Another gas gauge/ speedo question.

[Jimartina]

1977 c10 Scottsdale 350, 4brl, th350 automatic.
Gauge pegs at over full
Ok with that said. I have replaced the sending unit. Tested the gauge and the wiring. Everything seems ok. I get 12.7 volts on the left side connector behind the fuel gauge. Like 6is volts on the right side connector and 0 volts on the bottom. From my research it seems that the sending unit is giving me reading but the gauge pegs over full every time I turn the key on. I've tested the fuse link on the back of the fauge and it tested good.
Most of my other gauges work, dash lights and all signals work, as well as dimmer switch.

Speedo was working at highway speed and suddenly stopped. Used drill to turn speedo and it moves like it should. Bullet and gear appear to be ok. I can see the gear inside the output tail on trans and it appears ok, (can't really tell for sure)  before I take the tail piece and drive shafts off do I need to do something else?

Thanks in advance everyone.

[Big Dee]

 
Is the gas tank full ? that would make the gauge read full.

Have you serviced the cable. a drop of oil can cure a gallon of problems.
 

[bd]

The fuel gauge probably has an open somewhere in the sender circuit.  With the ignition switched on, jump the fuel gauge sender pin directly to ground.  The gauge should rapidly drop to Empty.  Check the PCB-to-gauge connections and the I/P cluster harness-to-PCB connections.  Burnish the copper foil and terminals at the points of electrical contact using a soft rubber eraser, if necessary.  But be careful not to peel the foil away from the plastic substrate; it's an easy mistake to make.

Now is a great time to lube the speedometer cable!  Make sure the cable is fully seated onto the back of the speedometer head.  When you stated that you "used a drill to turn the speedo and it moves like it should," was that with the drill motor attached at the transmission end of the cable?  It is very rare for the speedometer drive gear to fail without causing visible wear of the driven gear.  However, the drive gear may have shifted axially on the output shaft of the transmission if its retaining clip failed or was not properly installed during prior assembly.  Peruse the article, "Common causes of 'bouncy' speedometer needles" for additional insight.

[Jimartina]

Is the gas tank full ? that would make the gauge read full.

Have you serviced the cable. a drop of oil can cure a gallon of problems.
 

3/4 full, but good question.  Cable seems to move freely. 

[Jimartina]

The fuel gauge probably has an open somewhere in the sender circuit.  With the ignition switched on, jump the fuel gauge sender pin directly to ground.  The gauge should rapidly drop to Empty.  Check the PCB-to-gauge connections and the I/P cluster harness-to-PCB connections.  Burnish the copper foil and terminals at the points of electrical contact using a soft rubber eraser, if necessary.  But be careful not to peel the foil away from the plastic substrate; it's an easy mistake to make.

Now is a great time to lube the speedometer cable!  Make sure the cable is fully seated onto the back of the speedometer head.  When you stated that you "used a drill to turn the speedo and it moves like it should," was that with the drill motor attached at the transmission end of the cable?  It is very rare for the speedometer drive gear to fail without causing visible wear of the driven gear.  However, the drive gear may have shifted axially on the output shaft of the transmission if its retaining clip failed or was not properly installed during prior assembly.  Peruse the article, "Common causes of 'bouncy' speedometer needles" for additional insight.

Great thx. I'll try that and see what happens.

[MIKE S]

Put the rear end up in the air on jack stands. With the cable disconnected from trans and and out of park, rotate  the drive shaft and look at the end of the Driven gear and see if it rotates. Also make sure that the end of cable indexes into the gear properly. Have seen the square of the driven gear get rounded. Sometimes the inner cable does not go in the gear far enough and the very end of the square drive is worn so inspect gear carefully.

[Jimartina]

Ok great. I'll try that too. 

[Jimartina]

So I did as you suggested. While under the truck I realized that the sending unit wire Ran to the drivers side. Which made me think about the fact that my selector switch is missing. So as it turns out they wired the selector switch to drivers side tank, so they gauge was trying to read the drivers side tank sending unit. So I rewired the sending unit wires to use the passenger tank. So now my gauge reads properly.
I did use the tip with shorting to ground the sending unit wire to make sure the wiring was proper. Thanks for the tip.

[Jimartina]

OK so new problem. I installed the new sending unit in the tank. It works. Kind of. It shows just over 1/4 full when the tank is full and it won't move til it gets to a quarter tank then it works properly. I tested it in the tank and out the tank.  Did I just get a faulty unit? I know the gage will go to full because if the wires are disconnected it defaults to full. Any input is appreciated

Sent from my SM-S901U using Tapatalk

[Mr Diesel]

Might need to test it with sender out of the tank. Use some jumper wires between sender terminals and Truck harness and move sending unit arm up and down. Should tell you if it's dash gauge or not.

[bd]

I tested it in the tank and out the tank. ... I know the gage will go to full because if the wires are disconnected it defaults to full. ...

When the gauge wire is disconnected from the sender with the ignition switched ON, the fuel gauge needle should swing well past FULL to the 3:00 o'clock position.  When you ground the sender wire the gauge should quickly drop to EMPTY.

You can also perform a rudimentary check on the sending unit if you have a decent ohmmeter.  With the sender float in its lowest position (EMPTY) an ohmmeter with its leads clipped to the sending unit frame and sender wire pin terminal should indicate ~0 Ohm.  Moving the float to its highest position (FULL) the ohmmeter should indicate ~90 Ohms; midway (1/2 Tank) it should indicate ~45 Ohms.

If the sender float is closed cell nitrophyl foam, give it a firm squeeze between your finger and thumb nail.  If the float oozes fuel when squeezed, the float is saturated and the sender should be replaced with another brand that uses better quality materials.