fuel sender info needed

BakedAlaskan · #1 (**permalink**) 08-06-2003, 09:22 AM

I have a CJ7 (1979)

The guy before me installed a Holley projection and external fuel pump and screwed up with the sender. I get about 13 gal befores she chokes and runs out of gas. Of course like most jeeps neither the original gas gauge or the POS he installed works.

SOOOOO.........I have a stock (I believe) Jeep fuel sender rigged up with a in-tank GM fuel pump. It range from 0 to 90 ohm's empty to full. Can someone tell me what the original gas guage is. Has anyone ohm'ed one or can they ohm yours at the full mark?? If it is 90 ohms full then I will buy a new speedo set up from Quadratech and be done with this masking tape gas gauge.

Thanks

XJNick · #2 (**permalink**) 08-06-2003, 10:53 AM

Hi!

My '88 gas sending unit is:

0 ohms = Empty tank
69 ohms = Full tank

If you want to see what resistance is required to get the gauge to read, say full tank you can buy a Potentiometer (variable resistor). Plug on side of it to ground and the other to the fuel gauge "signal" wire. When you get the gauge to read full, remove the pot. and place an ohm meter across it's terminals. This will tell you the resistance required to make your gauge read full.

Hope that helps you

-Nick

Inspector-Gadget · #3 (**permalink**) 08-06-2003, 11:09 AM

Hello, Baked,

An excellent diagnostics writeup for all of the CJ dash gauges and sending units is located here:

http://www.jeeptech.com/electrical/gauges/

Regards,

Inspector-G

BakedAlaskan · #4 (**permalink**) 08-06-2003, 11:42 AM

Thanks for the link. It looks like I might be searching for a new sender to attach my in-tank pump to. I will double check it against the specs in the artical.

Inspector-Gadget · #5 (**permalink**) 08-06-2003, 12:58 PM

You are welcome, Keith.

Glad I could offer something of value.

I admire XJNick's troubleshooting technique, but I believe he might
have inadvertently reversed his empty vs. full tank resistance readings.

Thinking through the situation -- With a fuel tank full of gasoline, the gas gauge needle should be at maximum deflection toward the "Full" mark. Gas gauge needle deflection is proportional to the amount of current through the gas gauge deflection coil. No current, such as when the Jeep ignition key is in the "off" position, means no needle deflection. Hence the gas gauge needle points to "E."

This is a simple series circuit: +12 volt battery voltage is reduced to a nominal +5 to 7 volts by the vibrating contact power supply- voltage limiter contained within the gas gauge. Current flows from chassis ground, to and through the variable resistance of the fuel tank sending unit, to and through the gas gauge deflection coil, to the positive 5 to 7 volt output of the vibrating contact power supply-voltage limiter.

Maximum current through the meter movement deflection coil will occur when the variable resistance of the sending unit is minimum.

Ergo: Full tank = minimum fuel tank sending unit resistance (Whatever the designed minimum value). Empty tank = maximum fuel tank sending unit resistance (Whatever the designed maximum value).

Reduced to the essence, your gas gauge is an ammeter with a meter face calibrated in capacity.

Regards,

Inspector-G

XJNick · #6 (**permalink**) 08-06-2003, 01:50 PM

Quote:

I admire XJNick's troubleshooting technique, but I believe he might
have inadvertently reversed his empty vs. full tank resistance readings.

Thanks, but I got those from the factory service manual. The Cherokee gauges use three wires: 12v hot, ground, & "signal" (wire with a varied resistance to ground), and apparently the "signal" is connected to the float's varible resistor in a fashion that as the float rises in the tank, it is moving the resistor farther from the grounded end thus placeing more resistance between the input from the gauge and ground.

It is entirely possible that BakedAlaskan's gauges work differently. I was just giving some info and an example of a way to determine the resistance needed to make the gauge show full

Sorry for boring you, sound like your gauges work differently...
-Nick

Inspector-Gadget · #7 (**permalink**) 08-06-2003, 02:33 PM

Nick, if my second post came across as correcting you, I apologize. A correction was not my intention.

My intention was to prevent Keith from possibly purchasing a sending unit and/or gauge because of incorrect info.

There are, indeed, several common configurations of gas gauge circuitry. But Keith is working on a 79 CJ, and the web article I referred Keith to is specifically for CJs. My second post in this thread is valid only for mid-70s to 80s CJs.

That said, I do admire your technique. It is very ingenious. I wonder, given all the oil, temp and gas gauge related questions one sees on these Jeep boards, how many people realize that your potentiometer technique would be equally applicable to trouble-shooting those gauges as well.

Numbers aside, you have made people aware of a very good trouble shooting technique.

Regards,

Inspector-G

PS: Before somebody chimes in with the difference between a potentiometer and a rheostat. I know that a pot is a three-terminal device and a rheostat is a two-terminal device. No practical purpose would have been served here by drawing that obscure distinction.

BakedAlaskan · #8 (**permalink**) 08-07-2003, 09:02 AM

Shwwoooooooooooooooshhhhhhhhhh
(info flying over my head!!)

Well I will look you two up when I get serious about this. I came across some "Aftermarket" gauges for fuel and they had three choices of full/empty ohm's. So I promptly walked out and ohm'ed my sender he had rigged up and gave me. It was closest to 0-90 ohm's. Which sounds fine if I want another gauge other then my stock one. But I would rather have that space for a fuel ratio gauge. (due to my older style manual adjust injection system.) My stock fuel gauge doesn't not have the resistor between hot/ground. I swapped the one found on my POS gauge, and never got any readings on my gauge. (Also didn't check it against your check list)

My main focus is to get all 15 gallons from my tank, make my stock gauge at least "jeep" accurate, and get the constant whine of my fuel pump on the frame rail to go away.

I wish I could dive into this right away, but I am currently zoned in on screening in my porch. Now that my secret of being able to build something is out, my wife's honey-do list has grown.

Not bad, I kept it for 13 years. But I had to have a place I could sit outside and enjoy a beer without the mosquitoes. Coming from 30 yrs of soggy-wet-n-cold Southest Alaska these warm summer nights are AWESOME!!!!

Topless since April (The Jeep that is!!)

Thanks again.