TEMP AND VOLT GUAGES NOT WORKING

[Rkendrick05]

I have two gauges not working. Temp and volt gauge.
Both have 12 volts on the clips in the gauge cluster.
I took the volt gauge and applied 12 volts directly from the battery to the two posts that are supposed to be used and no luck. If i move one of the wires(I forget which one) to the post that is not supposed to be used the needle maxes out at 18 volts. Not sure why that is.
My temp gauge has 12 volts being supplied and a new sending unit but doesn't work. I ran a wire directly from the sending unit to the back of the gauge where the sending unit connects and no luck.
Is there anything else I can do to diagnose or is it safe to assume I need new gauges?

 

[Rkendrick05]

After buying new gauges, the volt gauge now works.
I checked and there is voltage on the two clips at the gauge cluster. The other clip that goes to the sending unit has continuity to the connector that attaches to the sending unit.
If I ground the wire that attaches to the sending unit the gauge pegs out at max temp.
I got the sending unit from O'Rieleys. Took it back and got another and still no luck.
Is there a certain sending unit I should be using that has more or less resistance? At this point I don't know what to do next.
Anyone out there that knows more about this than me? I am out of ideas.
Truck is an 84 GMC with a 350 ci engine.

 

[Jgonick]

Hopefully someone will be able to help you. I haven't had to personally deal with this issue (yet), but I saved this from an older post just in case I had too.

from post-- Link to post
COOLANT TEMPERATURE GAUGE PARAMETERS

For Truck Years: 1967 - 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω

For Truck Years: 1974 - 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω

For Truck Years: 1979 - 1990
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω

as you can see there are three different ohm ranges depending on the year. there are also 1/2" and 3/8" pipe threads so you need to match the size.