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1970 XR-7 351C FMX
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Discussion Starter · #1 ·
I had recently installed a new gas tank in my ’70 XR-7, but used the same sending unit after checking that is would generate voltage with a multimeter. I put the tank back in the car and saw that the gas gauge was showing on ‘E’. I also noticed that the temperature gauge was not registering either. I did some research and purchased the gage1 voltage tester from Desert Valley Auto parts.

When I hooked up the tester, I saw some movement on the gauges, but not near the where they should have sent the needle to. When I put the tester setting to full, the gauges completely shut down. This happened on both the fuel and temperature gauges. I am trying to figure out if the instrument voltage regulator is bad, the gauges need to be calibrated or both. Any assistance would be helpful.
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I'm not familiar with how the Gage1 tester works, for example - where do you connect it such that it is able to move both gauges at the same time? If you are sure it is hooked up right and the fuel gauge never movers that tells me bad gauge.

My old school method for the gauges is to start at the connection to the sender. Check for voltage, which on an analog meter there should be about 5 VDC if you have a OEM style instrument voltage regulator. On a digital meter you will probably pick up a fluctuation centered on about 5 VDC. I know there are also more modern solid state IVRs out there. I would guess those would be constant 5 VDC on both meter types. With the feed wire disconnected at the sender the guage should be fully at one end of the range. If you have the 5 VDC on the wire then ground the wire and the gauge should sweep to full scale to the other end of the range. Don't leave it there too long or you could damage the gauge. If the gauge sweeps from one end to the other of the scale then bad sender. If it does not sweep to full range then bad gauge. If there is not 5 VDC in the first part of the test then bad IVR or bad wiring, if there is substatially more than 5 VDC then also bad IVR (but I've never seen one fail that way).

And for what it is worth, the sender provides resistance, in Ohms, not voltage so I am not clear what you mean when you say the fuel sender generates voltage?

Greg C
Longmont, CO
 

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1970 XR-7 351C FMX
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Discussion Starter · #3 ·
I'm not familiar with how the Gage1 tester works, for example - where do you connect it such that it is able to move both gauges at the same time? If you are sure it is hooked up right and the fuel gauge never movers that tells me bad gauge.

My old school method for the gauges is to start at the connection to the sender. Check for voltage, which on an analog meter there should be about 5 VDC if you have a OEM style instrument voltage regulator. On a digital meter you will probably pick up a fluctuation centered on about 5 VDC. I know there are also more modern solid state IVRs out there. I would guess those would be constant 5 VDC on both meter types. With the feed wire disconnected at the sender the guage should be fully at one end of the range. If you have the 5 VDC on the wire then ground the wire and the gauge should sweep to full scale to the other end of the range. Don't leave it there too long or you could damage the gauge. If the gauge sweeps from one end to the other of the scale then bad sender. If it does not sweep to full range then bad gauge. If there is not 5 VDC in the first part of the test then bad IVR or bad wiring, if there is substatially more than 5 VDC then also bad IVR (but I've never seen one fail that way).

And for what it is worth, the sender provides resistance, in Ohms, not voltage so I am not clear what you mean when you say the fuel sender generates voltage?

Greg C
Longmont, CO
I'm not familiar with how the Gage1 tester works, for example - where do you connect it such that it is able to move both gauges at the same time? If you are sure it is hooked up right and the fuel gauge never movers that tells me bad gauge.

My old school method for the gauges is to start at the connection to the sender. Check for voltage, which on an analog meter there should be about 5 VDC if you have a OEM style instrument voltage regulator. On a digital meter you will probably pick up a fluctuation centered on about 5 VDC. I know there are also more modern solid state IVRs out there. I would guess those would be constant 5 VDC on both meter types. With the feed wire disconnected at the sender the guage should be fully at one end of the range. If you have the 5 VDC on the wire then ground the wire and the gauge should sweep to full scale to the other end of the range. Don't leave it there too long or you could damage the gauge. If the gauge sweeps from one end to the other of the scale then bad sender. If it does not sweep to full range then bad gauge. If there is not 5 VDC in the first part of the test then bad IVR or bad wiring, if there is substatially more than 5 VDC then also bad IVR (but I've never seen one fail that way).

And for what it is worth, the sender provides resistance, in Ohms, not voltage so I am not clear what you mean when you say the fuel sender generates voltage?

Greg C
Longmont, CO
Thank for getting back to me and sorry for the delayed response. Your suggestions were really helpful. On the tester, I included the link below: I put the tester on a cheap multimeter so results may vary a bit and got the following:
High - 9 - 10 ohms
Med - 21 - 22 ohms
Low - 70 - 71 ohms
Gauge Testers | Product categories | Desert Classic Parts

After reading you post, I went back and retested the car. After testing again, I got a blinking light which indicate the regulator is not solid state. I was also getting reasonable response from both the temperature and fuel gauges. I ran the car for a bit (10 - 15 Min) and did see the gauge move but not out of the low range. I will need run the car longer to see if the temperature rises. On the fuel gauge, the tester registered readings close to the where they should be. I must have had bad connections on my first tests.

I used the multimeter, and I am not getting anything from the fuel sending unit. I had tested it when I replaced the tank, so it must have gone bad since. At least I have a direction to go in. Thank you.
 
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