need help with temp guage - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Rescue 1 · 03-25-2002, 06:02 PM

Okay I need help figuring out a problem with the temerature guage on a 1972 Chevy 3/4 ton. The truck has a guage package and it appears all the other guages are working okay. The problem is the temperature guage goes clear to the right, indicating a hot condition. The engine is not hot, it will go over there with just the key turned on.

I assumed it was a grounded wire, nope. I checked wire with ohm meter with no shorts indicated. So I assumed the sending unit was bad, nope, changed with new one, no difference. So I went after the guage itself, changed it, no difference. Go figure all the major players changed and no difference.

I have had it all apart several times. I have checked the sending unit resistance, it has less resistance as the engine warms up. I don't know how many ohms it is supposed to have cold or hot, any help on numbers?

I took the dash apart again today and checked the printed circuit, nothing unusual. (no burns, breaks, etc.) I started probing the guages themselves, here are the results.

Temperature guage four posts: all four have continuity on new guage, only three on old one, none to case itself.

Fuel guage three posts: all three have continuity, only lower post has continuity with outside case, does this indicate a shorted guage? When hooked up normal all three have ground to cluster, when hooked up so case doesn't touch cluster, fuel guage doesn't work. Temp guage still goes over.

When fuel guage is hooked up the temp and fuel guages have grounds through the cluster, is this correct? If fuel guage is unhooked temperature guage does not ground with cluster. When I say cluster I mean the metal part of the guage/dash assembly.

Could it be the fuel guage is shorted and the temp guage is showing it by grounding through the sending unit? I have checked the grounds on both the dash and the engine, they appear to be okay.

Okay electrical wizards give me your best advise. I have been told by many apeople just to change it to a mechanical guage, but I don't think it would look right. After all it is a 1972 3/4 ton 4X4 that has only 16,000 original miles...

**JIMs70GMC** · 03-25-2002, 07:37 PM

Well all I can add to this is, when i purchased my truck a couple of years ago the factory temp gauge did not work. The previous owner installed one to the right of the steering column that was mechanical. Well I was bored and found a green wire under the hood that was run with the ignition and starter wires, appears this was for the factory temp gauge, after getting board input it was determined to be that and was hot (12 volts) in run/start only. I purchased the correct sending unit and installed. (the mechanical gauge had actually quit working last thursday, did all this over the weekend). Before I hooked up the wire to the sending unit. I started the truck to check the thermostat, upper rad hose, and sending unit for leaks. I accidentially touched the green wire to ground which pegged the temp meter. I then hooked the wire up to the sending unit and after a couple seconds the meter came down and works great. I don't know if this helps.

------------------
1970 GMC K25 Custom
Chesapeake, VA
My 70 GMCor Webshots http://community.webshots.com/user/70gmc

blue72stepside · 03-25-2002, 07:54 PM

Do u have a cerimac resistor on the back, if so take it of and check resistance should be about 100 Ohms a open in the this resistor will make the neddle go to hot. if it is bad go to radio shack or other and get a 100 ohm ceramic resistor put it in place of the bad one. they cost about $2.00 for two. here is a chart i made on resistence of sending units.
[img]http://communities.msn.com/_Secure/0OAAoD*ESmQaHPm!b8CAPX6IBRKsZXAixiCR4qhGBXw4Z6yKVDH9mDt4XpQcqk83cRZxIR8XkGtFs1RH*WbO*DHQAcAA6AC8A/Temp-gauge.bmp[/img]

[This message has been edited by blue72stepside (edited March 25, 2002).]

Rescue 1 · 03-25-2002, 09:48 PM

Thanks for the chart. There is a resistor on the back of the old guage. The new guage did not come with one so I assumed it did not need it. I will try the new resistor and see what happens. Thanks again.

Cameron · 03-25-2002, 09:57 PM

The same thing happened to me and I took the same steps as you. I found out that the problem came from the voltage regulator that is mounted on the radiator support. When it started going out, the trucks volts would be way higher than normal. I also noticed that when I loaded the electricity up by turning on the A/C, lights, radio, wipers, etc: the tempurature guage would go down. So I replaced the voltage regulator and it fixed my problem. It still shows hotter than I would like though :0(

------------------

03-25-2002, 06:02 PM	#1
Rescue 1 Registered User Join Date: Mar 2002 Location: Delaware Posts: 6	need help with temp guage Okay I need help figuring out a problem with the temerature guage on a 1972 Chevy 3/4 ton. The truck has a guage package and it appears all the other guages are working okay. The problem is the temperature guage goes clear to the right, indicating a hot condition. The engine is not hot, it will go over there with just the key turned on. I assumed it was a grounded wire, nope. I checked wire with ohm meter with no shorts indicated. So I assumed the sending unit was bad, nope, changed with new one, no difference. So I went after the guage itself, changed it, no difference. Go figure all the major players changed and no difference. I have had it all apart several times. I have checked the sending unit resistance, it has less resistance as the engine warms up. I don't know how many ohms it is supposed to have cold or hot, any help on numbers? I took the dash apart again today and checked the printed circuit, nothing unusual. (no burns, breaks, etc.) I started probing the guages themselves, here are the results. Temperature guage four posts: all four have continuity on new guage, only three on old one, none to case itself. Fuel guage three posts: all three have continuity, only lower post has continuity with outside case, does this indicate a shorted guage? When hooked up normal all three have ground to cluster, when hooked up so case doesn't touch cluster, fuel guage doesn't work. Temp guage still goes over. When fuel guage is hooked up the temp and fuel guages have grounds through the cluster, is this correct? If fuel guage is unhooked temperature guage does not ground with cluster. When I say cluster I mean the metal part of the guage/dash assembly. Could it be the fuel guage is shorted and the temp guage is showing it by grounding through the sending unit? I have checked the grounds on both the dash and the engine, they appear to be okay. Okay electrical wizards give me your best advise. I have been told by many apeople just to change it to a mechanical guage, but I don't think it would look right. After all it is a 1972 3/4 ton 4X4 that has only 16,000 original miles...

03-25-2002, 07:37 PM	#2
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	Well all I can add to this is, when i purchased my truck a couple of years ago the factory temp gauge did not work. The previous owner installed one to the right of the steering column that was mechanical. Well I was bored and found a green wire under the hood that was run with the ignition and starter wires, appears this was for the factory temp gauge, after getting board input it was determined to be that and was hot (12 volts) in run/start only. I purchased the correct sending unit and installed. (the mechanical gauge had actually quit working last thursday, did all this over the weekend). Before I hooked up the wire to the sending unit. I started the truck to check the thermostat, upper rad hose, and sending unit for leaks. I accidentially touched the green wire to ground which pegged the temp meter. I then hooked the wire up to the sending unit and after a couple seconds the meter came down and works great. I don't know if this helps. ------------------ 1970 GMC K25 Custom Chesapeake, VA My 70 GMCor Webshots http://community.webshots.com/user/70gmc __________________ 1970 K25, 8' stepside bed 350/465/205 44 up front, 60 in the rear 4.10s rolling on 33" Dunlop MTs 1986 K5, 350/465/208 Dana 60/14 bolt from a cucv 36" Super Swampers TSL/SX 1983 K20 w/ CUCV axles, 350/700R4/208 sitting on 37" Goodyears 1986 M1031 6.2 diesel, TH400/NP205 locker in the rear and a LS in the front, all stock for now..... 1986 K30, 350/400/205 dana 60 and 14 bolt. I kept the drivetrain. Body/bad and chassis are gone. 1981 K30, 350/465/205 dana 60 and dually 14 bolt. Has a G80, and a flat bed. Going to replace the flat bed. 1985 K20, 350/400/208 10 bolt and SF 14 bolt. I wonder where I can find some 1 tons. Hmmmmm

03-25-2002, 07:54 PM	#3
blue72stepside Registered User Join Date: Aug 2001 Location: Morgan Hill, California Posts: 309	Do u have a cerimac resistor on the back, if so take it of and check resistance should be about 100 Ohms a open in the this resistor will make the neddle go to hot. if it is bad go to radio shack or other and get a 100 ohm ceramic resistor put it in place of the bad one. they cost about $2.00 for two. here is a chart i made on resistence of sending units. [img]http://communities.msn.com/_Secure/0OAAoDESmQaHPm!b8CAPX6IBRKsZXAixiCR4qhGBXw4Z6yKVDH9mDt4XpQcqk83cRZxIR8XkGtFs1RHWbO*DHQAcAA6AC8A/Temp-gauge.bmp[/img] [This message has been edited by blue72stepside (edited March 25, 2002).] __________________ 72 Cheyenne Stepside 357/TH350 3.07 Posi 379HP@5500 412FT#@4000 96 1500 Work Truck 88 Chevy Van 3/4 97 Tahoe 8.5 Lift (Sons) 00 1500 LT 4x4 (Sons) 98 Ford Cobra (Wifes Car)

03-25-2002, 09:48 PM	#4
Rescue 1 Registered User Join Date: Mar 2002 Location: Delaware Posts: 6	Thanks for the chart. There is a resistor on the back of the old guage. The new guage did not come with one so I assumed it did not need it. I will try the new resistor and see what happens. Thanks again.

03-25-2002, 09:57 PM	#5
Cameron Registered User Join Date: Apr 2001 Location: Tucson, Arizona, USA Posts: 44	The same thing happened to me and I took the same steps as you. I found out that the problem came from the voltage regulator that is mounted on the radiator support. When it started going out, the trucks volts would be way higher than normal. I also noticed that when I loaded the electricity up by turning on the A/C, lights, radio, wipers, etc: the tempurature guage would go down. So I replaced the voltage regulator and it fixed my problem. It still shows hotter than I would like though :0( ------------------



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.