Piston Slap: It’s a long, long run to a full gauge reading


Larry writes:

I have a 1965 Chevrolet Impala SS, and I have replaced the float in the gas tank twice. I have also checked the gauge (by putting 3 volts to it and it does go to full as intended), but when it is full the gauge only reads half a tank. What is the problem here?

Sajeev answers: 

Hi, Larry. There are normally three trouble spots to check when it comes to an issue like this: malfunctioning fuel gauge, a new sending unit with incorrect resistance calibrations, or too much resistance in the wiring causing a subsequent voltage drop. You tested the first issue, and confirmed it needs 3 volts to read full, so let’s look at the other two trouble spots.

Sending unit: Unlike the units in my kooky 1980s Fords, which require third-party rebuilding services to get them back to spec (or years of patiently waiting for an NOS part to surface on eBay), it appears new units for your Impala are plentiful. Lucky you! Classic Industries has them (here and here), and Rockauto has a wider variety too. You can test your sending unit’s resistance to ensure it is calibrated correctly. This article states General Motors’ sending units are calibrated to read “about 90 ohms” when in the full position, and the rest of the article might be even better than this handy video.

That said, maybe we can assume the sending unit is not causing the problem, because we have one last item to check.

Wiring: This is a 56-year-old vehicle, and at that age decayed wiring causes all kinds of bizarre problems. The first thing to do is to look for signs of corrosion at each end of the circuit and underneath the wire’s protective insulating skin. (Look for bubbles, splits, or any sign of decay in the insulation.) Then, using a multimeter, check for voltage drop between the sending unit and the fuel gauge. Our man Rob Seigel explains the process in perfect detail, and because of the distance between the gas tank and the dashboard, extended leads (like these) are a wise purchase. Remove the sending unit (or fill up the tank) and check for a voltage drop: if you need 3 volts to read full and the wiring is losing it as it travels to the dashboard, it might be time to replace the wiring. 

Now a final question: Which item has failed to provide you with accurate fuel level readings? I’m gonna guess that the wiring is fine, and that these reproduction sending units are poor quality and/or incorrectly calibrated. Furthermore, they might not work correctly with aftermarket gauges, if applicable. But the only way to know for sure is to test the wiring first. Best of luck in your diagnosis!

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com, give us as much detail as possible so we can help! If you need an expedited resolution, make a post on the Hagerty Community!

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