I am having a small problem rewiring my 1985 Jeep CJ7. I am putting in a set of aftermarket gauges and the fuel sender unit that is supplied is going to be a bear to install it. On the Jeep, my fuel pickup tube goes through the same part as the oem sending unit does. The aftermarket sender does not have this capability. So...I was planning on using the OEM sending unit with the aftermarket guage. When I hook everything up, the aftermarket gauge reads full with the sender in full and 3/4 full with the sender in empty. The guage is a 240-33 ohm. From what I can figure out, the sender reads from 73-10.

How can I get this to work? A resistor somewhere?
Thanks
Mike

I'm thinking you need to put resistance in the line under the dash... One of these lectric magicians will have to tell you how much... I cannot.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

Hi BJ,
Yah...I got that figured out. I went up to Radio Shack and bought some resistors. Without any resistors the gauge would fluxuate between 3/4 and full when moving the sender. A 150 ohm resistor moved the guage between empty and 1/4. A 47 ohm resistor moved it from 1/4 to 3/4 and a 270 ohm didn't move it at all.

I am thinking I need a smaller resistor.

Thanks
Mike

Seems like a 20/25 or so might getcha in there don't it.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Actually, I was thinking maybe a 30. I am hardly an electronics genius, but the 47 ohm resistor seemed to be 47% of the total I needed it to move. If I took 47% of 240-33, I get 112.80 and 15.51. This would be close to where the gage was reading (just over the 1/4 and around the 3/4 mark. If I take 31% of 240-33, I get something like 74.40-10.23 (I need 75-10). I figure 30% would be close enough. Hope this made sense.

That is how I came up with a 30 ohm resistor. I am probably completely wrong.

I also see there are variable resistors available. I might see if I can get something in my range (5-50 ohms???) Otherwise, I'll just pick up another selection or resistors in the 20-30 range this time.

Thanks for the reply.
Mike

Well the way I am reading this here, you have

a sender that spans 10 to 73 ohm
a gauge that was designed for a sender that spans 33 to 240 ohm (Roughly 3.3 times what you have)

If you just put a resistor in line (Series), all you are doing is summing the resistance, so you will never get the right reading over the whole range, you will only be able to set it up to be accurate at one specific tank level. What you really need is to multiply resistance. I am drawing a blank on how to resolve this issue, but then I am not a ôlectric magicianö.

Sean

Hmmmmm...that is something else to think about. The only thing....31% of 240 is close to 75 and 31% of 33 is close to 10. I figured I just had to cut the ohms down 31% across the board and it "should" work. I don't think I need to multiply the ohms. I tried a larger resistor and the gauge does not move at all then. The 47 ohm made the best improvement so far.

I've been searching around the internet for a couple of hours over the last two nights and I have found nothing on this.

Maybe it cannot be done....but, I am going to try some resistors in the 20-30 ohm range and see what happens.

I personally don't like wiring. I probably won't like working on my Jeep for the next day or two though. I am rewiring it with an EZ-Wiring kit. I hope it is EZ.
Mike

I gave up this stupid idea....LOL

No matter what resistor I put in the line, the gauge would not read correctly. I even went over to one of the electronic stores and when the counter guy gave me a blank look, I knew this was hopeless.

I finally took a new sender that I had on hand and took out the electronics. I then took out the electonics from the aftermarket sender (luckily it bolted together) and I am making one sender out of the two. I had to do a little machine work on the OEM sender housing to get a little clearance for the aftermarket guts, but it should work. I need to drill two holes and I should be good to go.
Mike

Well shoot, I should have said that in the first place. I just assumed since you didn't even mention it, you had already looked at combining the new electronics into the old sender, and it wasn't possible.... LOL

Good deal, that will be the better way to do it anyway, so the proper electronics will match.

Sean

And good luck getting the sweep to work from the bottom (full) to the top (empty).. I never had much luck getting a true 1/4-1/2-3/4-full reading.
But I always learned to interpret the gauge.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />