Fuel gauge and IVR diagnosis

[LTDScott]

I posted this in the LTD specific section here, but the system is the same as Mustangs and I'd like to get some more opinions.

For the last few years the fuel level gauge on the dash of my '85 LTD LX has been reading about 1/4 tank low, meaning when it's full it reads in between 3/4 and full. More recently, the gauge has stopped working intermittently, and sometimes is just slow to respond. I got some suggestions that the issue is likely the fuel level sender in the tank or the instrument voltage regulator (IVR) on the back of the instrument cluster. The LTD doesn't have a factory volt meter or oil pressure gauge, so only the fuel level gauge uses the IVR and I can't use the action of all of the gauges to narrow down the problem.

I set out on doing some diagnostic tests to narrow down the problem. Here's what I've done so far, from the beginning.

-With an estimated 1/3 of a tank of gas (based on gallons added and miles driven) and the original IVR, the gauge displayed a tick under 1/4 full. This is consistent with what I've seen normally - the gauge reads about 1/4 tank low.

-I grounded the fuel sender wire (yellow w/ white stripe) right near the tank which resulted in the gauge pegging full with the key on. This confirms the wiring, gauge, and most likely IVR are good.

-Removed the IVR from the cluster and bench tested by connecting it to good power and ground, and was able to see the IVR output pulse on and off as it's supposed to. No way for me to measure the average voltage, but at least I can see it's not stuck constantly on or off.

-Measured the resistance between the fuel sender wire and ground with an estimated 1/3 of a tank of gas in order to see what resistance the fuel level sender (and associated wiring) has. I tried a few times and got varying results between 60-70 ohms. The EVTM says 60-86 ohms = empty and 8-12 ohms = full, so in that case I'd expect to see something like 40-50 ohms with 1/3 tank of fuel.

-Measured the resistance of the gauge itself. I don't know what the spec is, but in the course of investigating this I found a classic Mustang site that talked about diagnosing the IVR (same exact system as my car has) and they said that all gauges should measure about 13 ohms. Mine was 12.5 ohms, so I believe it's good.

-At that point I decided to buy a new solid state IVR just to see what happened. First I bench tested it and it output a consistent 5.02 volts, so it appears to be good. I then installed it in the cluster and reconnected the cluster to the car. The gauge needle just barely moved from the pegged at the bottom (way below empty) to just below empty. I thought that was odd, so I measured the output of the IVR while it was connected to the cluster and still got 5.02 volts. Just to eliminate bad ground issues, I connected the case of the IVR to a good ground using alligator clips and the voltage still showed 5.02 volts. So that tells me that the IVR, ground, and related wiring to/from it are good.

Out of all of these tests, the only result that seems wrong is the resistance reading through the fuel level sender, so I think it's bad but that doesn't explain why the gauge showed a bit less than 1/4 full with the original IVR. My best guess is that the sender in the tank is bad and the original IVR is outputting something other than an average of 5 volts? So replacing the IVR fixed one issue, and now the gauge is showing empty based on the incorrect resistance reading from the sender. Thoughts? I really don't want to drop the gas tank but I already bought an NOS fuel pump assembly and everything else checks out, so I think it's the likely cause of problems.

[homer302]

You asked for thoughts so that is exactly what I am giving, my THOUGHTS. I don't have any knowledge of what the sender should read on a meter. I appreciate all the troubleshooting you have done. It sounds like you are very smart. I totally understand not just throwing money at something. Judging from your sig, this is a pretty nice car and you have spent at least a decent amount on it. The fuel level sender is over 30 years old. Why not just replace it? They are less then $100 aren't they? I put one in my GT Turbo last year in 40 minutes start to finish. Don't you have more than 40 minutes in just testing alone at this point. What is your time worth? Maybe the LTD has an "un-obtanium" sender that is some kind of diamond and impossible to find. I just THOUGHT (again, just my thought) since it was a Fox it would be a part that you could get.

Yes before you say it, people like you who actually will spend the time and effort to take something apart and put the meter to it are rare and that is honestly cool. I seriously DO understand your desire to determine a root cause. It's just to me at some point, if the part is cheap enough and easy enough to install I just do it. I guess maybe I am older. Good luck on your quest. I hope you solve it soon!

[LTDScott]

Because dropping the tank with a panhard bar in place is a pain in the ass.

[conmech]

Hey Scott. .....
Question regarding the float.....
Might need to see if any gas made its way inside, end result could be weighing the float down.....throwing your reading off. Seen it before... in brass floats as well as plastic. Best of luck.

[Tigger]

My 67 has the same problem. Is has never read full since I've had it. It's has not bothered me enough to fix it as I'm more concerned on it reading properly when it's empty. About 10 years ago it stopped working all together. I bought a repro sending unit thinking the original one was toast. When I took it out I found out it was just the float that went bad. In comparing the original unit to the repro, the repro looked cheap. The float was plastic and looked crappy. I dug thru my pinto part stash and found a good float and put the original one back in. The float didn't fix the problem. It still reads a hair above 3/4 when full. I'm kind of thinking the circuit board is bad at the full end. Either that or the gauge is out of calibration.

My wife's explorer had the little circuit board go bad in the instrument cluster. It's a little card that plugs in(I don't remember what they are called ). It caused the gauge not to read at all but maybe your LTD has something similar that is causing the gauge not to read full? I know the SN95's have them so maybe the fox body's do too. I have not had the cluster out on my 86 yet so I'm not sure.

Good luck to you.

[KevinK]

Typically, there is a circuit board in the fuel sending unit that gives the variable output resistance. Sometime you can get lucky and find a similar unit to swap it out the board. The float should be checked but really check the resistance when you have the unit out. Make sure you get the full range. If the sending unit is hard to find, there are companies who rebuild them. Look for gas gauge repairs.

[homer302]

My wife's explorer had the little circuit board go bad in the instrument cluster. It's a little card that plugs in(I don't remember what they are called ). It caused the gauge not to read at all but maybe your LTD has something similar that is causing the gauge not to read full? I know the SN95's have them so maybe the fox body's do too. I have not had the cluster out on my 86 yet so I'm not sure.

Good luck to you.

That card is called an "Anti-Slosh" module. The SN95 Mustangs had them to and I replaced one in mine. I have the old one in the box still somewhere because my issue turned out to be something in the charging system instead. My understanding was the main function of it was to keep the needle more steady and not to jump up and down as the gasoline "sloshes" around in the tank while you drive. Apparently, you discovered it has more function then just that.

[Tigger]

Typically, there is a circuit board in the fuel sending unit that gives the variable output resistance. Sometime you can get lucky and find a similar unit to swap it out the board. The float should be checked but really check the resistance when you have the unit out. Make sure you get the full range. If the sending unit is hard to find, there are companies who rebuild them. Look for gas gauge repairs.

That is what I think is wrong with my 67 coupe. I think the full end of my circuit board is not good.

I followed this from JACOOK when I dropped the tank on my Saleen. http://www.foureyedpride.com/content...-Sender-Repair

I wanted to swap out the noisy fuel pump. While I was there I changed the circuit board on the sending unit because before I parked the car the fuel gauge would randomly peg way past full. That and the low fuel light would come on at 3/8 of a tank instead of at the 1/8th like it used too. I had no problems doing the repair per the above instructions. I have not really driven the car since so I am not sure if it solved the pegging problem but my low fuel light is not on anymore and it currently has a 1/4 tank.

That card is called an "Anit-Slosh" module. The SN95 Mustangs had them to and I replaced one in mine. I have the old one in the box still somewhere because my issue turned out to be something in the charging system instead. My understanding was the main function of it was to keep the needle more steady and not to jump up and down as the gasoline "sloshes" around in the tank while you drive. Apparently, you discovered it has more function then just that.

Thanks, that is what it is called. Yep when it went out, the fuel gauge did not read. I Thought it was the sending unit at first. After reading about it and doing some simple checks, I pin pointed it was the board. Do you know if the foxbody have them as well? Just curious.....

[homer302]

Thanks, that is what it is called. Yep when it went out, the fuel gauge did not read. I Thought it was the sending unit at first. After reading about it and doing some simple checks, I pin pointed it was the board. Do you know if the foxbody have them as well? Just curious.....

They do not have them. I guess our needles "slosh" around, haha, though I have never noticed.

[LTDScott]

As others have said, the early Fox cars didn't have the anti-slosh module, but I believe later ones did. It's a pretty simple system - fuel level sender in the tank send, IVR, and gauge - that's it.

I never thought about the float being bad, but I suppose it's possible. The LTD uses a module where the pump and fuel level sender are in one unit so I bought a NOS Ford unit before I even learned about the IVR. Once I learned about the IVR, I decided to replace it, but without much success.

I've been avoiding trying to drop the tank, but I think I have to bite the bullet and figure it out.