Alright I'll line up for some abuse & confess my total ignorance on all things electronic. My son is coming down from college this weekend to help me with the truck - our project this time is to get the Fuel Gauge to work.

We've got a 1950 F1 with an EZwire kit. The Fuel tank is a 1970 Mustang tank. The Gauge was kind of "working" before - registered half of what was actually in the tank - i.e. never showed over half full. We've got Autometer "Classic" gauges.

The Gauge shows nothing currently as my friendly garage cut the wire
labelled "third brake light" when they were fixing the turn indicator lights.
That wire actually went to our fuel gauge. We spliced the wires back together & got nothing... hmmmm...

How would you tackle this challenge? What steps to take to hunt down the problem? We have a Multimeter but have no clue how to use it - picture attached. Do we need to have bare wires for the multimeter to read whether it's hot? What do you turn the dial to? Looking for kind electronic problem solving lessons....

I would guess once we get the wires to work - we'll need to put some sort of "restrictor" in place to get the sender to talk correctly to the gauge...

Thx
Ben in Austin
1950 F1 (351W/AOD)

 

This may be helpful.

How to use Innova Multimeters - YouTube

Also, if you got a User Manual with your meter, it will be helpful to understand where the leads go for taking different types of readings (ohms / volts DC / volts AC / amps).

Always set the range to higher than expected reading to start, and work your way down to a lower scale if desired.

 

My first question is - Why is the wire labled "third brake light" if it's actually providing power or signal for your fuel gauge?

Step one - identify the purpose of the object you're trying to troubleshoot.
Step two - identify what it needs to do it's job.
Step three - verify what it actually has versus what it's supposed to have, locate the problem.
Step four - follow that back and find where the issue has occured.

Ok so step one, your fuel gauge uses a signal from the sender in your tank to display your current fuel level using a float system not unlike the one in your toilet tank. It needs power and ground to light the gauge, and it needs the sender signal in order to discern what the level of fuel is.

Step three - verify that you have 12v at the power terminal of your fuel gauge and ground at the ground terminal. If you have power and ground at the the gauge, the last thing is the sender signal. The gauge is looking for a resistance range, as the float moves up and down, the reistance changes, and tells the gauge what to read accordingly. You appear to have a mismatch, where your gauge has a slightly difference range of resistance than your sender is expecting, which could be why your gauge is only reading half full or less. OR your sender could be faulty...

Either way, you need to test the actual signal coming from the sender. You do this by measuring resistance in "ohms", the symbol on the multimeter looks kinda like an upside down horse shoe. I am sure there are some guys that could tell you what that sender should be reading off the top of their head. Since that fuel gauge is aftermarket, you should be able to research the specs on that gauge pretty easy. That way you'll know what the gauge wants to see.

 

In most cases the tank sending unit and gauge must be matched or at least compatible. Different sending units use different ohm's to send the signal to the gauge. You may have to replace the mustang sending unit with a Autometer sender.

Was'nt there a "fuel sender" wire in the EZ wire kit? Why use the third brake light wire? The third brake light wire will only be hot when you step on the brake. Try stepping on the brake and see if the fuel gauge works.

 

Here's the owners manual for yours if you don't have it.

http://www.equus.com/Content/Support/Manual/3300.pdf

I would start at the fuse box to make sure your fuse isnt blown from coming into contact with bare metal when the truck was on. Then i would check the splice to make sure its good, maybe cut it off again and redo to make sure its good, then i would move to the tank to make sure your guage is getting power. This would be done by putting the red needle on the positive connector (has to be the actual wire or metal on the connection) then put the black on a good ground like the frame bolt or something near. Put your multimeter on DCV and 20 and if your not getting a reading then I would work my way up to the gauge. If your getting a good reading (around 12) I would say bad sender or guage, don't know how to help with that one.

As far as the "restrictor" part I have no clue there, just now how to use the multimeter.

 

Joe - I went through the video so my knowledge level is now slightly above zero. I'll search on-line & find the manual to this thing.
I do see the Ohm section on the meter...

Unfortunately I went with local garage "experts" on the EZ Wire installation & now I'm trouble shooting. I could take it back to them but they are the ones who did it wrong in the first place - I'd prefer to fix it myself & learn. I don't know "why" it was done the way it is.

JD/53fopar/Joe - When checking the Ohms of the sender wire - let me ask a stupid question - Can I just check it from the outside of plastic coated wire or do I need to put the multimeter lead on bare wire?

I'll check to see if the gas gauge moves when I put the brake on - I don't believe it does. Thanks guys.

Ben in Austin

 

Ben,

As 53Fopar said, the gauge and sender have to be compatible. If you are running your stock gauge on a newer sending unit, they don't match up as you discovered. One is I believe 0-30 ohms and the other is 0-90 ohms. Voltage, 6 vs. 12, can be an issue also.

OK, looking at my Classic Hauler catalog, they have a stock 6v only unit (which looks like 0-30 ohms) and a 12v unit w/stock tank unit. The 12v version is 14-70 ohms, where I believe most newer sending units (probably which would explain your reading being 50% off).

On my old Chevy, you could test them. First, disconnect the wire from the sender and the gauge should go to over full (open resistance). Then, if you grounded the wire, the gauge would go to empty. This would at least tell you that the gauge is good.

To test the sender itself, you can pull it out of the tank and connect your ohmmeter wires - one to the sender stud and the other to it's base (ground). If the sender is working, the resistance should change as you cycle the float arm through it's range of motion. If working properly, it should range from 12-83 ohms per a mustang forum.

 

Yes, the multimeter probe does need to touch the copper in the wire, not just the insulation.

 

Thanks HKYswim - That gives me a place to start. It's always challenging to solve the mystery of why someone else wired it a certain way. I know it doesn't work so now dig until we get it working..

This is a picture of the deal we used to splice the third brake light wire
back together - Is this the way you guys would do a splice? Or would you do the screw cap thing or solder it?

Ben in Austin

(I'm not typically this dense - I've done mechanical things just not electrical - thought I'd take the opportunity for some on the job training).

 

Usually those splices work fine. With the varying resistance of the gauge, they could add a bit of resistance, but I doubt enough to notice. Solder is generally best, but the crimp on splices are easy and fast.

 

Thanks Dmack
I appreciate the help - I've always found electrical stuff to be frustrating but I've decided to tackle some to get my knowledge level up a bit - tired of using shops. Too many fingers in the pie...

Let me digest the info you posted & look into the ohm range of the Autometer after market guages we have used to see if they will speak with the sending unit we have.

The gas gauge doesn't move when I press on the brake. It does move a bit when turning the key - comes up off of dead empty but not much.

Ben in Austin
1950 F1 (351W/AOD)

 

The spike when turning on the key is probably just that, a spike when power is first turned on. I would check your splice, but probably just as easy to check the whole circuit. Check for resistance at the stud of the sender to ground - if there are ohms registering, move up the line. Then check the wire from the sender at your splice - it should have the same reading as at the stud. Then probe the wire from the other side of the splice (that leads to the gauge). Again, it should read the same. Then check the end of the wire at the gauge. Same thing. If you start with a reading at the sender, wherever you lose the reading is where the open circuit is. If you have no ohms at the sender, that is probably your problem.

I guess I should mention that the easiest way is if you have an alligator clip type jumper wire, connect it to the sender stud and the red multimeter probe, then move up the circuit with the black probe and check readings.

 

Ben,

Those splices are OK for a quick fix, but the spade inside does actually cut into the insulation of both wires to connect them together. Basically the wires have a lousy connection and you will have exposed the copper conductors to the elements. I don't use these....in fact, I usually throw them away.

You will find these at a U-haul store....the rental trailer guys use them to cut into your wiring so that they can get you out the door with the rental trailer brake lights working. They don't care that your brake lights stop working gradually over the next couple of years!

I usually make my splices by soldering and then putting heat shrink tubing with sealant over the soldered joint.

There are some crimp connectors on the market now that are waterproof. You crimp as usual, then use a heat gun....the outer jacket is heat shrink tubing with sealant that acts like hot glue...so strip, splice, heat it up, and viola....a sealed connection that won't corrode by this time next year.

Dan

 

Thanks Dan - That's the basics that I need to know - I assume
Autozone might have these heat shrink wire connectors? Being lazy - that's the closest parts place to me - Napa is 10 miles.

Ben in Austin

 

You may want to read the later parts of this thread as it discusses the different "protocols" used between the sending unit and the gauge.

https://www.ford-trucks.com/forums/1...correctly.html