Weird needle movement on temp gauge... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ChevLoRay · 07-07-2004, 10:57 AM

I have noticed that the temp gauge on my '69 slowly moves from the cold position, over towards the hot position (but not all the way), then moves back to where it should be as the engine warms up. This happens within the first couple of miles of driving, after it has been sitting over night. I don't doubt the eventual reading on the gauge, and the sender is relatively new.

About three years ago, the wire to the gauge, from the sender, would short out against the valve cover and the needle on the gauge would ricochet from hot to cold....really, really, fast. This may have had an effect, but I have new wiring under the hood, now.

Just curious to know if anyone else has ever noticed this on their vehicle?

grebus1 · 07-07-2004, 11:17 AM

Ever since I replaced the sending unit on my truck, it does the same thing as what you described. It never behaved that way before and it's pretty annoying. The needle climbs and climbs to almost hot until the thermostat opens and then it finally drops. I'm guessing the sending unit is not the correct calibration even though the manufacturer guaranteed it was.

**JIMs70GMC** · 07-07-2004, 11:27 AM

My 70 does the same thing, even in the dead of winter. I figure it has something to do with coolant flow through the head.

ChevLoRay · 07-07-2004, 02:27 PM

Whoa....hadn't given much thought to the gauge actually doing what it says it's doing....reading the coolant temp. Makes sense, though. Until the thermostat opens, the coolant temp is increasing, as indicated by the gauge. Then, the thermostat opens, and the coolant temp drops, as indicated by the gauge. So, everything is working normally.

Now, I know why idiot lights were invented.....so I wouldn't post stupid questions!!! Guess I need idiot lights, instead of gauges....as long as the light is off, everything is okay, right??

Shane65 · 07-07-2004, 10:17 PM

This is the normal cycle of the cooling system until the engine has reached it's operating temperature. The cure for this (not that's it's a problem) is to drill 3 evenly spaced 3/32" holes in the thermostat. These would be in the flat area surrounding the valve. Now the coolant can bypass the thermostat and reduce the cycling effect. The thermostat still funtions to bring the temperature up to running temperature. I been doing this to all my cars for years. Some thermostats have a bypass hole(s).

07-07-2004, 10:57 AM	#1
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Weird needle movement on temp gauge... I have noticed that the temp gauge on my '69 slowly moves from the cold position, over towards the hot position (but not all the way), then moves back to where it should be as the engine warms up. This happens within the first couple of miles of driving, after it has been sitting over night. I don't doubt the eventual reading on the gauge, and the sender is relatively new. About three years ago, the wire to the gauge, from the sender, would short out against the valve cover and the needle on the gauge would ricochet from hot to cold....really, really, fast. This may have had an effect, but I have new wiring under the hood, now. Just curious to know if anyone else has ever noticed this on their vehicle? __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was. Last edited by ChevLoRay; 07-07-2004 at 10:59 AM.

07-07-2004, 11:17 AM	#2
grebus1 Registered User Join Date: Feb 2000 Location: Edmonds, WA Posts: 322	Ever since I replaced the sending unit on my truck, it does the same thing as what you described. It never behaved that way before and it's pretty annoying. The needle climbs and climbs to almost hot until the thermostat opens and then it finally drops. I'm guessing the sending unit is not the correct calibration even though the manufacturer guaranteed it was. __________________ '72 Chevy Suburban Custom Deluxe 10, 402/TH400

07-07-2004, 11:27 AM	#3
JIMs70GMC user # 2756 Join Date: Jun 2001 Location: Chesapeake, Virginia Posts: 4,612	My 70 does the same thing, even in the dead of winter. I figure it has something to do with coolant flow through the head. __________________ 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

07-07-2004, 02:27 PM	#4
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	Whoa....hadn't given much thought to the gauge actually doing what it says it's doing....reading the coolant temp. Makes sense, though. Until the thermostat opens, the coolant temp is increasing, as indicated by the gauge. Then, the thermostat opens, and the coolant temp drops, as indicated by the gauge. So, everything is working normally. Now, I know why idiot lights were invented.....so I wouldn't post stupid questions!!! Guess I need idiot lights, instead of gauges....as long as the light is off, everything is okay, right?? __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

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07-07-2004, 10:17 PM	#5
Shane65 Registered User Join Date: Oct 2003 Location: Ontario Posts: 108	This is the normal cycle of the cooling system until the engine has reached it's operating temperature. The cure for this (not that's it's a problem) is to drill 3 evenly spaced 3/32" holes in the thermostat. These would be in the flat area surrounding the valve. Now the coolant can bypass the thermostat and reduce the cycling effect. The thermostat still funtions to bring the temperature up to running temperature. I been doing this to all my cars for years. Some thermostats have a bypass hole(s).



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