Fuel gauge running in reverse.

[Dickinson County Comanche]

Ok, so my fuel gauge is running in reverse, this is due to me having a 91-92 sender on an 89, this being said, I’m wondering if I can’t just cut and splice the wires in reverse, would that fix it? I’ve seen people change them on the sender itself, and on the gauge but I wasn’t so sure that wouldn’t work also. Thanks.

[Eagle]

I've been pondering whether there's an easy way to correct this for years. Can't make up my mind if it's easy, or impossible.

 

What happened is that Chrysler reversed the polarity of the sender. On the Renix models, the fuel gauge range was 0 ohms, for Empty and 88 ohms for full. The Chysler sender's range is (IIRC) 105 ohms for Empty to 5 ohms for full.

 

I think what would work is cutting the traces on the cluster printed circuit and connecting the two leads from the wire bundle to the "wrong" terminals on the back of the gauge. But I'm not certain about that. I guess the way to test would be to pull the gauge out of the cluster and connect it to the appropriate two pins on the wire header corrector, see what the gauge reads, then reverse the two wires and see if the gauge reads in reverse. For this to be most useful, you would want to do it with the tank either 3/4 or more full, or 1/4 or less.

 

If you're good with connectors, it might be possible to remove the two wires from the header that plugs into the cluster and switch their positions.

 

I don't think switching the leads at the sender would have any effect. The resistance would still be the same.

 

[Minuit]

A fuel gauge potentiometer (if you wanna be picky, a rheostat) is actually a very simple thing. It consists of a strip of resistive material and a conductive "wiper" that is free to move along this strip. On a Jeep sending unit, one end of that strip is connected to ground (note that I didn't say which end), current is passed through the wiper, through the resistive strip, to ground.

 

Depending on the fuel level, the position of the wiper along the resistive strip varies. The actual resistance between the ends of the potentiometer's resistive strip is a fixed number that depends on the design of the potentiometer. That number happens to be 88 on a Renix sending unit and 100 on an HO sending unit, keeping in mind that "0" for a HO sending unit is still 5 ohms to ground. The variable resistance is created by the position of the wiper along the strip.

 

Let's say you have half a tank of fuel, so your fuel sending unit is in the middle of its sweep. Let's also pretend that the resistance sweep is linear from full to empty. It might be, it might not be. The highest resistance to ground of an HO sending unit is 105 ohms, which occurs when the tank is empty. Half of 105 is 52.5, so let's say there is 52.5 ohms to ground.

 

- - 105 ohms

- -

- - 

- -                                                        

- -  <- wiper, connected to gauge. R = 52.5 ohms.

- -

- -

- -

Ground

 

The amount of resistance between the sense line and ground is indicated by the amount of dashed lines between the wiper and the ground terminal which I have arbitrarily placed at the bottom.

 

Here's the big difference between the Renix and HO senders, besides the difference in full range sweep. The ground is connected to the "empty" side on a Renix and the "full" side on an HO. What this means in physical terms is probably that there's a ground wire soldered to one side of the strip on an HO sender that's soldered to the opposite end of the strip on a Renix sender. It might be possible to switch this around. Electrically, it's as simple as that, but I don't know if there's enough room for you to desolder the ground wire from one end and solder it on the "wrong" end to correct the gauge polarity.

 

I don't know if switching the terminals on the gauge cluster will work. Never tried it. If I come across a broken gauge cluster I might try it.

 

Here's a way I know would work, assuming the potentiometer is designed in such a way that the resistance strip is accessible:

 

Wait until you're almost out of gas. Pull the fuel pump hanger assembly from the tank. You don't have to drop the tank or pull the drive shaft. Gently remove the potentiometer from the assembly and let it dry. One end of the potentiometer's resistance strip will be connected to ground. Use a multimeter to identify which end has continuity to the fuel pump ground wire on the 3-pin connector. If you're lucky, that ground is provided by a wire soldered to the potentiometer that you can desolder and solder to the other end. If you're super lucky, it's just a spade connector you can switch from one position to the other. That'd flip around the polarity of the gauge, so you'd have 105 ohms at full fuel and 5 ohms at empty. Still not quite what the gauge expects, but at least it's the right way around and it would be accurate-ish. Now, the all important question: can you actually do this? I have no idea! I want to say the HO fuel potentiometer has a pretty open design, so it's probably possible with only minor frustrations.

 

I'd hate to see someone do this to a real HO Comanche fuel pump assembly though.

 

To be honest, I'd probably just take the HO hanger out and sell it for big bucks to an HO Comanche owner and just get the correct part for your truck which, by the way, are available as new productions now from MTS Company, but that's just me. You could also buy a used Renix fuel pump hanger for your engine and tank, switch the float out with one of these, get a new Bosch fuel pump, and you'd be in business with an OEM part that's going to fit right, an accurate gas gauge, and all without hacking up an extremely rare and very expensive 2-year-only part. Even if you do commit murder on an HO sender, your gauge is still going to be off.

 

[Dickinson County Comanche]

4 hours ago, Minuit said:

A fuel gauge potentiometer (if you wanna be picky, a rheostat) is actually a very simple thing. It consists of a strip of resistive material and a conductive "wiper" that is free to move along this strip. On a Jeep sending unit, one end of that strip is connected to ground (note that I didn't say which end), current is passed through the wiper, through the resistive strip, to ground.

 

Depending on the fuel level, the position of the wiper along the resistive strip varies. The actual resistance between the ends of the potentiometer's resistive strip is a fixed number that depends on the design of the potentiometer. That number happens to be 88 on a Renix sending unit and 100 on an HO sending unit, keeping in mind that "0" for a HO sending unit is still 5 ohms to ground. The variable resistance is created by the position of the wiper along the strip.

 

Let's say you have half a tank of fuel, so your fuel sending unit is in the middle of its sweep. Let's also pretend that the resistance sweep is linear from full to empty. It might be, it might not be. The highest resistance to ground of an HO sending unit is 105 ohms, which occurs when the tank is empty. Half of 105 is 52.5, so let's say there is 52.5 ohms to ground.

 

- - 105 ohms

- -

- - 

- -                                                        

- -  <- wiper, connected to gauge. R = 52.5 ohms.

- -

- -

- -

Ground

 

The amount of resistance between the sense line and ground is indicated by the amount of dashed lines between the wiper and the ground terminal which I have arbitrarily placed at the bottom.

 

Here's the big difference between the Renix and HO senders, besides the difference in full range sweep. The ground is connected to the "empty" side on a Renix and the "full" side on an HO. What this means in physical terms is probably that there's a ground wire soldered to one side of the strip on an HO sender that's soldered to the opposite end of the strip on a Renix sender. It might be possible to switch this around. Electrically, it's as simple as that, but I don't know if there's enough room for you to desolder the ground wire from one end and solder it on the "wrong" end to correct the gauge polarity.

 

I don't know if switching the terminals on the gauge cluster will work. Never tried it. If I come across a broken gauge cluster I might try it.

 

Here's a way I know would work, assuming the potentiometer is designed in such a way that the resistance strip is accessible:

 

Wait until you're almost out of gas. Pull the fuel pump hanger assembly from the tank. You don't have to drop the tank or pull the drive shaft. Gently remove the potentiometer from the assembly and let it dry. One end of the potentiometer's resistance strip will be connected to ground. Use a multimeter to identify which end has continuity to the fuel pump ground wire on the 3-pin connector. If you're lucky, that ground is provided by a wire soldered to the potentiometer that you can desolder and solder to the other end. If you're super lucky, it's just a spade connector you can switch from one position to the other. That'd flip around the polarity of the gauge, so you'd have 105 ohms at full fuel and 5 ohms at empty. Still not quite what the gauge expects, but at least it's the right way around and it would be accurate-ish. Now, the all important question: can you actually do this? I have no idea! I want to say the HO fuel potentiometer has a pretty open design, so it's probably possible with only minor frustrations.

 

I'd hate to see someone do this to a real HO Comanche fuel pump assembly though.

 

To be honest, I'd probably just take the HO hanger out and sell it for big bucks to an HO Comanche owner and just get the correct part for your truck which, by the way, are available as new productions now from MTS Company, but that's just me. You could also buy a used Renix fuel pump hanger for your engine and tank, switch the float out with one of these, get a new Bosch fuel pump, and you'd be in business with an OEM part that's going to fit right, an accurate gas gauge, and all without hacking up an extremely rare and very expensive 2-year-only part. Even if you do commit murder on an HO sender, your gauge is still going to be off.

 

I bought this one from Rockauto. So I guess if anybody needs a HO sender, you should look there. I guess I’ll see about reversing the posts on the gauge. I don’t want to have to re-pull the pump. So I might just have to live with it being backwards.

[MiNi Beast]

Mines been backwards for years. The more I drive the more gas I get in the tank.  I just think of it as a anti theft detarent. 

or for $#!&amp;s and grins, load up some drunk fellas and tell them you need gas money for driving them home.