Ammeter not working

[ray_mcavoy]

Originally Posted by leftybass209

Minimum voltage required to charge a lead-acid battery is 2.15 volts per cell. That equates to 12.9 volts required to start charging a standard car battery. ....
In your case, the difference between the charging rate and the battery voltage isn't much, so you shouldn't expect to see hardly anything on your gauge.

I've always heard that it was closer to 2.3 volts per cell to start charging, hence the 13.6 to 13.8V I posted. But anyway, it looks like we both agree that in this case the stock battery gauge should be reading very close to center with the 13.5V he measured at the battery.

Originally Posted by leftybass209

A properly set up charging system with a functioning gauge really doesn't register much in regards to needle movement because the source voltage (battery) and alternator output should be as little as possible to maintain the battery charge. Of course, added electrical components can change the demands.

I agree. The needle will normally stay near the center when the battery is fully charged and the alternator is able to supply enough current to meet the demands of the truck's electrical system. If the needle is reading towards the discharge side that means the alternator isn't keeping up with demand and the battery is supplying the extra current to meet that demand (discharging in the process).

[71 LS SUB]

I have been fighting the same issue, I thought because I went to a LS motor maybe that had something to do with it. ( newer alt. to older gauge )
I've attached a few pictures for reference.

[VetteVet]

One thing [I]'m curious about is how the ammeter wires are wired on the battery and alternator end. The 12 wire should start near the positive battery terminal and the B/W no.1 wire should start at the main junction of the truck harness, with a shunt wire between the two points. The length and size of the shunt wire is also a factor in the accuracy of the ammeter reading.

Here is the original factory wiring with the external regulator in place.





Notice the battery and alternator are wired on the ends of the shunt wire.
When converting alternators or going to a one-wire alternator These rules still apply.

[ray_mcavoy]

Originally Posted by 71 LS SUB

I have been fighting the same issue, I thought because I went to a LS motor maybe that had something to do with it. ( newer alt. to older gauge )

Ditto what VetteVet just said about how the system is wired. It doesn't matter what type of alternator is being used so long as it's output is wired into the system at the factory location as shown in the diagram VetteVet posted.

And now that I think about it, an alternator that is connected directly to the battery (instead of the factory junction) could be a possible explanation for a battery gauge indicating discharge all the time. I had overlooked that in 70STOVEBOLT's case since he mentioned having all new wiring harnesses but I suppose it's still a possibility if an extra wire was run directly from the alternator to the battery.

When the alternator output is connected to the battery (instead of to the factory junction point) it allows current to flow directly to the battery, bypassing the shunt wire & battery gauge. So the battery can still receive a charge but the current won't be flowing through the shunt wire and therefore won't be shown on the gauge.

The battery gauge will still show current that flows from the battery (through the shunt wire) to the truck's electrical loads as a discharge (same as it normally does). However, with the alternator output being on the battery end ("wrong end") of the shunt wire, current that flows from the alternator (through the shunt) to the truck's electrical loads will also be shown as a discharge (normally this current is not routed through the shunt wire and doesn't register on the gauge).

[71 LS SUB]

Originally Posted by ray_mcavoy

Ditto what VetteVet just said about how the system is wired. It doesn't matter what type of alternator is being used so long as it's output is wired into the system at the factory location as shown in the diagram VetteVet posted.

And now that I think about it, an alternator that is connected directly to the battery (instead of the factory junction) could be a possible explanation for a battery gauge indicating discharge all the time. I had overlooked that in 70STOVEBOLT's case since he mentioned having all new wiring harnesses but I suppose it's still a possibility if an extra wire was run directly from the alternator to the battery.

When the alternator output is connected to the battery (instead of to the factory junction point) it allows current to flow directly to the battery, bypassing the shunt wire & battery gauge. So the battery can still receive a charge but the current won't be flowing through the shunt wire and therefore won't be shown on the gauge.

The battery gauge will still show current that flows from the battery (through the shunt wire) to the truck's electrical loads as a discharge (same as it normally does). However, with the alternator output being on the battery end ("wrong end") of the shunt wire, current that flows from the alternator (through the shunt) to the truck's electrical loads will also be shown as a discharge (normally this current is not routed through the shunt wire and doesn't register on the gauge).

All that being said, what would be the recommended fix then...?
thanks

[ray_mcavoy]

Originally Posted by 71 LS SUB

All that being said, what would be the recommended fix then...?
thanks

The fix would be to run the alternator's output wire to the stock connection point in the charging system (not directly to the battery). See the diagram VetteVet posted above in post #28.

In other words, you should have the output of the alternator connected to a junction point (typically located on the driver side of the core support). The junction can be the factory type in-harness splice or a separate junction block.

Then you should have a battery charge wire (that also serves as the ammeter shunt) running from the driver side junction, across the core support, and connecting to the junction block on the pass side fender near the battery. And circuit protection (such as a fusible link) connecting that pass side junction block to the positive battery terminal.

One leg of the ammeter wiring (black with white stripe) should be connected (through an inline 4A fuse) to the driver side junction. The other leg of the ammeter wiring (black) should go (also through a 4A fuse) to the pass side junction block.

Other items you'd want to connect to the driver side junction point include:
(1) Alternator voltage sensing wire.
(2) Main feed wire to the firewall bulkhead connector (feeds the fuse box, ign switch, etc).
(3) Feeds for any other add-on electrical circuits.

[71 LS SUB]

[QUOTE=ray_mcavoy;7436640]The fix would be to run the alternator's output wire to the stock connection point in the charging system (not directly to the battery). See the diagram VetteVet posted above in post

Thank you for the reply. I was thinking that might be the fix, but I'm sure no electrician. I will try it this weekend and post results. Other members have the same issue.
Thanks again
Jim

[gable74]

So from the alternator output to the j-block is only a #12 wire? I always thought that should be a #4 at least. Where then does the starter feed come from in the system and what size wire should that be? Not trying to hyjack, I hope my question is still relavant to the topic.

Thanks,
Jimmie

[ray_mcavoy]

Originally Posted by gable74

So from the alternator output to the j-block is only a #12 wire? I always thought that should be a #4 at least.

That wire was 12ga from the factory. But when using a higher output alternator, you can (and should) upgrade to a heavier gauge wire.

Originally Posted by gable74

Where then does the starter feed come from in the system and what size wire should that be?

The starter wiring should consist of:

(1) Positive battery cable to the starter solenoid "BATT" terminal.
(2) Negative battery cable to the engine block.
Stock battery cables were usually 6ga or 4ga.
(3) Purple 12ga wire from ignition switch "SOL" terminal to starter solenoid "S" terminal. This circuit runs through the neutral safety switch (auto trans) and through the firewall bulkhead connector.
(4) Mulit-colored cloth-coated resistance wire from firewall bulkhead connector to starter solenoid "R" terminal. Then a yellow 20ga wire from there to the coil +. This circuit is typically updated to a 12ga pink wire running from the firewall bulkhead connector to the distributor when changing to HEI.

Note: Some GM vehicles utilize the large "BATT" stud on the starter as a junction point. These 67-72 trucks on the other hand have that junction point built into the wiring harness.

[gable74]

Great, thanks for the quick reply. What size wire would be recommended for the "shunt" or charging wire in the diagram listed? Would a #10 or #8 be sufficient?

[ray_mcavoy]

Originally Posted by gable74

Great, thanks for the quick reply. What size wire would be recommended for the "shunt" or charging wire in the diagram listed? Would a #10 or #8 be sufficient?

You're welcome!

The stock 12ga wire will be fine for charging the battery.

However, with the engine off (or if the alternator can't keep up with demand), current to power the vehicle's electrical system will be drawn from the battery and run through that wire. So bumping it up to 10ga might not be a bad idea if you have some high draw loads that might run with the engine off. In fact, I think there were some factory applications that used 10ga here instead of 12.

The only issue with using too large of a wire for the "shunt" is that it's lower resistance will decrease the sensitivity of the ammeter.

[gable74]

Okay, I got it. That makes sense about the shunt as well. The only extras I have on my truck are the pillar spot lights. I will stick with the #12 for now. My Aux battery has #10s on it so when the new harness goes in, I will hook any "extras" like that to the Aux battery J-Block to keep the demand off the main battery.

Thanks again for the help!

[70STOVEBOLT]

Quote:
Originally Posted by Jeffvega View Post
maybe my gauge is working then since mine also is disconected at the bat. side. the gauge always reads discharging, but I know its charging. would power to only the junction side make it always read this way?


I would say so because you would not be reading a differential between the battery and the alternator rather you would only be connected to the alternator side. A long enough piece of wire to reach from the ammeter + to the junction on the passenger fender well with a small fuse inline should make it work. As you have seen in the thread a 4 amp fuse is the size you need.
__________________
VetteVet

Could this be why my ammeter is always showing discharge, even though it is charging? That would be odd since I changed the underdash harness, the underhood harness, and the entire gauge cluster. I had the problem before changing all of those, and still have the problem.

[ray_mcavoy]

Originally Posted by 70STOVEBOLT

Could this be why my ammeter is always showing discharge, even though it is charging? That would be odd since I changed the underdash harness, the underhood harness, and the entire gauge cluster. I had the problem before changing all of those, and still have the problem.

How is your alternator output wire configured?

As I mentioned in an earlier reply, if the alternator output is routed directly to the battery (instead of the stock junction point), the charging current will bypass the ammeter shunt and not register on the meter. But current flowing from the alternator / battery to the rest of the truck's electrical system will be flowing through that wire that serves as the ammeter shunt. And that will cause it to indicate discharge all the time.

[70STOVEBOLT]

Output wire is connected to the stock junction block. If that wire was not making a connection at the junction block, would it act that way? I will take a pic when I get home from work of the junction block.

[ray_mcavoy]

Originally Posted by 70STOVEBOLT

Output wire is connected to the stock junction block. If that wire was not making a connection at the junction block, would it act that way? I will take a pic when I get home from work of the junction block.

Okay, that's good. I don't think it's a bad connection issue ... otherwise you'd be experiencing charging problems.

One other thing I thought of ... does the needle return to the center of the meter when everything is turned off?

As leftybass209 and I discussed earlier, it's normal for the needle to stay near the center when the alternator is meeting the demands of the electrical system and the battery is nearly or fully charged. The needle will only move toward the charge side when the battery is a little low and is accepting a charge from the alternator. So if (for whatever reason) the needle is bent / offset slightly toward the discharge side, it could look like it's indicating some discharge even when things are operating properly. And if that were the case, you'd also see the needle staying off toward the discharge side (instead of centered) when everything is off.

[70STOVEBOLT]

Needle does not return to center, stays on the discharge side. I will take a pic in a little bit and post it. As I stated in past posts, I have had two different ammeters do this, before and after changing all the harnesses and the cluster. Before I put the refurbished cluster from Tom in the truck, the needle on that ammeter was in the center, but once I installed the cluster and started the truck, it went to the discharge side and has stayed there ever since. It does go to the charge momentarily after starting the truck, but returns to the discharge side shortly after. It does move like normal when accessories are turned on.

[ray_mcavoy]

Originally Posted by 70STOVEBOLT

Needle does not return to center, stays on the discharge side. I will take a pic in a little bit and post it.

Okay, that confirms my theory that the meter (for whatever reason) is "leaning" toward the discharge side when it should be centered. Yeah, posting a pic to show how far it's off would be helpful.

Originally Posted by 70STOVEBOLT

As I stated in past posts, I have had two different ammeters do this, before and after changing all the harnesses and the cluster. Before I put the refurbished cluster from Tom in the truck, the needle on that ammeter was in the center, but once I installed the cluster and started the truck, it went to the discharge side and has stayed there ever since. It does go to the charge momentarily after starting the truck, but returns to the discharge side shortly after. It does move like normal when accessories are turned on.

The fact that your meter does go to the charge side momentarily after starting is a good sign. And combined with the fact that it moves normally when accessories are turned on would seem to indicate that your wiring is okay and configured properly.

It seems rather unusual that you've had this same problem with 2 different meters, but it does sound as though you've ended up with a couple that have "leaning pointers".

Just out of curiosity, what happens if you disconnect the meter (such as by removing both of the 4A inline fuses)?

If it still points toward the discharge side when unhooked, that would pretty much confirm a "leaning pointer". But if disconnecting it allows the needle to return to center, that would seem to indicate an electrical issue that is biasing the meter toward the discharge side. I'd think that would show up as a parasitic drain on the battery though.

[70STOVEBOLT]

Here is with key off

[70STOVEBOLT]

Her is with key on, lights on

[70STOVEBOLT]

I have let it sit for two weeks without starting it recently and it cranks over at normal speed and fires up. I have not tried unhooking the fuses. I will go do that and post the results shortly.

[70STOVEBOLT]

Unhooked fuses on both sides (broke one $&"*^#!!) and no change. What would have caused the leaning needle in the first place is the mystery. Both gauges were centered prior to the start of the problem. Very frustrating!

[ray_mcavoy]

Originally Posted by 70STOVEBOLT

Unhooked fuses on both sides (broke one $&"*^#!!) and no change.

Sorry, I should have warned you that those little fuses are rather delicate. When you put them back together, putting a little bit of dielectric grease on the rubber will make them easier to separate and reduce the chances of breaking a fuse should you ever need to service them again in the future.

Originally Posted by 70STOVEBOLT

What would have caused the leaning needle in the first place is the mystery. Both gauges were centered prior to the start of the problem. Very frustrating!

Well, with it disconnected and still leaning, we can at least rule out an electrical issue.

I've never torn one of these meters completely apart to investigate their inner construction. But they feel like they may have a permanent magnet (instead of a spring) inside that is responsible for returning the pointer to the center. I don't know if it would be possible for that magnet to loosen up and shift sideways or not. Or maybe the vane that moves the needle has developed some residual magnetism and is keeping it from centering? You've got me curious now! I'll have to look through my spare parts and see if I can find a junk meter to take apart.

[70STOVEBOLT]

I knew they were delicate, I broke one before when I was changing them the first time. They are not easy to find since they are "shorties". If you figure something out, please post in the thread. Thanks for taking the time to help.

Thanks,
Mark

[ray_mcavoy]

Okay, I found a 73-75 truck ammeter with a rusty face plate and took it apart.

The 73-75 ammeters are wired up in the trucks the same way as 67-72's and I'm pretty sure the internals are the same too.

Here is what is inside after pulling off the needle and drilling out the rivets holding on the face plate. Basically just a coil of wire wrapped around a plastic form with the pivot shaft in the center:


And here is another view of the internals after taking them out of the case:


Removing the wire coil and the tape underneath reveals this metal "shoe" on the back:


And finally, pulling off the rear half of the plastic coil form allows the pivot shaft to be removed. It's attached to a small disc shaped magnet (2nd in from the left):



When current flows through the coil, it creates a magnetic field that causes the disc shaped magnet to turn (and move the attached needle to one side or the other). The direction of the current determines the direction of needle movement. And the amount of current determines the amount of deflection. Since these meters are wired in parallel with a shunt, the majority of the current flows through the shunt and only a small fraction goes through the meter coil.

When no current is flowing through the coil, the disc shaped magnet aligns with the metal "shoe" on the back (3rd pic) and that is what returns the needle to the center position. Given the way it fits into notches in the plastic coil form and is held fairly tight under the tape & coil windings, it's unlikely that shoe would shift out of position and cause the needle to lean to one side. Also, the pivot shaft appears to be permanently attached to the disc shaped magnet so no chance for unwanted movement there either.

The needle is just a press fit onto the tiny pin-sized pivot shaft. So that seems to be the most likely place for needle mis-alignment to occur. But unless the needle is loose, I don't really have an explanation for why it is now leaning to the left when it used to be centered. Fixing this type of mis-alignment is fairly simple though. Pull the needle off and press it back on so it is centered, being careful not to damage the needle or pivot shaft.