My 68 C20 has a 305, temp sender won't work - The 1947 - Present Chevrolet & GMC Truck Message Board Network

gerretw · 09-04-2022, 01:31 AM

Hi I have a 68 C20 long bed - it doesn't have the original motor, it has a 305 out of a 83 buick. I replaced the temp gauge from brothers, and new sensor, but nada. When I ground out the green wire at the block the gauge moves so I know the circuit works. Any idea how to fix it?

Likewise the voltage gauge doesn't move, sits in the middle. When I ground it, it moves. What's the story here?

Any help appreciated.

G

**pjmoreland** · 09-04-2022, 01:55 AM

Good evening. Here are a few things that come to mind:

1) The original temperature gauge would have had a ceramic resistor mounted to the back of it. Did you transfer that resistor to the new gauge? Some reproduction gauges are designed to work without the external resistor. I don't know if that's the case for the Brothers gauge. Might require a call in to them to confirm.

2) Is the new temperature sender the correct style for a gauge? There's another style that is meant for base model trucks with warning lights.

3) The stock BATTERY gauge does not display volts. It is an ammeter that displays the amount of current flowing to or from the battery. The needle doesn't usually deflect very far. A good test is to turn the headlights on with the engine off. The needle should deflect slightly to the left. If not, then check the two 4A fuses located in the front two corners of the engine compartment. This assumes the original wiring harness is still in the truck.

**Accelo** · 09-04-2022, 02:01 AM

Any Teflon tape on the sensor?
It has potential to mess with the sensor ground.
Cheers

gerretw · 09-05-2022, 02:58 AM

When I tested the green wire today, it is putting out 2.3 - 3 v at idle. It is a possibility, I guess, that the sender that fits a 305 is incompatible with the 68 wiring - the sender for the 68 is a larger thread size. Maybe I can find a reducer and see if that fixes the problem? Wasn't the 307 the stock motor? The 305 is out of a 83 buick.

The sender has red gunk on it.

If there was a resistor on the original gauge, it's long gone, my mechanic swapped it out for me. There is however, an original chip of mica (?) wrapped in wire on the back of the gauge - maybe that's the resistor?
G

**pjmoreland** · 09-05-2022, 03:06 AM

Member A1971Blazer sells modified senders for newer engines. I purchased one from him for the 1997 Vortec engine I have in my truck, and it works great.

https://67-72chevytrucks.com/vboard/...d.php?t=726700

09-04-2022, 01:31 AM	#1
gerretw Registered User Join Date: Jul 2021 Location: Morrison Colorado Posts: 13	My 68 C20 has a 305, temp sender won't work Hi I have a 68 C20 long bed - it doesn't have the original motor, it has a 305 out of a 83 buick. I replaced the temp gauge from brothers, and new sensor, but nada. When I ground out the green wire at the block the gauge moves so I know the circuit works. Any idea how to fix it? Likewise the voltage gauge doesn't move, sits in the middle. When I ground it, it moves. What's the story here? Any help appreciated. G

09-04-2022, 01:55 AM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,975	Re: My 68 C20 has a 305, temp sender won't work Good evening. Here are a few things that come to mind: 1) The original temperature gauge would have had a ceramic resistor mounted to the back of it. Did you transfer that resistor to the new gauge? Some reproduction gauges are designed to work without the external resistor. I don't know if that's the case for the Brothers gauge. Might require a call in to them to confirm. 2) Is the new temperature sender the correct style for a gauge? There's another style that is meant for base model trucks with warning lights. 3) The stock BATTERY gauge does not display volts. It is an ammeter that displays the amount of current flowing to or from the battery. The needle doesn't usually deflect very far. A good test is to turn the headlights on with the engine off. The needle should deflect slightly to the left. If not, then check the two 4A fuses located in the front two corners of the engine compartment. This assumes the original wiring harness is still in the truck.

09-04-2022, 02:01 AM	#3
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,295	Re: My 68 C20 has a 305, temp sender won't work Any Teflon tape on the sensor? It has potential to mess with the sensor ground. Cheers

09-05-2022, 02:58 AM	#4
gerretw Registered User Join Date: Jul 2021 Location: Morrison Colorado Posts: 13	Re: My 68 C20 has a 305, temp sender won't work When I tested the green wire today, it is putting out 2.3 - 3 v at idle. It is a possibility, I guess, that the sender that fits a 305 is incompatible with the 68 wiring - the sender for the 68 is a larger thread size. Maybe I can find a reducer and see if that fixes the problem? Wasn't the 307 the stock motor? The 305 is out of a 83 buick. The sender has red gunk on it. If there was a resistor on the original gauge, it's long gone, my mechanic swapped it out for me. There is however, an original chip of mica (?) wrapped in wire on the back of the gauge - maybe that's the resistor? G Attached Images Last edited by gerretw; 09-05-2022 at 03:10 AM.

09-05-2022, 03:06 AM	#5
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,975	Re: My 68 C20 has a 305, temp sender won't work Member A1971Blazer sells modified senders for newer engines. I purchased one from him for the 1997 Vortec engine I have in my truck, and it works great. https://67-72chevytrucks.com/vboard/...d.php?t=726700



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