Thought I'd start a new thread on my BigBeast rebuild, as the old one was getting kinda long.

I've finished rebuilding the engine, but it won't start (runs on a dual fuel system, with only lpg for the last fifteen years); so I decided to resurrect the petrol system (EFI) and see if that works.

My current problem is the fuel gauge. I can't remember if it ever has worked...but it's not working now. I've put a new sender in the tank: resistance reads 90 ohms empty, 60 ohms mid range, 30 ohms full. Ground from sender is good, and resistance of circuit from sender (immediately downstream of resistor) to IVR (instrument voltage regulator) is just 0.8 of an ohm. Gauge reads full when grounded. When connected to sender, gauge needle moves about an eighth of an inch from empty at mid range of sender, and about the same again when sender at full. Total movement brings it just a little past the empty mark.

I'm thinking it has to be the IVR; however the other two gauges fed by it (oil and temperature) were functioning normally before the truck broke down, as far as I recall.

Any thoughts, fellas?

 

All I can add is the link to the EVTM, Hopefully you have that already,
https://www.garysgaragemahal.com/fue...auges-gas.html

 

30 ohms is too high for full. Officially it's supposed to be 10, but anything in that neighborhood should work. Empty is officially 73 ohms, but 90 is close enough to work.

If you want to check the wiring and the gauge, go back to the sender and take the sending wire off it and ground it. The fuel gauge should swing to full. Then take the wire off and let it hang in the air. The gauge should swing to empty. If that works, the wiring and the gauge are ok.

 

Dave I think he said the gauge goes to full when grounded but we dont know if it goes to empty when not hooked up or was that "little bit below empty? with the wire off?
It sound like the sender is not working as it should with the ohm readings Dave gave.
Also note with the readings Dave gave the halfway in ohms is not half on the gauge, most are like this so dont think that way only look at empty & full based on the gallons or liters in the tank.

If it is easier to pull the bed then go for it. It is not so easy with a flare side as you dont have a inner bed rail lip to hook anything to for lifting, only on the outside and you can end up pulling the sides of the bed in.

On mine I saw when both tanks were bone dry the needle was just below the empty line.
With 5 gallons in each tank it put the needle on the empty line. So I know when the needle on the line I should have 5 gallons left in each tank.
With tanks filled the needle is above the full line and that is ok with me. As long as the needle reads right at the empty side I am good with that as I have run out of gas with a gauge showing 1/4 tank
After the 3 time I fixed it and never a problem again.
Dave -----

 

Doing the ground, ungrounded test, the gauge needle should move full scale. I should not have said "full". And this fuel gauge is a general indicator, like the other gauges. It is not known to be super accurate, but gives you a general ideal how much fuel you have left.

 

Hi, guys. Did all you suggested...nothing worked.

I have a pulsing current of around five volts at the end of the circuit before it connects to the sender, so I'm guessing the IVR is okay. As far as I can see, there's nothing left to check. Five pulsing volts is clearly going through the gauge to the sender, where it meets a resistance ranging from 90 ohms down to 30 ohms, before going to ground. This does produce movement of the needle in the gauge, graduated as you would expect according to the change in resistance as the sender goes down the resistor range...except that said movement is much reduced, barely getting past the empty mark on the dial.

So what's going on? How do these gauges actually work, anyway? I'm not much on physics, but I would guess that less resistance would mean a greater flow of electricity through the gauge. So does that mean that this particular gauge is less sensitive in some way, even though it went very quickly to full when grounded? Would the higher low of my sender (30 ohms instead of 10 ohms) account for my gauge needle not even reaching quarter full?

I've heard you can get universal gauges with an adjustable signal...anybody tried that?

 

We told you in a previous post, 30 ohms is too high. It should be closer to 10 or less. That is why you are not getting enough movement in the gauge. If you short that wire that has the pulsing 5v to ground, then the gauge should move full scale past full. That would prove the gauge is capable of moving to full or beyond, and you have a faulty sending unit. You may be able to take the sending unit out and simply bend the arm to get 10 ohms out of if when it's resting on the upper stop.

 

Seems to be some issue with communication here. I've said three times now that I've already tested the gauge by grounding it. Also I realise that 30 ohms is too high (thanks for the info there)...I'm just wondering if something else is involved also, as it seems to me that a 20 ohms difference should still have the gauge up to at least three quarters full, rather than just past the empty mark.

I've actually checked the voltage output after putting a new battery into my volt meter, and now I find that the pulses are sometimes significantly less than five volts, so I'm thinking it could be the IVR after all.

 

Nothing else involved. You have proven the gauge and the wiring to the gauge works. If you happen to have a problem with the IVR like you said, then that is going to make ALL the gauges vary and not perform correctly, it feeds all of them. Just get the sending unit to read closer to 10 ohms and you should be good.

 

Couple things to think about. How are you measuring the resistance? Directly at the sender? At the circuit including wiring? What is the resistance of your test leads? A poor connection anywhere in the circuit can raise resistance. With the sender in the empty position if you are reading high resistance your guage should approximate the ratio of delta R over total R. Won't exactly be linear with tank level as the sender float swings an arc. You get less swing per inch of tank level full than when empty where the last inch of level gives much more arc swing thereby changing the R more when closer to E.

 

Sorry, fellas, been away for a bit and haven't checked my messages.

Dave, the float arm on my sender already takes the resistor mechanism to full: I can't see how bending the arm would therefore decrease the resistance any further.

BigBlue2, I don't understand what "ratio of delta R over total R" is...too technical for me, I'm afraid.

Tell you where I am at the moment. I'd already ordered in a new IVR, so I thought I might as well put it in. It did make a difference. The fuel gauge now reads half full instead of empty when the sender arm is in the full position, so that's an improvement...but I don't get why it doesn't now go to full.

Got to thinking about that. Anybody know the electrical conductivity of petrol? I was just wondering if you tested these senders dry, or immersed in a bucket of petrol? Would the petrol bring the resistance between the contacts down from 30 ohms to the desired 10 ohms?

Another problem with my instrument cluster: I replaced all the blade type light bulbs in the printed circuit board (the instruments were always hard to read at night), but the lighting now is if anything even dimmer. I'm thinking now that special bulbs might be required instead of the standard 12 volt bulbs I installed, as the output from the IVR to the bulbs is a pulsing 5.5 volts?

 

Ok think about this for a minute.
I bet you are checking the resistance when the sender is on the bench and it reads right at both ends right?
Now lets say the arm got bent when being stuffed in the box so now when it is installed in the tank it will not read right when full.
If the arm was bent up this would lower where the rub arm is on the resister and read a lower level.
So Dave is saying to bend the arm down so the rub arm runs higher on the resister with the tank full. Just remember when the arm is bent to rear right at full it can change the empty level.
I would want the gauge to rear good when empty and so what if it is a little off at full.

I am not going back over posts for the "delta R" as I dont need to know what it was all about

Dont know why the IVR changed the gas gauge did it change the other gauges too? If they read right did you ever thing that the fuel gauge is bad?
If you have the cluster out swap the fuel for 1 of the others and see if it reads different.

Dash lights, try and turn the switch back & forth a few times and see if that changes it. Did you remove the bulbs and clean them? The colored covers can also kill the light.
My fix was to remove the covers and replace the bulbs with colored, green, LED's and I am happy with the light out put.
Dave ----

 

Quote...
I'm thinking now that special bulbs might be required instead of the standard 12 volt bulbs I installed, as the output from the IVR to the bulbs is a pulsing 5.5 volts?

The IVR does not power the lights, just the gauges.
I just took off the blue covers and used stock bulbs.
Good luck, Jim

 

Wet or dry makes no difference. I've calibrated many fuel tank senders dry and they performed the same once installed and submerged in fuel.

 

Calibrating a sender is a two-stage process.

First, you calibrate the sender electrically to the gauge. I've never worried about the specific resistance of the sender. I'll typically use an ohmmeter to test for a smooth sweep of the sender, but didn't worry about the actual values as long as the gauge responds properly. I simply connect the sender to the vehicle harness via some test leads, to mimic it being installed. Turn on the ignition to power the circuit. Move the arm to the low position and let it sit for a minute or so, as the gauge is deliberately slow-acting to dampen normal oscillations. Most senders have a little tab you bend to adjust the range of travel. If so equipped, adjust the tab so the gauge reads E. Next, move the arm to the high position and let it sit for another minute. Make sure the gauge shows F, and adjust the tab as needed. Just remember, when making any electrical adjustments, give the gauge a minute to respond.

The next step is to calibrate the sender physically to the tank. During this step, you bend the arm but must be careful not to disturb the electrical calibration in the previous step. If done properly, the float will rest just a hair above the tank bottom when the sender travel is all the way to the E position. If the arm is too long, you may have to make an S bend to shorten it. I've found it easier to make a mock-up on a sheet of cardboard showing the depth of the tank. If the tank has an internal baffle, make sure the arm is well clear of it. Once you've got the arm physically adjusted to the bottom of the tank, there's not much else you can do to calibrate F. The indication may be off a little at higher levels, but it's only critical when the tank is nearly empty.