Ammeter not working

[70STOVEBOLT]

That is probably what I will do, as I believe the ammeter is working properly. I just wish I could satisfy my curiosity as to WHY it is leaning. I really appreciate you taking the time to help.

Thanks,
Mark

[ray_mcavoy]

You're welcome!

Yes, at this point, I think it's safe to conclude that your meter is working properly and it's simply the pointer that has somehow become mis-aligned with the inner workings of the gauge. It's definitely a mystery as to how that happened though (especially with 2 different meters).

[Captainfab]

Subscribing for later

[70STOVEBOLT]

UPDATE! The ammeter is now working as it should. I took my cluster apart to paint the needles and add a vac gauge, and I noticed the ammeter needle seemed to be "spring loaded", so I wound it around in a clockwise fashion until it was straight up and put the cluster back together, and now it shows a slight charge when idling, and the needle moves left slightly when I turn the headlights on. If I rev it up the needle moves to the right slightly. The truck has been sitting for about 2 weeks w/o being started, so I expected the needle to move to the charge side, as it did before when the needle was leaning to the left at rest. I wish I had an explanation for it, but I don't. Thank you to all who offered help and made suggestions.

Thanks,
Mark

[1995flstc]

I just want to extend thanks to all who posted. Without this forum and it's members, I never would have figured out my battery gauge issues. The diagram was priceless. My issue was a corroded connection where the right side fuse tied in +b/w wire. Thank you!

[pilotXC]

My ammeter also has not been working since I've owned the truck and I'm curious to get it going... I was looking around today and want to confirm these are the right fuses in the images. The older one is on the battery side and the fuse was broken when I opened it. Is there supposed to be wires connected to the back of the meter? If so, I have some digging to do to find them, but wanted to make sure. Thanks for all the help the previous posts have already offered!

[ray_mcavoy]

Originally Posted by pilotXC

My ammeter also has not been working since I've owned the truck and I'm curious to get it going... I was looking around today and want to confirm these are the right fuses in the images. The older one is on the battery side and the fuse was broken when I opened it. Is there supposed to be wires connected to the back of the meter? If so, I have some digging to do to find them, but wanted to make sure. Thanks for all the help the previous posts have already offered!

Yes, those appear to be the correct fuses. The black rubber one on the battery side is a factory fuse holder that should take an SFE-4 fuse once you get the old broken pieces out. The white plastic one is an aftermarket replacement fuse holder that might need a longer fuse, but don't put anything larger than 4 Amp in there though.

No, the wires don't run directly to the back of the meter. They go to pins #1 and #12 of the rectangular plug on the back of the instrument cluster. From there, traces on the printed circuit (that's on the back of the cluster) make the connection to the meter.

[pilotXC]

Finally got the fuses I ordered and put them in, ammeter seems to be working on a very brief test... Thanks for the info! I love this site for this stuff, makes life just slightly easier. Much appreciated!

[Grandma’s 68]

I found this explanation of a shut that helped me understand how the ammeter works https://www.bluesea.com/support/arti...o_a_DC_Ammeter

“
How a Shunt Works. It is useful to think of water flow in a pipe when thinking about current flow in a wire. The way that a shunt works is analogous to a restriction to the flow of water in a pipe and a bypass around the restriction. Some of the water flow in the Main Pipe is diverted through the Bypass. A Paddle Wheel in the Bypass measures the water flow through the Bypass. If the Bypass allows 1/100th of the water in the main pipe to flow through it, and the Restriction allows the remaining 99/100ths of the water to flow through it, the ratio of flow in the Bypass to the Restriction is 1 to 99. Using this ratio, the Paddle Wheel could be calibrated to indicate the total flow through the Main Pipe.


A DC ammeter and shunt works in a similar way—a small amount of current that flows through the Main Wire is diverted to, and measured by, the Meter. Analog meters have very fine internal wires that flex to enable the needle to move. Because the wires are fine, they carry only a very small current. Therefore, the current in the meter must be a tiny fraction of the total current to be measured.
In order to obtain an accurate reading of the current flow through the main wire, the shunt and meter are very precisely calibrated at fixed resistance values—the meter resistance is typically 50 Ohms and the shunt resistance is a fraction of an Ohm.
“

[ray_mcavoy]

Originally Posted by Grandma’s 68

I found this explanation of a shut that helped me understand how the ammeter works https://www.bluesea.com/support/arti...o_a_DC_Ammeter

Yes, that is a good description of how ammeters use a shunt to measure current without having it all flow through the meter itself.

The system used in these trucks is very similar, but not as refined or precisely calibrated. It's designed to let the driver know if current is flowing to or from the battery (charging or discharging) and roughly how much (more needle deflection = more current) but not marked with any numbers to measure the current in amps. So instead of using a calibrated shunt resistance, they simply rely on the inherent resistance of the charging system wire. And the meter movement itself has a bit heavier gauge wire than what you would typically find in an ammeter used with a shunt resistor. The principle of operation is the same though.

The 4 amp fuses (mentioned earlier in this thread) serve two purposes. (1) They protect the wiring leading to the meter if there is a short to ground. And (2) they also protect the meter movement and it's wiring if there is an open circuit in the shunt wire. An open circuit in the shunt wire blocks current flow (like completely closing off the restriction in the water flow analogy). That leaves the meter (or paddle wheel in the bypass) as the only other path for current (or water) to flow. But it's not designed to safely carry that much current so the fuses provide protection in that situation too.

[Grandma’s 68]

I’ve been trying to figure out why my ammeter doesn’t work in my 68 c10. It’s had an engine/transmission swap in the past. I found a few smoking guns, only to find the ammeter still not working. For example completely missing 4A fuse. I hoped that would be the simple fix, but no luck. I traced the black wire from the junction box to bulkhead connection and wire was good. The b/w wire back to the fuse was also good. Paring back the insulating tape wrap to trace the b/w wire further, the wire just stopped about 4” from the fuse.

From the circuit diagram it’s supposed to connect to a red wire from the 12v junction box and 12v wire from the alternator. My alternator red wire does not seem to join the party at this location. I have not traced that red wire yet. The red wire arrangement at the junction box also does not match the circuit diagram. There seems to be a lot of red wires going to the starter. That stuff may be my next hurdle to overcome/ tidy up/ make look like the drawing.

My immediate question however is, what does the connection between these multiple red wires and the b/w wire look like? Is it some other type of junction box like the one near the battery? The two red wires near the fuse are currently just joined with aftermarket crimp butt connectors.

Thanks.

[ray_mcavoy]

Originally Posted by Grandma’s 68

My immediate question however is, what does the connection between these multiple red wires and the b/w wire look like? Is it some other type of junction box like the one near the battery? The two red wires near the fuse are currently just joined with aftermarket crimp butt connectors.

Sounds like you're on the right track with troubleshooting your wiring.

The factory connection between the multiple red wires and the b/w ammeter wire was made using a metal clip crimped around the wires. The connection was then soldered and wrapped with tape (in addition to the harness wrap).

[VetteVet]

Originally Posted by ray_mcavoy

Sounds like you're on the right track with troubleshooting your wiring.

The factory connection between the multiple red wires and the b/w ammeter wire was made using a metal clip crimped around the wires. The connection was then soldered and wrapped with tape (in addition to the harness wrap).

Thanks for your input Ray. It is so helpful.

[Grandma’s 68]

Yes, thanks for the help Ray. Unfortunately more is required.

I think I’ve established what’s going on with the red wires. The main battery positive goes to the starter, which is effectively used as a another junction box. A pair of red wires come from the starter +ve and go to the original factory junction box. The pair of red wires from the alternator (the bolt on connection and the double spade type connector) also connect at the starter +ve.

Will this arrangement allow the ammeter to work okay if the b/w wire is just spliced into the the red wire (previous picture with the current crimped butt connector) or do I need to move the pair of red wires that originate at the alternator to connect at the current crimped butt connector near the b/w fuse?

Thanks again.

[VetteVet]

Here is what my junction looks like. I do not have the metal crimp but the junction is soldered. I also do not have the black wire with white stripe for the ammeter because I have a warning light dash not a gauge dash.









Here is a diagram like yours that shows the ammeter shunt on the left side of the diagram. It tells the ammeter whether the voltage is greater on the battery or the alternator. In other words whether the alternator is charging or the battery is providing the power to the circuits.







In order to do this the ammeter wires have to be connected to the main junction and to the battery positive side. See the diagram. If you connect the alternator to the starter post then the ammeter will not get a correct reading of the voltage difference between the alternator and the battery.

The shunt wire also serves as the charging wire for the battery which would be accomplished as well by connecting the alternator to the solenoid with the large cable from the battery positive.

A common misconception is that the battery supplies the power for the truck and the alternator just charges the battery. They always want to run the alternator charge wire back to the battery positive post or to the starter.
It should run to the main junction to feed all the circuits with a small wire connected to charge the battery.






When GM went to the next generation trucks with the voltmeters they ran the alternator charge wire to the solenoid with the big battery cable and then ran two feed wires off that, with fusible links to feed the rest of the circuits.

Here's a diagram I like to post that shows a real good way to wire our trucks when we go to the high draw loads like cooling fans. I modified it to show how to wire it and keep the ammeter.

[Grandma’s 68]

Thanks VetteVet and Roy.

To avoid going to the hassle of routing the end of the alternator red wires from the starter to the tie-in point at the b/w fuse, I thought I’d just tie-in the b/w wire to the red wire already there below the left side headlight and see what happens. Fired the truck up and nothing on the dash ammeter. Oh well.

With better understanding from this post and elsewhere, I’ve come to better understand electrical theory/circuit diagrams and what it looks like in reality. An image that helped is the analogy to water and gravity and that this ammeter is really displaying direction of flow, depending on which side has greater voltage potential. Most of the flow will got to the ‘shunt’ side as the ammeter probably has a quite high resistance/flow restriction. So it probably doesn’t matter too much where the ammeter black and b/w wire tie into the ‘shunt’ wire, there will still be a little bit of flow through the ammeter parallel line (limited by the resistance of the ammeter).

It finally dawned on me that my voltmeter is also an ammeter (never actually used that part). So I hooked it up to the junction block where the black wire ties in and at the b/w fuse. I am getting .2Amp flow with the alternator running. Now that leads me to believe there is something wrong at the dash wiring/ammeter.

Any easy way of checking that, or do I need to take the dash gauge cluster off?

[Grandma’s 68]

Further developments. I disliked the idea of working on behind the dash wiring, so realized I can start with the disconnected bulkhead connector.

I first measured the resistance, expecting to see it was a failed open circuit. It wasn’t. It measures 1.8 Ohms, which doesn’t seem very much, but I’ve no idea.

I then hooked up a regulated power supply and put in 0.5V. The gauge moved! It seemed to take about 0.48Amps to move it. I turned the supply up to 1.0V and the current was 1.1Amps. I swapped over the wires and the gauge also worked in the other direction. With 0.5V it took 0.5A and at 1.0V it took 1.2Amps.

Previously I measured the current flow from the alternator to the battery and it was 0.2Amps, so there was probably some small movement of the gauge, I just didn’t notice it.

So the ammeter appears to work, but I’m not sure these values are ‘normal’. If they are, it’s just that in normal operation there is almost no discernible movement of the needle. Is that correct functionality?

I’m ready to finish this quest and hope that if the alternator isn’t charging I’ll get some sign on the ammeter.

Thanks for everyone’s help.

[ray_mcavoy]

It's been a while since I've measured one, but the values you posted look pretty close to what I remember. So I think your meter is okay. But like VetteVet described earlier, it's not going to work correctly with the alternator output being re-routed to the batt stud on the starter solenoid. You'll need to return that back to it's original connection point.

Attached is a simplified diagram of the charging system / ammeter circuit. As VetteVet pointed out, it is a common misconception that the battery provides all of the power to the truck and the alternator just charges the battery.

Under normal operation, the alternator supplies all of the power to the truck's electrical loads as well as some current to charge the battery. So I_alt = I_load + I_batt (flowing to the battery). But since the output of the alternator is connected directly to the loads via the main power distribution point, only I_batt flows through the shunt / ammeter. So the meter only indicates the charging current flowing to the battery. This is usually noticeable on the meter after first starting the engine. But as the battery reaches full charge, the charging current drops off and the meter goes back closer to the middle.

In a situation where the load current exceeds the capacity of the alternator, the alternator will provide as much as possible and the battery will make up the difference. I_load = I_alt + I_batt (flowing from the battery). But again, only I_batt flows through the shunt / ammeter. This time it's flowing in the opposite direction so the meter will indicate the battery being discharged. This could be normal if the engine is idling (alternator output low) and there is a lot of load on the system (lights, heater fan, wipers, etc. all running). But would indicate a problem with the alternator if it's happening under higher RPM and/or lower load conditions.

With the output of the alternator re-routed to the starter batt post (effectively the same as hooking it to the positive battery terminal), you now have the battery & alternator connected together on one side of the shunt (instead of one at each end). So any current going to charge the battery will be going direct (bypassing the shunt/meter) and therefore not registering on the meter. While the load current (as supplied by both the alternator and battery) would all be going through the shunt/meter and therefore showing up as a constant heavy discharge.

[Grandma’s 68]

I can see the logic that the alternator wired to the starter post that is connected to the battery basically ‘shorts’ out the rest of the power circuit. It’s all still live and current flows to the devices but not much to cause a differential over the ammeter. Basically negligible flow across the ammeter, so not moving.

I tried a quick test by cutting the alternator wire at the starter and jumping it to the ‘correct’ location at the b/w fuse.

However it doesn’t look like I’ve got any change in ammeter results. I get about 0.17A between black and b/w wires. That about the same as before (0.22A). Now that may be coincidence, so is it normal operation of the gauge basically sitting in the middle with a fully charged battery and functioning alternator? Is 0.2A flow what to expect?

I also noticed that the ‘shunt’ wire that goes along the front of the truck looks a heavier gauge. Is the ‘shunt’ wire some sort of special wire?

I may still rehash the wiring to get it like the original diagram, but I’m curious now to functionally test the gauge and would appreciate some guidance on how it’s supposed to look under normal charging and how to simulate alternator failure.

Thanks.

[ray_mcavoy]

Originally Posted by Grandma’s 68

I can see the logic that the alternator wired to the starter post that is connected to the battery basically ‘shorts’ out the rest of the power circuit. It’s all still live and current flows to the devices but not much to cause a differential over the ammeter. Basically negligible flow across the ammeter, so not moving.

Yes, having the alternator output re-routed to the starter batt stud still results in a fully functional charging system. But the altered current path prevents the ammeter from working as intended.

Originally Posted by Grandma’s 68

I tried a quick test by cutting the alternator wire at the starter and jumping it to the ‘correct’ location at the b/w fuse.

However it doesn’t look like I’ve got any change in ammeter results. I get about 0.17A between black and b/w wires. That about the same as before (0.22A). Now that may be coincidence, so is it normal operation of the gauge basically sitting in the middle with a fully charged battery and functioning alternator? Is 0.2A flow what to expect?

Yes, it's normal for the gauge to stay at (or close to) the middle with a fully charged battery.

Originally Posted by Grandma’s 68

I also noticed that the ‘shunt’ wire that goes along the front of the truck looks a heavier gauge. Is the ‘shunt’ wire some sort of special wire?

No, the shunt wire isn't anything special. Just regular stranded copper automotive wire. The gauge and length of the wire does determine it's resistance so if it has been replaced with something other than the stock equivalent, it might alter the sensitivity of the meter. For example, a heavier gauge wire will have less resistance (for an equivalent length) so a greater majority of the current will go through the shunt and less through the meter, making it less sensitive.

Originally Posted by Grandma’s 68

I may still rehash the wiring to get it like the original diagram, but I’m curious now to functionally test the gauge and would appreciate some guidance on how it’s supposed to look under normal charging and how to simulate alternator failure.

It is normal for the needle to be toward the charge side (especially after the battery has been discharged a little by starting up the engine). The exact amount of deflection will vary depending on the battery's state of charge. As the engine runs and the battery charges, it's normal to see the needle slowly return back to (or close to) the center.

To simulate an alternator failure, simply leave the engine off and switch on some loads (like the headlights, heater fan, wipers, etc.) ... that should cause the needle to swing toward the discharge side. And more load should result in more needle deflection.