C Hevrolet Truck Temp Gage Reads High

• #2

If the truck was actually overheating within 1-2 minutes of startup, the motor would not have survived a 300 mile trip.

I would verify that the cooling organization if operation properly by:

1. Make sure fluid is topped off in radiator and to full common cold line on reservoir.
2. Brand sure at that place are no oil spots or residue floating on top of fluid.
3. Make sure engine oil is dark brownish or black and does not wait like weak hippie coffee.
4. Start truck and let idle for ten-15 minutes
5. Verify no fluid leakage
6. Feel tiptop and bottom radiator hoses. The superlative hose should be hard and hot. Bottom hose should be slightly less hot.
7. Close engine off and let cool . Open up radiator cap and verify full.

Once you lot know the system is working correctly and non actually overheating, outset troubleshooting the gauge and sensor. It is possible the sensor is for the incorrect year truck, incorrect engine, or that the wire is broken somewhere. A sort in the wire would cause the judge to read 0. An open(cleaved wire) would cause the gauge to go all the way to max.

• #4

If the thermostat wasn't opening and the gauge was maxing out for existent... you'd be seeing some steam and it would not be driving well at all

• #v

Welcome to the forums!

The fact that information technology drove 300mi means you don't have a mechanical outcome with the engine or the cooling organisation (every bit has already been stated above).

Typically, coolant temp gauges are fed a little less than vehicle organization voltage, and so perhaps 9-10 volts, instead of 13-14v. So to make the temp gauge piece of work, the coolant temp sensor provides a variable ground point. So when the engine coolant is cold, the temp sensor provides a poor ground (high resistance), and that keeps the needle in the lower position. Every bit the coolant warms upward, the temp sensor improves the ground office, causing the estimate to read higher. Then in the case where a gauge needle moves to full calibration correct away, you typically have a ground wire (the wire from the sender to the judge) that is shorted (touching) ground somewhere. This is ordinarily where the wire passes through the firewall, or through some tight area along the engine...similar maybe it'due south gotten likewise shut to the exhaust and has melted to the manifold, etc.

• #7

Does your temp sending unit have a "nailhead" connector, where you slide the wire connector onto the terminal from ane side? Your '74 should be similar to my '75, and its a challenge to detect a sending unit that really matches the gauge. If you purchase the ane they advise at the parts counter it might not have the nailhead connector, and your gauge will always read very low if it has the resistance profile for gauges in the afterward year trucks.

The original Delco sending unit is no longer available, and the replacement Delco 12334869 is difficult to find at local parts stores. IIRC, the sending unit of measurement I finally constitute that seems to have the correct resistance is a Standard Motor Products TS6, and they had it in stock at Autozone.

The wire for the temp estimate is dark greenish and it runs forth the driver'south side valve cover then connects to the sending unit in the commuter'south side head. Its supposed to have a heat-resistance sleeve on the last 8-10" to protect information technology from the exhaust manifold.

Bruce

• #eight

A couple tools that are handy for this type of diagnostic piece of work:
- Infrared temp gun
- Digital Voltage/Ohm meter

You can use the temp gun to shoot the thermostat housing on the engine, and ostend engine coolant temp. You lot tin can shoot all over the radiator and confirm that coolant is uniformly beingness cooled towards the lesser, AND that there are or aren't whatsoever dead spots in the radiator due to clogged, or partially clogged cadre tubes. You can also shoot the hoses.

The digital voltage/ohm meter will allow you lot to test the back of the approximate for voltage and for the level of the ground signal during the changing coolant temp, as the engine warms up. Similarly, you can check the irresolute resistance to ground, correct a the sender, every bit the engine warms up.

A quick test of a temp gauge is to simply disconnect the wire from the sender. With the key "on" the gauge should read nothing. If the gauge does read, yous know that sender wire is touching ground somewhere forth it's path. The second test is to impact the sender wire to a metal part of the engine, thus giving the estimate "full ground." Yous should run into the gauge peg to it'south highest reading. If the gauge passes this test, you know it'due south receiving power, AND you know it probably works.....so your result at that point, would solely be in the sender.

• #11

Blue Ox

Turning Diesel fuel Into Fun

You have the expected resistance readings for the sensor. Put information technology in a pot of water with a thermometer, hook a meter up to it and cook it.

• #12

The next steps I would use require a digital multimeter and possibly a temp gun like austinado suggested. If y'all dont have the temp gun, you tin can merely use the multimeter. I would bank check the resistance to ground through the sensor with the engine cold. Get-go the engine and bank check again when fully hot. If it is within the range, you either have a actually bad connection on the back of the temp guess in the cluster that is causing the resistance readings to exist off, or your approximate is bad. I would definitely pull the cluster and check/clean the connections on the back of the cluster and the grounds on the parking restriction body

• #15

Your upper hose will be the hot coolant directly from the thermostat. The lower hose is later on it has been cooled through the radiator and entering into the water pump. I would think anywhere from 180-220 on the upper... no idea on the lower, merely definitely should be lower. I would actually put it right on the temp sensor on the driver side head. It should be ambience temp when cold and probably 195ish when hot.

The previous tests verified that the signal wire is connected to the judge and isn't a direct short to basis or a break to open. It did not verify that the gauge is properly reading inside the resistance range that the sensor is supposed to provide. Now that you know information technology has a connection, I would cheque the connections to the dorsum of the approximate for corrosion and bank check the grounds, they are common issues that can practice strange things to electronics

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Source: https://www.gmsquarebody.com/threads/temperature-gauge-maxes-out.30500/

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