Hello all, my chassis is a newer mono number 169. Issue is simple I need to correct the ohms reeding the gauge gets from the senders. I was given modified VDO sending units to run my original style Smith's fuel gauge. The gauge reads no more the 3/4 when the tanks are full and it reads emty when the tanks are about 1/2 full. Any suggestions short of changing to the VDO gauge would realy help.
CAV fuel gauge help
- Thread starter rohach
- Start date
Seymour Snerd
Lifetime Supporter
You might be able to roughly correct one side (E or F) but not the other with the addition of a resistor. Do you know the resistance range of the sender, and what resistance range the gauge expects?
Also, what was the modification to the VDO senders?
Finally, what do the VDO senders look like? Do you have a picture?
Hello Allen,
I will do some reaserch and find out the parameters for both senders and the gauge itself. The mods to the sender are to slightly shorten the cylinder shaped body only to safely fit inside the fuel bladder(no pics,sorry).
another note....the board you put together for the one wire alternater is still functioning perfectly after 1500 miles... Thanks again Rohach
I will do some reaserch and find out the parameters for both senders and the gauge itself. The mods to the sender are to slightly shorten the cylinder shaped body only to safely fit inside the fuel bladder(no pics,sorry).
another note....the board you put together for the one wire alternater is still functioning perfectly after 1500 miles... Thanks again Rohach
Seymour Snerd
Lifetime Supporter
If it's the usual type with an arm and a float then sometimes a lot can be done just bending the arm. So a picture would help judge the likelihood of that.
Ian Anderson
Lifetime Supporter
Mike
Before you start playing with resistors etc have a check and see if your floats are able to move through the comptete swing required to get a full reading.
There could be a baffle / foam etc in the way to stop the sender float moving.
Or the float itself may be hitting the top of the tank belfore the unit itself is reading full.
Putting a bend in the wire that hold the float ball may be all that is required
Ian
Before you start playing with resistors etc have a check and see if your floats are able to move through the comptete swing required to get a full reading.
There could be a baffle / foam etc in the way to stop the sender float moving.
Or the float itself may be hitting the top of the tank belfore the unit itself is reading full.
Putting a bend in the wire that hold the float ball may be all that is required
Ian
Seymour Snerd
Lifetime Supporter
Good points. And of course another option is a solid state sender that can be custom calibrated to your gauge's requirements, eg CENTROID PRODUCTS - Computerized Tank Display - Electronic Senders - capacitive fuel gauges. at ~$65 it's worth a fair amount of hair pulling.
Originally Posted by rohach 
Hello Allen,
I will do some reaserch and find out the parameters for both senders and the gauge itself. The mods to the sender are to slightly shorten the cylinder shaped body only to safely fit inside the fuel bladder(no pics,sorry).
Sounds like when they modified these they are reading near the top ( not totally shorted ) and open part way down??? Are these the cylindrical one's with the ring around them ??? For linear movement ??? If so they may be the wrong OHM units. If you have a spare sender find out what Resistance is required to make your gage sweep full scale then check the sender against that. Wally
Hello Allen,
I will do some reaserch and find out the parameters for both senders and the gauge itself. The mods to the sender are to slightly shorten the cylinder shaped body only to safely fit inside the fuel bladder(no pics,sorry).
Sounds like when they modified these they are reading near the top ( not totally shorted ) and open part way down??? Are these the cylindrical one's with the ring around them ??? For linear movement ??? If so they may be the wrong OHM units. If you have a spare sender find out what Resistance is required to make your gage sweep full scale then check the sender against that. Wally
Mike:
Before you get into removing senders or other major stuff try this.
Go to radio shack or some other electronic supplier and get a 1000 or so ohm potentiometer, you will need to hook up one terminal to your gauge sender terminal, and the other to ground, use two adjacent terminals on the pot.
This will put you in parallel with the sender, and you can now lower the resistance of the output thereby raising the reading to the guage and possibly correcting it. If that works you can remove the pot, measure its resistance where it worked, and get a fixed resistor that is close.
If it doesn't make the changes you need, you have issues with your sender and you've only spent a few bucks to find out.
The formula for parallel resistance on DC is the sum divided by the number of resistors... adding the pot allows you to lower the resistance of the circuit.
Good luck
Phil
Before you get into removing senders or other major stuff try this.
Go to radio shack or some other electronic supplier and get a 1000 or so ohm potentiometer, you will need to hook up one terminal to your gauge sender terminal, and the other to ground, use two adjacent terminals on the pot.
This will put you in parallel with the sender, and you can now lower the resistance of the output thereby raising the reading to the guage and possibly correcting it. If that works you can remove the pot, measure its resistance where it worked, and get a fixed resistor that is close.
If it doesn't make the changes you need, you have issues with your sender and you've only spent a few bucks to find out.
The formula for parallel resistance on DC is the sum divided by the number of resistors... adding the pot allows you to lower the resistance of the circuit.
Good luck
Phil
All good points, I am going to Radio Shack to find out exactly what I have for sender output and gauge demands. The senders are the cylindrical style with an internal linear movement( no way to adjust or bend). The correction will most likely have to be electrical.
This will be Sunday's chore.
Thanks All
This will be Sunday's chore.
Thanks All
Seymour Snerd
Lifetime Supporter
You can also do this with a $2 ~500 ohm potentiometer (aka "pot" aka variable resistor) such as this:
T7YB471MB40 Vishay/Sfernice | Mouser
Turn the car on with the potentiometer temporarily wired in place of the sender. Twist the pot around until the gauge reads empty, disconnect the potentionmeter, and measure the resistance between the same two terminals on the pot. Then do the same after getting the gauge to read full. Those two resistance figures are the key numbers for the gauge. If you're planning on installing a Centroid or similar sender and want it really accurate, take measurements at 1/4, 1/2 and 3/4. and gives to them when ordering.
Typical figures are (courtesy of Summit Racing)
0-90 Ohms - Works on most GM cars, 1965 to present
73-10 Ohms - Works on Fords prior to 1989 & most Chryslers
240-33.5 - Industry standard, works on many popular cars
0-30 Ohms - Works on most GM cars prior to 1965
16-158 Ohms - Works on most Fords, 1989 & newer
I dont think you'll find that range pot at Radio Shack, I'm sad to say. I think the closest they have is 5,000 ohm, which probably does not have the resolution you need to do this properly.
Last edited:
Hello All,
After doing some research. It turns out that CAI(Smith's) gauges use the European Standard for fuel gauge demands.
10 ohms at empty and 180 ohms full.
At this point I am going to have custom solid state senders made. The accuracy factor won't be an issue and they make a version that fits my wire harness without modification.
Thanks for all the help,
Rohach
After doing some research. It turns out that CAI(Smith's) gauges use the European Standard for fuel gauge demands.
10 ohms at empty and 180 ohms full.
At this point I am going to have custom solid state senders made. The accuracy factor won't be an issue and they make a version that fits my wire harness without modification.
Thanks for all the help,
Rohach
