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Does anybody know the fuel gauge sender ohm readings for an empty tank and a full tank? I know modern GM products are 90 and 0. My gas gauge bounces twixt F and E while driving and sits at F when parked (key on).

Sounds like a loose or bad wire between the sender and the gauge, or a bad ground or loose ground on the sender.

Any idea what brand instruments in 38? Only for sure info I have is for King-Seely type 51-6 and that is about 10 ohms full and 70-75 empty using a regular resistance type sender. Kevs suggestion of wire is very valid as a short on the 51-6 type would cause a full reading but it would be a fairly smooth but slower movement, not what I would call a bounce. The type that uses a magnetic movement (GM, I think is one) is very fast.

There are several different type senders though for different brand gauges and all different--some use resistance, others have a bimetal heater that is just an on-off. The duration of the on period (and how much the gauge moves) depends on how much deflection the float causes to tank element.

The gauges are SW and the gas gauge is a resistance type. I did try redoing the wiring and now it read 1/2 full. At least that seems about the right reading , I'll find out if it works on my next drive. If the tank is really half full it means that I'm getting about 6 MPG OUCH.

The SW is magnetic so fast movement, don't have a specific ohm range to share at moment, but to test, grounding tank wire should indicate full and not connected should read empty. If you had a loose or bad wire that would certainly cause the bounce.

I own a 1939 Packard 110 Coupe. I had all the instruments renewed, repainted, and calibrated by John Wolf & Co. last year. The fuel gauge is indeed SW with a range of around 75 ohms empty and zero to 5 ohms full. My problem is now the tank sender. Does anyone know of a source for a NOS or new sending unit with the above values? Thanks

Only options I know of for those values is either OEM from a vendor, the 57 Ford unit (which I don't know if it would or could be made to fit your mounting arrangement easily) IF still available to be ordered from a local ford store or the JC Whitney universal retrofit as detailed in the Tech tips section on the website.