Fuel tank electrical question

unibodynewbie · Post by **unibodynewbie** » October 23, 2011, 6:25 pm

I am trying to hook up my sending unit to my fuel gauge. I know that I have power coming into the truck and to the gauge. I have a wire running to the sending unit. I connected the power to sending unit wire and to the gauge, but nothing happened. Can anyone offer some help?

Thanks.

Shane

Toyz · Post by **Toyz** » October 23, 2011, 6:40 pm

"Normal" gauge setup has a wire from terminal on gauge (may be labeled "s" or similiar) to the grounded sender. Ignition "power" would only be provided to the gauge itself, not directly to the sender.
There's plenty of hook-up diagrams and info available through on-line searching.
Paul

unibodynewbie · Post by **unibodynewbie** » October 23, 2011, 6:45 pm

I have installed a Mustang fuel tank and I have a new wiring harness by Rebel Wire. So gonna be different I think. Sorry should have said that in the first post. So I have a wire going to the sender. Should that wire have power? I know the power should go to the gauge, but how do I transfer the "signal from the tank level" to the gauge?

Uncle Skip · Post by **Uncle Skip** » October 23, 2011, 6:54 pm

You do not hook up a positive battery feed to the fuel sender.
The fuel sender is a ground.
The sending unit in the tank is a variable resistor and changes the ohms as the float goes up and down, which is recorded at the gauge.
Some gauges require a lower voltage at the + terminal. If you're using a stock gauge, the 12 v. power goes thru the little square resistor under the speedometer cable hookup and sends (I think) 4 volts to the gauge.

Battery 12v+ to resistor 4v - out to the gauge. The other wire out from the gauge goes to the sending unit on the tank which is ultimately a ground.

unibodynewbie · Post by **unibodynewbie** » October 23, 2011, 6:58 pm

Kool thanks.

bobenhotep · Post by **bobenhotep** » October 23, 2011, 7:00 pm

you got it? If not you got my #

Toyz · Post by **Toyz** » October 23, 2011, 7:14 pm

As far as "power" to the gauge, you should "see" some voltage from the gauge as Skip explained. He also made a good point as to the factory gauge, if that's what you are using. It should not have a full 12 volts under any condition. The voltage is regulated by the instrument panel voltage regulator.
You should be able to test with a voltmeter positive lead; negative lead grounded, and find the gauge output terminal voltage to be close to what is shown at the power wire to the gauge, after the regulator.
That brings us to the sender in tank.
You need to assure that the sender assembly is well grounded externally.
Do you know the history of the Mustang tank?
The sender resistance may not be a match for your factory gauge.
I know this all sounds complicated; it may be to your advantage to find a good schematic to guide you through this.
Also, is your replacement harness specific to your truck? Sometimes aftermarket harnesses may be incompatible with your particular set-up.

Paul

unibodynewbie · Post by **unibodynewbie** » October 23, 2011, 7:15 pm

Checked to see if I get continuity between the wires from the little transformer and all is well. Getting power to the transformer. Took an alligator clip from the gauge to the wire to the tank. Nothing. I guess the gauge could be bad too right?

The wiring harness is fine. The tank is brand new as well as the sender. I'm sure it's easy. I just need to make sure I have it grounded.

Toyz · Post by **Toyz** » October 23, 2011, 7:19 pm

Make sure the sender is well grounded when checking. Run the float arm through it's travel; you should see voltage variation (change in gauge reading). You may have damaged the sender with the full 12v applied.

Paul

Toyz · Post by **Toyz** » October 23, 2011, 7:33 pm

I am not sure what you are referring to on your continuity test.
Check for power at the gauge + terminal with a voltmeter or testlight. If you have voltage there, ground the gauge terminal going to the tank with your jumper.
You should see gauge movement.
If you don't you will need to check actual voltage at that terminal when ignition is on. If you don't find ~ 4 v, and you still have ~4v coming into gauge, it's probably fried.
Paul

bobenhotep · Post by **bobenhotep** » October 23, 2011, 7:37 pm

hook your multimeter between the post that the wire hooks to on the sender, and the grounding spade on the sender, or whatever the sender has to hook a ground to. set meter to ohms. what does it say?

BarnieTrk · Post by **BarnieTrk** » October 26, 2011, 2:48 pm

Toyz wrote:I am not sure what you are referring to on your continuity test.
Check for power at the gauge + terminal with a voltmeter or testlight. If you have voltage there, ground the gauge terminal going to the tank with your jumper.
You should see gauge movement.
If you don't you will need to check actual voltage at that terminal when ignition is on. If you don't find ~ 4 v, and you still have ~4v coming into gauge, it's probably fried.
Paul

Can he be simply hooked up to the wrong terminal at the gauge?
Isn't there a right/wrong terminal to bring the ~4volt wire into the gauge as well as a right/wrong terminal for the ground wire coming from the sender to the gauge?

BarnieTrk

TxSlick66 · Post by **TxSlick66** » October 26, 2011, 8:14 pm

EAsy way to test gauge... The Constant Voltage Unit under the dash should be sending a pulsed voltage of about 5 volts. Hook that to the + side of the gauge. Take a wire from the other side of the gauge (temporarily) and ground that wire, momentarily only - do not allow the gauge to peg out to max. If the gauge moves, you know the gauge works. Then the wire you were holding to ground gets run to the sender. The sender acts as a variable ground. The more fuel in the tank, the better the ground.

To test the sender in the tank, hook an ohmeter between the sender wire and chassis ground. You should see about 75 ohms when the tank is full (float at top of stroke) and 10 ohms when empty (float at bottom of stroke) If you get those readings and the gauge moves when tested, it should work...
UNLESS...
Alot of the aftermarket gauges are set up like GM gauges to work on reverse reading of the Ford senders. If you got one of those, then you need to swap the sending unit for a GM type.

Toyz · Post by **Toyz** » October 26, 2011, 9:05 pm

TxSlick66 wrote:EAsy way to test gauge... The Constant Voltage Unit under the dash should be sending a pulsed voltage of about 5 volts. Hook that to the + side of the gauge. Take a wire from the other side of the gauge (temporarily) and ground that wire, momentarily only - do not allow the gauge to peg out to max. If the gauge moves, you know the gauge works. Then the wire you were holding to ground gets run to the sender. The sender acts as a variable ground. The more fuel in the tank, the better the ground.

To test the sender in the tank, hook an ohmeter between the sender wire and chassis ground. You should see about 75 ohms when the tank is full (float at top of stroke) and 10 ohms when empty (float at bottom of stroke) If you get those readings and the gauge moves when tested, it should work...
UNLESS...
Alot of the aftermarket gauges are set up like GM gauges to work on reverse reading of the Ford senders. If you got one of those, then you need to swap the sending unit for a GM type.

Just to clarify, when testing the sender, hook the ohmeter between the sender Terminal (without the wire) and the sender ground lug. If this gives a reading in the listed resistance range, then re-connect the ground terminal wire and go to chassis ground as stated. Often the ground circuit from the sender to chassis ground is incomplete or otherwise sporadic. If the two readings vary, tie an additional ground wire into the sender body to ground circuit and secure it. I believe you will see LESS resistance on a full tank for the reason stated, thus the movement to full with a direct ground.
12 volts sent to the sender gauge terminal more than likely damaged the sender if it was in fact grounded at the time. 12v sent to the ungrounded gauge MAY not have had a detrimental effect. If the gauge voltage found it's way to ground, it is probably damaged, too.
I can not remember the early slick's OEM gauge terminal configuration; (I've got Dakota Digital VHX), but can check later this week on the old gauge assembly if needed.
Also, this is assuming a factory gauge. If you have an aftermarket gauge as mentioned above, tying it into the instrument regulator curcuit does not apply. Since you have not stated the year of Mustang tank used, the 73-10 ohm resistance readings also may not apply to your application.
My concern about your replacement harness was in regard to the resistance wire in the OEM ignition circuit. If it is not present with the OEM ignition system, the distributor points will have a considerably shorter life span.
Paul

Uncle Skip · Post by **Uncle Skip** » October 26, 2011, 9:08 pm

Ohms not volts.
To check the sender you put your multimeter on the terminal of the sender and the other probe on the flange and check the ohms as you move the float up and down.
The gauge is the same thing as a ohm meter. I can't remember the ohm range resistance to ground for the Ford gauge, but it's pretty low. Like 7-20 ohms or something. I'll check for you.

Toyz · Post by **Toyz** » October 26, 2011, 9:17 pm

TXSlick is correct on the resistance range on the sender for our slicks and the early Mustangs. I am not sure when the later change in resistance took place.
I am not sure what the resistance across the gauge itself might be, but I would expect it to be extremely low.
Paul

TxSlick66 · Post by **TxSlick66** » October 26, 2011, 10:06 pm

Toyz... Good point about checking resistance from the ground lug on the gauge sender first, then checking to chassis ground. If they are different, you then need a ground wire to the sender.

Alan Mclennan · Post by **Alan Mclennan** » October 27, 2011, 1:13 am

While we are on Fuel gauges, can I run my fuel gauge from my LPG sender?, the gauge is a Autometer fuel and is by memory the 73/10ohms one.

Toyz · Post by **Toyz** » October 27, 2011, 5:05 am

Alan,
The geographic opportunities provided by your quest overwhelmed my DSHG. I should know better than to leave these Gen 1 models on Autostart

.
OK, back to facts:
As long as the sender matches the gauge, no problem.
Are you running dual fuel, and wanting the Autometer gauge to monitor petrol as well?
I believe the Autometer site lists the correct resistance range for each gauge model number.
You can check the sender as indicated in earlier posts. Be aware there are some 10/73 units which will read backwards, even Down under.

(Dam thing Autostarted again!)
Paul

Alan Mclennan · Post by **Alan Mclennan** » October 27, 2011, 3:29 pm

Geez Paul!, you talk Prudy!

, I`m running it on straight gas, I`ll have to check the sender unit as you suggested will be a lot easier than topping up the 200ltr tank every now and then!