View Full Version : Gas gauge

09-19-2006, 06:06 PM
I'm back at the gas gauge problem. According to the manual you can short out the sending unit wire directly at the gauage to check the gauge. If it is ok, the gauge will go to full. Mine did and the resistance at full is 0 ohms. I obtained a new SW sending unit. Per the service manual I just moved the lever to full and the least resistance I could get was about 5 ohms. The original one in the tank gave the same 5 ohm reading, with an empty reading about 40 ohms. Both original and new SW give the same readings. As a result of this it explains why I only get to the 3/4 mark on the guage. Anyone know why the sending units dont go to 0? I checked and re-checked the new one.
Eariler postings had suggested that the resistance was highest when full. Am I missing something?
(p.s I got the speedo working so I dont have to count telephone poles. by the way. I used my laptop computer in the car with a cheap microsoft GPS system to check the speed. Any portable GPS can do this to easily check the speedo and not have to find a constant 1000 rpm to check for 60mph. My speedo runs slow by about 7 mph at 60 but not going to fool with adjustments as it now works fine.)

Mike DeLapp
Barrington, IL
53 Commander Hardtop

09-19-2006, 10:09 PM
Mike, I'm no engineer, but I can speculate with the best of them so let me try this:
The gages SW makes are electric self-compensating. That is they adjust themselves for the current that is being supplied which can be anywhere from 12 to 14.5 volts. If they didn't then the needles would move down everytime the engine dropped to idle and then back up when engine speed was higher. So why didn't they make the sending unit so that it had no resistance at the high point? Maybe they figured that it was a little insurance for the gage. If the tank was full and the voltage high the gage could risk burning itself out if the compensation mechanism wasn't calibrated correctly? I don't know, just a theory. What I do know is that the fuel gage in my Hawk has never ever gone over 7/8's even when the fuel is spilling out of the filler.

Tim K.
'64 R2 GT Hawk