Temp Gauge issue

[custom10nut]

Just recently, my temp gauge has been registering near the “Red Zone“, under normal operating conditions, and will “Peg out” if I sit in traffic.
I’ve checked the water temps with a thermometer and it’s not over heating.
The sending unit is new, and the thermostat is working fine.
The gauge is it he original, that the truck rolled off the line with in 1970.
Is it possible for them to “Go Bad” and start reading “Hot”?
Would it fix the problem if I clean the contacts from the the Printed Circuit tothe gauge?
I just wanted to ask, before I go and buy a new gauge.

[gmc684x4]

Question 1 did the old sending unit do the same ?

Q2 do you have a clutch fan ?

Q3 fan shroud?

I had a 70 sb 4x4 4 blade fan with no fan shroud it would over heat in traffic when having lots of parked time i added fan shroud and 6 blade fan made a big difference

[custom10nut]

Question 1: No. It read normal. Just past half way
Question 2: no. I have a non-clutch fan
Question 3: yes. I have the Fan Shroud in place.

Is it True that there’s are different sending units. As in, one type for a Gauge , and one type for the “Temp” dash light?

[dmjlambert]

Originally Posted by custom10nut

Is it True that there’s are different sending units. As in, one type for a Gauge , and one type for the “Temp” dash light?

Yes, gauge type sending unit has a variable resistor inside to send different voltages to the gauge at different temperatures, while the temp dash light sending unit is a switch that grounds the wire to turn on the light bulb at a specific high temperature (250 to 260 F)

[custom10nut]

Originally Posted by dmjlambert

Yes, gauge type sending unit has a variable resistor inside to send different voltages to the gauge at different temperatures, while the temp dash light sending unit is a switch that grounds the wire to turn on the light bulb at a specific high temperature (250 to 260 F)

That’s what I was wondering!
You wouldn’t happen to have a Part Number for the correct one, would you?

[custom10nut]

I just looked for a Part Number
The AC Delco P/N is showing 12334869
Is this correct?
I also found a number 1852, for a “with factory gauges” application

[dmjlambert]

I think you may have an actual cooling problem if you notice the gauge moves just because you sit at a stop light, and reading goes lower when you are moving. Mine reads very steady at the same reading regardless of whether the truck is moving down the road on a 50 degree day, or sitting in my garage just running for 30 minutes or more on a 100 degree day.

[gmc684x4]

Yes there is a difference the light has smaller range it eorks in and the guage sending unit works over a broader range

Do you have acess to a coolant pressure tester ?
Many parts stores may do loan a tool on these

You pressure up the system and leave it for awhile see if it drops in pressure over time
This will confirm any leaks in the system

Higher flow water pump and 6 blade fan or fan cutch fan can help
Clutch fan needs to be proper rotation for water pump
Example 88 98 the water pump pully goes in reverse direction

Sender could be a bit out of range as well

[custom10nut]

Originally Posted by dmjlambert

I think you may have an actual cooling problem if you notice the gauge moves just because you sit at a stop light, and reading goes lower when you are moving. Mine reads very steady at the same reading regardless of whether the truck is moving down the road on a 50 degree day, or sitting in my garage just running for 30 minutes or more on a 100 degree day.

I don’t think so. The temp of the coolant never gets critical. Sitting at idle for 20 minutes never gets over 195, as I’ve checked it with a thermometer.
I think my issue is with the type of sending unit,

[dmjlambert]

The ACDelco part number is G1852 for the gauge type sender.
https://www.amazon.com/dp/B000C9SM22

[dmjlambert]

I don't think thermometer in the radiator is all that useful to tell engine temperature. If your thermostat is bad, the engine could get hot and radiator temperature stay cooler. Perhaps one of those laser temperature guns pointed at a head near the temperature sensor would be a good reading. Those things cost just a few dollars nowadays ($20 to $35).

[custom10nut]

Yeah I know, but at least I know the coolant is circulating. I’m just gonna get the new send unit, because I think that’s my issue

[custom10nut]

So I went and ordered an AC Delco unit. I’m hoping that fixes the issue

[Boog]

These things can be troublesome for sure. I guess using any aftermarket brand sender or thermostat can cause the gauge to read erroneously. I had an 85 blazer, all stock, the gauge read normally until I changed the thermostat. It then began going nuts like yours. Upon cold start up the gauge climbed and didn't stop until it pegged out. All I did was change from a 185 stat to a 160. I bled until I was sure all the air was out and it still read the same. I changed the thermostat, sender and even the gauge.
I put the 185 back in and it once again read normally.
Apparently changing the range it was designed to operate in doesn't always work well.

[custom10nut]

So, now I’m really stumped.
I changed out the T-stat and Sending unit with Delco stuff and still get the gauge running all the way to the right. I grounded the wire and it pegged out, so I know the wire has continuity.
The water is not boiling, and the truck runs fine.
Could it finally be the original gauge gone bad? Or maybe just a little oxidation breaking the connection?
Any insight is welcomed with open arms

[dmjlambert]

When the key is in the on position, does the gauge read hot all the time, even when then engine has not been started for the day?

[custom10nut]

Nope. I start it up and it all the way to the left. After about 5-10 min the needle just makes a steady (not fast) sweep all the way over to the right. As I said numerous times, the truck doesn’t run hot, and this just started doing this one day out of the blue. I still drive the truck, but it just irritates me that the gauge shows hot all the time. I think I’m just going to order a new gauge and see if that fixes it

[Basketcase889]

I have read reviews about sending units as I need to replace mine too. I know you looked up the model number and such as most did that as well. I read that you need the correct year of sending unit for your gauge and not the engine itself. There are a bunch of different years of units that all look the same but have different resistance inside the sending unit itself for different year gauges. A lot of people that bought the wrong ones had the gauge show as it was overheating and it wasn't. They put the stock unit back in and it ran normal again.

So seems to me the resistance may be off on your unit? Not sure if it is a year specific issue or if you had the right part both times and there is something off elsewhere. But wanted to share my experience just as an extra bit of knowledge.

[tdangle]

Last year, I had the same issue. The sending unit went bad, I replaced it and started acting the same way. Cold when you start and then would climb to hot peg and then drop to "normal" Temp gun verified engine was not overheating.
Tried 3 different sending units, verified the sending units with water heating on the stove and charting the resistance vs heat of the water. All of them were OK, verified the operating range of the gauge with a resistor box to ground. Totally stumped. Even went so far as to remove the head thinking I had a blockage in a water passage. Also disassembled the water pump looking for the cause. Then started worked all by itself Still don't know. Now have a new motor with vortec heads, got a turned down sending unit and all works perfect. Only thing different with my new motor is vortec heads require a coolant bypass hose setup.

[custom10nut]

Well, I ordered a Delco unit for my year so I would hope that it would be the right one. I have to go through mo old parts bin and see if I still have the original one. Incidentally, I spoke with a guy with a 73 truck, and he’s having the same issue.
Would it be worth it to buy one on these resistors for the gauge, or are they only for electronic ignitions.
But now that I breached that thought. Could the fact that I put an Electronic Conversion Module in the distributor make it act like this? I don’t know, just a thought

[54blackhornet]

Guys I have the opposite issue. I can drive all day and my temp gauge hardly gets to the middle mark. Takes forever in colder conditions. Should I worry about it ? Truck has a shroud and clutch fan.

[HO455]

Originally Posted by 54blackhornet

Guys I have the opposite issue. I can drive all day and my temp gauge hardly gets to the middle mark. Takes forever in colder conditions. Should I worry about it ? Truck has a shroud and clutch fan.

That sounds like my truck. It is normal operation. On the cold mornings does your heater start blowing warm air after about 5 minutes? For me I fire the truck up, let it idle it for about 30 seconds, then turn around ( I live on a one way street.) drive 5 blocks at 10 to 15 mph, wait at a stop light for a minute or 2. From there it is 3.more blocks to the freeway onramp. By the time I'm on the freeway the heater is blowing warm air. The guage at that point is still down in the bottom of the part of the guage.

[maynardogle]

Check to make sure all the ground steps between the engine block, frame and cab are intact. Bad grounds screw up gauges.

[custom10nut]

Originally Posted by maynardogle

Check to make sure all the ground steps between the engine block, frame and cab are intact. Bad grounds screw up gauges.

Good point! I’ll check them

[HO455]

Where is your sender installed in the cooling system? It may be that you have steam pockets building up in the system. Steam will build up in pockets and then burp out into the coolant flow. When steam passes the sender it will cause the temperature at the sender to rise temporarily then drop after the steam has passed. Sometimes this can be cured by drilling a 1/8" hole in the thermostat . That is if your sender is in the thermostat housing.
This condition can start happening due to a failing head gasket. You can test your coolant for the presence of exhaust gasses to rule out this as a cause of your problem.
Just information in case the grounds prove to be fine.