pile of resisters.

 

[fast 99]

A cheap heat gun is a very useful tool. Last I checked about $20.

 

[Buck69]

Just a follow up. The cluster I have is of the correct vintage along with the AC Delco sensor purchased.

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Gauge and circuit confirmed to be functioning correctly with a few of the resistors (67,80 and 99 ohm) I had in my collection. Sensor also found to stay open while heating in a container of water.
Thanks again @StickyLifter for sharing above info.