Neg ground conversion

[rdean7]

Hello all, I acquired a B3B in Dec and am trying to go thru previous repairs and changes performed by PO.

My battery is hooked up as a 6v neg ground. I think it was originally a + ground ? What changes would have had done to make this work? 

My only real current issue is no fuel gauge reading and - reading on amp gauge when running.

 

This is a 51 with working foot starter.

 

[Dave72dt]

Original would have been + ground.  That may have been the only change made since you are getting a neg reading on the amp gauge.  swapping the terminal wires on the amp gauge would have been one of the changes made if the swap was done correctly as well as the coil leads.  If the coil wires weren't swapped, simply change the battery back to + ground.  That may or may not get the fuel gauge working again.  Fuel gauge problems have been somewhat troublesome and may have had nothing to do with the battery ground polarity.

[B1B Keven]

Disconnect one of the wires from the fuel gauge.

Take a reading across the two posts with a multi meter.

You're looking for an ohm reading of about 20-50 ohms

for a good gauge.

[rdean7]

Ok thanks I'll check on these hopefully today if it will warm up, dang it.

[rdean7]

Ok so maybe my gauge is ok. I read 24 ohm  across the two terminals. The tank and sending unit are new so maybe the wire from tank is bad or hooked up incorrectly idk. Guess I'll have to drop the tank to sort this out.

Where does the other terminal wire go to? I have one new wire from the tank, and the other is original.  

[B1B Keven]

Should only be one wire. From sending unit to the gauge.

Some folks add an extra wire from the body of the sending

unit to the frame for a better ground.

[rdean7]

Sorry I probably didn't ask it clearly.

Where does the other wire on the  fuel gauge go to? One goes to the tank sending unit. where is the second wire on the other terminal on fuel gauge go to.

I tested it for ground and nothing. 

[Veemoney]

It is important to understand which gauge you have to correctly hook it up. The link below should help and is one of the many on this subject that can be found by doing a search on this site. Using the search box at the top right corner of the page and then I usually go to advanced search and check the topics box to narrow the search to topics that deal with the subject I'm searching. Hope this helps. 

 

 

[B1B Keven]

5 hours ago, rdean7 said:

Sorry I probably didn't ask it clearly.

Where does the other wire on the  fuel gauge go to? One goes to the tank sending unit. where is the second wire on the other terminal on fuel gauge go to.

I tested it for ground and nothing. 

Sorry. To power. Usually the ignition switch.

 

Edited January 9, 2024 by B1B Keven

[B1B Keven]

[rdean7]

Ok thanks. I got my shop manual back out and yep I see that now. 

 

Thanks for the help!

[Merle Coggins]

Understanding how the gauge operates helps to troubleshoot it. As you can see in this image, the gauge has 2 electro-magnetic coils inside. Both receive voltage from the Ignition Switch. The Constant Field coil is grounded, through the gauge mount, to the dash. This coil magnet is trying to pull the needle towards the 'E' position. The Variable Field coil is grounded though the sender in the tank. The sender is a variable resistor. The higher the fuel level the lower the resistance will be through the sender, which will create a higher amperage through the Variable Field coil. This will make this coil magnet stronger and will pull the needle towards the 'F' position. As the fuel level drops the resistance gets higher in the sender, the amperage is reduced in the Variable Field coil, and it looses strength. The Constant Field coil will then start to pull the needle back towards the 'E' position. 

 

If you have good voltage supply to the gauge, the next checks are to check the continuity of the wire going to the sender. Check resistance between the wire terminal, at the gauge, to a good chassis ground. You should get a reading between 10 and 90 Ohms, depending on fuel level. If you have an Open Circuit to ground on this wire you then need to be sure that the wire is in tact and connected to the sender. Also, a check to be sure that the sender body has continuity to ground is warranted. If you temporarily ground the GA terminal to chassis ground the needle should peg to the 'F' position. This indicates the gauge is working properly and the problem lies in the fuel sender, or the wire to it. 

 

Another failure I've seen is the gauge not properly grounding through it's mount. This will cause the gauge to be pegged to 'F' all of the time. Some of the gauge mounting screws have insulating washers. You don't want that on at least 1 of the fuel gauge screws so that it can properly ground to the gauge housing, and ultimately the cab. The insulating cup washers are usually installed near the 'hot' terminal to help prevent an inadvertent short circuit to the adjacent wire terminals. 

 

[rdean7]

Great input thank you!

So I did some more testing and this is what I have found.

The terminal on the sender has 94ohms to ground with 7+ gal fuel. So that's my E reading. If I go sender wire to gound the gauge pegs F  so I guess wire and gauge good. I have power on the SW term on gauge as well.

So this means the sender is faulty or bad ground idk.  The top plate around the one terminal on sender unit does have continuity to ground.