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OK, I bought a '64 PV544 with a non-working fuel gauge. I initially thought the gauge was at fault (since none of the other gauges worked either!) so I got a gauge from a 122. Same gauge, except it has a small coil on it back side.
After I installed it the gauge still read 'dead' so I pulled the sending unit, only to find the windings and armature were missing. Basically it was a plug used to keep gas from spilling out of the tank, nothing more!
I got a sending unit, supposedly from a 12V PV544, and inspected and tested it before installing it.
Now the problem was that when I filled the "1/2 full" tank, it took 8+ gallons of gas. Apparently 1/2 full means EMPTY.
Back home again, I swapped the new gauge for the original gauge, then took a short road trip to my Dad's place - a little over 90 miles.
The gauge read 3/4 when I got there, which COULD have been more or less correct. I filled the tank before returning home, and again, 1/2 full was nearly running-on-fumes empty.
The needle didn't reach the 1/2-way mark and stop...it read smoothly all across the top half of the markings, so the armature isn't getting hung up or stuck.
Now I need to figure out a way to modify the sending unit (maybe a resistor?) to get a corect reading.
Any thoughts or suggestions?
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Steve,
If that's the sending unit that came from my salvage yard, I agree with Dennis that the arm is likely to be bent. The post office probably banged it around pretty good on it's way from Vermont to California.
By the way, that sending unit's from a 122 wagon. Is the tank depth the same on a 544? Arm length could be a factor, too.
Neil
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It sounds like the wire that the float is on is bent. If you get your tank down to an accurate 1/2 way point ( You can use a dipstick through the opening to check this) Then see what the sender causes the gauge to read, bend the wire so the gauge reads 1/2 and you should be OK. Word of caution............ Take a spare can of gas with you for awhile while you "Fine Tune" the sending unit.
Dennis
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I checked the drop on the arm before I installed it. The tank is six inches deep, and the float drops to exactly six inches on the low side.
In addition to adjusting the angle of the arm, you can also adjust the stop tabs on the sending unit so you can define the travel limits.
I think that I need to pull the sending unit and ground it to the body, then move the arm while my wife watches the gauge.
Since I've tried the gauge from a 122 and a PV544, the problem MAY BE a bad ground on the sending unit. If there is additional resistance, the gauge will never read 'empty.'
Wish me luck!
I'll be back when I've figured this mess out.
Although, having a gauge that reads ANYTHING is better than no gauge at all!
At least right now I know to fillup when it show 1/2 full!
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Quote-
"Since I've tried the gauge from a 122 and a PV544, the problem MAY BE a bad ground on the sending unit. If there is additional resistance, the gauge will never read 'empty.'"
Isn't it just the opposite? That if there is additional resistance, the gauge will read lower?
Maybe I'm not understanding the circuit, but here goes: The gauge has battery positive voltage on one side of the coil. Current flows through the coil, through the wire to the sending unit, through the sending unit to ground. Sending unit resistance (at least on 122s) is high when the float is in the "empty" position and low in the "full" position. Current in the circuit (and resulting gas gauge needle deflection) would therefore be lowest when resistance is highest.
Seems to me you NEED additional resistance in the circuit to lower the gauge reading. It's easy enough to check by putting an additional resistor in series. Maybe an adjustable resistor (0-500 ohms or so).
Neil
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If I run the wire from the gauge to ground, it reads exactly on the EMPTY mark. If I disconnect the lead, the gauge reads FULL. This leads me to believe that the less resistance, the higher the gauge reads.
:-)
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That is correct. Not all gauges read higher when the resistance to ground is decrased. Many GM models will read empty when the wire is grounded and full when the wire is run through some resistance such as the variable resistance from a sending unit.
So the gauge seems to be working fine.
Next step might be to see if the gauge reads full with more or less resistance, then which way does the sending unit cause the resistance to change. Do they work as a pair or do you in fact have a miss match due to two different types of gauges ?
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Um, no. Ground = minimum resistance. Open = maximum resistance. The less resistance, the LOWER the gauge reads.
The gauge in question here reads high, so it can theoretically be corrected by adding a resistor in parallel with the existing circuit. This lowers the resistance "seen" by the gauge.
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Hmmm. The evidence seems to contradict everything I thought I knew about these circuits. It's throwing me for quite a loop. Either I'm completely wrong about the way these things work or there's something wierdly wrong with Steve's car.
I understand that a direct connection to ground equals zero, or nearly zero ohms and that an open equals infinite ohms. In the older gas gauge systems I have experience with, grounding the sender side of the gauge will make the gauge read "full" and disconnecting that side of the gauge will make the gauge read "empty".
I guess tomorrow would be a good time to look at a schematic, then grab some parts and do some testing. This is bugging me.
Thanks for your patience and kindness.
Good night, all.
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Guys;
In order to be highly insensitive to fluctuations in the power system, the fuel gauge was designed as a "balanced magnetic instrument" ...so it not as simple as "more current means a higher meter indication" (which would be highly sensitive to power fluctuations).
For an engineering data sheet on the (ingeneous!) VDO "Drehspulinstrument" that was used on many of the early Volvo models, see: http://www.intelab.com/swem/VDO%20Drehspulinstrument.htm ...it's in the original German but there are useful graphs. I'll have to translate it sometime when I get bored...(fat chance!).
Cheers
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Thanks for the VDO gauge link, Ron. Yep, it's way more complicated than I thought.
So yesterday I got a gas gauge and sending unit out of a '65 122 wagon and set it up with a battery.
The sender measured 19 ohms "empty"; 100 ohms "half full"; and 179 ohms "full".
Gauge connections were: 12 V+ to the "+" terminal, "G" terminal to the sending unit, and the terminal in the middle that's connected to a little flat bar was connected to 12V-, as was the case of the sending unit.
The circuit worked, the gauge pointer went up and down just as intended, then the sender lead was disconnected. Gauge read full. The sender lead was grounded and the gauge read empty. So the sending part of it works the opposite way I thought, too.
Since Steve's gauge behaves the same way this one does, his gauge is probably OK.
So I agree now with Steve and Phil that what is apparently needed is less resistance on the sender side of the gauge. If Steve has a 20-179 ohm sending unit, maybe the wire between the sender and the gauge has a bad connection somewhere or maybe there are some weak connections within the sender. The current has to travel through the copper resistor wiper to the steel float arms through the float arm pivots to the sender case to a screw connection to the tank, etc, etc. Lots of connections- 100 extra ohms would be enough to throw the gauge reading off by 1/2 tank.
Hope this has been helpful...
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OK, an update on my findings.
I pulled the sending unit out of the tank and hooked my ohmmeter to it to read the resistance as teh lever was moved and the contact slid across the windings.
It should have been an even drop all the way across, but I found that it stopped changing readings when it got 1/2 way cross.
Reluctantly I opened up the sending unit - I knew that I would find very delicate wire windings inside, and I have big clumsy fingers!
I ran my ohmmeter probe across the windings themselves and got the same readings as I got from moving the arm. Putting on my magnifier-glasses, I looked closely at the windings and found them to be crusted over with dried gasoline residue. I sprayed the works liberally with carburetor cleaner, then carefully (!!!) used an engraving tool to cut through the encrustation. I tried the ohmmeter again. Voila! About 60 ohms resistance on the low end!
I put it all back together and it appears to be working correctly now.
I wonder if there was enough lead in the dried residue to short out the windings?
This isn't something I'd want to do again, but I could probably have gotten away wiht just soaking he works on carb cleaner for a couple of days and brushed the residue away wiht an old toothbrush.
Thanks for the help & advice!
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posted by
someone claiming to be Joe in St Louis
on
Fri Oct 15 02:16 CST 2004 [ RELATED]
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What I'd like to know is how most fuel gages we see are damped, avoiding the wild fluctuations we see on our fuel gages caused from the sloshing of fuel in the tank.
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In the old days of 6 volt systems the gas gauges did fluctuate when starting and stopping. Kinda like the old "Model A" which was a float that read directly on the dash as the fuel tank was mounted under the windshield and all was strictly mechanical. Later when everything went to 12 volt gauges actualy ran at about 8 volts and used a "voltage regulator" that dampened the reading by delaying fluctuations. I am not sure what they use as of late , possibly an electronic version doing the same. Chrysler products used a unit that looked like a rectangle flasher unit that plugged into the circut board of the dash while GM used a resister built into either the fuel or oil gauge that supplied both.
Hope that helps explane how they did it.Most gauges over the years have a power applied to one side of the gauge and the other side goes to a sending unit, in the case of a gas gauge it has a reostat that changes the resitance to ground when the float moves, the temp gauge changes with temprature changes and the oil is a pressure operated reohstat going to ground if it is electrical. Some gauges work just the oppisite and the gauge will read inverse of the above but the operation is all basicly the same.
Idiot lights were used as most people don't scan their gauges and will only lok at the fuel gauge, the oil and temp are forgotten unless strange noises are heard then it's usualy too late. Piolts on the other hand are trained to monitor the gauges as their lifes depend on things going smooth. My self, I like the gauges and a light both hoked up in the circut for the oil pressure. At one time Buick tried a switch that killed the engine if pressure dropped below a certain point or didn't come up within a certain period after starting, bad thing was if your engine quit in traffic, you lost power steering and brake booster at a very bad time so they discontinued it after a short time.
Dennis
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I know mine sure isn't kidding when it hits empty. I've run out a couple of times trying to push it a little bit. When the guage stops 'sloshing' I'm just about 1/2 mile from walking. I only seem to get about 9 gallons of fuel between full and empty on mine. As it has gradually gotten faster the MPG has gone down to where it now has a pretty pathetic range.
--
I'm JohnMc, and I approved this message.
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Hmmm..sounds like I'd be adjusting that sending unit to give me a little more 'reserve' gas.
I had a big Toyota Landcruiser that had a 10-gallon tank under the passenger's seat. When it got to empty the vacuum would cause the tank to go WHANG! and about 10 seconds later I'd be out of gas. There was no pushing that beast, either!
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