Ammeter acting up

[Dave WM]

took the car out for a drive, while on the road constant speed smooth highway, the ammeter starts going nuts, 60+ then about -30 amps. then settle down then starts up again, lasting several seconds.

I heard NO odd noises (but a 70mph prob would not hear over road noise anyway). No other indication of problems (radio fine, lights seemed normal (I turned on the HL to soften the charging down to about 30 amps indicated).

I was not far from home so just disconnected the VR and drove on home, no more intermittent anything, just a constant to be expected discharge rate. Installed a new backup VR that had been tested on a running engine, same thing random wild swings but mostly, like before.

I tried unplugging the connector from the back of the alt, shot it with some deoxit, same with the shunt (actually removed it to inspect the wiring inside, pretty basic the shunt IS the plug no way it can have a solder issue) the plug for the meter is soldered to the shunt, all looked good. I moved the small fuses around inside the shunt that I presume are there to protect the gauge. reinstalled everything, checked over the battery ground (wire from batter post to fire wall) and the engine harness ground (wire from harness that goes to the alt, has a ground lead). All seemed good and clean. checked the wire that goes from the starter positive back to the engine harness, that looks a bit suspect, some rust on that nut, will give that a good wire brush treatment. made sure the E and B connections at the alternator were solid. oh forgot also deoxit the spade connectors of the two fuse links mounted by the VR.

Anyway after the above (but for the last large wire at the starter), took it for a test drive (70 miles, lots of revving, lots of rougher roads to shake it up) I had NO more jumping around +/- like before, and yes the ammeter is working as I will show a drop when loaded momentary until the alt adjust for it back to center.

If it happens again I will take the time to install a lead at the battery terminal to attach a volt meter to. IF it truly was seeing a 60 amp charge I would expect there to be a noticeable increase in battery voltage. Frankly I doubt there was a 60amp charge as I was only turning about 2.5k when I noticed it. I should have thought to push in the clutch and drop the RPM to see what happened, I doubt the alt could make that kind of power to a load at idle, heck it will not do that when testing and the VR is bypassed.

 

The battery is a napa legend 24f less than a year old, test good with a digital tester (not the carbon pile type, the Internal resistance type) resting voltage is about 12.65-12.75. the batter cables looks good, have molded on contacts, no corrosion and are on tight. I really should have gone to that white lead on the positive side of the starter. From what I read, the VR looks at the voltage of the battery (as determined by this connection for the positive) and the body ground (that mid point ground lead I presume for black ground lead of the VR), to establish an overall state of charge, then cycles a zero/med/high field current thru the VRs contact point, one side is high, one side is zero and when off both contacts there is a voltage divider that sets up a med current flow. Pretty neat. But if it "sees" a voltage that is low (presume a poor connection at that starter or ground) then it would assume a low state of charge and attempt to correct by cycling max field current. Still I don't see how it could swing SO much, but that is the nature of intermittent faults. Oh I did a through visual exam of the wire harness looking for any rub thru ect… I am lucky this car is totally unmolested as far as PO work to the electrical I all seems fine and well supported with the correct tie downs etc.

I looked at the plug in connector of the VR to harness, it looked fine, no corrosion was noted, but I still should have hit it with some deoxit. will go back and do that as well, and reinstall the orig VR as a test. I don't the VR was the issue after all.

 

 

 

Edited January 4, 2021 by Dave WM

[Captain Obvious]

Hmmm... My first guess is that your shunt connections are (were?) intermittent.

The ammeter is supposed to sense a tiny tiny fraction of the total current that is flowing in the system. It does that by letting the vast majority of the current go through the shunt, while the tiny remainder heads off to the gauge coil. It's a current divider based on the resistances of the shunt and the ammeter coil.

But if the shunt goes open-circuit in the right (wrong?) way, all the current will try to go through the ammeter coil. Probably pegging the ammeter in the process.

[Dave WM]

10 hours ago, Captain Obvious said:

Hmmm... My first guess is that your shunt connections are (were?) intermittent.

The ammeter is supposed to sense a tiny tiny fraction of the total current that is flowing in the system. It does that by letting the vast majority of the current go through the shunt, while the tiny remainder heads off to the gauge coil. It's a current divider based on the resistances of the shunt and the ammeter coil.

But if the shunt goes open-circuit in the right (wrong?) way, all the current will try to go through the ammeter coil. Probably pegging the ammeter in the process.

That seems logical, but not sure why the ammeter worked perfectly after disconnecting the VR (thereby preventing the alt from charging) seems like a defect that would allow less shunting of current thru the man bar would still be there and present as intermittent large discharge amounts as well.

There is not a lot to it so I agree seems like the only possible explanation. Perhaps the intermittent nature of the defect just did not happen while I was driving home with the VR disconnected.

Needless to say I will be keeping a very close eye on it. will make another 70mile run later today to see if it happens again.

One point to note was the shunt did have two very small fuses built into it as part of the circuit that drives the gauge, so it must have been a consideration (that the shunt could some how fail in a way that you described) otherwise why would the engineers bother with adding those fuse to the gauge circuit.

 

 

[Captain Obvious]

I haven't studied the wiring diagram really thoroughly, but it looks like the one side of the shunt (the white wire side) only goes to the fusible link that goes to battery (+), and the voltage regulator. I'm thinking that because of the way things are wired, with the VR out of the circuit, most of the current will find another path instead.

Basically, the wires to the ammeter are self-contained. Those two wires originate in the shunt and go to one place and one place only... The ammeter. They do not make connection to anything else anywhere in the car. So one of three things is going on...

1) The shunt goes open and all the current tries to go through the ammeter instead of the shunt.
2) You really DO have 60A flowing through something. Since you didn't see smoke and everything seems to be working correctly, I doubt this one.
3) One of the two wires leading to the ammeter coil are shorting out to something else. Since the polarity seems to flip, I doubt this one too.

Of course troubleshooting is always difficult from a distance, and that goes double for electrical issues, but that's what I got.  

[Dave WM]

intermittent is the worse part of it. Well after 150 miles of testing and NO spurious +/- amperage to report I am going to have to call it. Just no way to trace it down. IF it continued I think I would get a voltmeter on the battery and see if the voltage varies with the indicated charge/discharge. The battery is a big load/supply so I figure it would absorb the flux well but still 60 amps would have to show up on the voltmeter. That way 2) could be confirmed or denied.

 

[Captain Obvious]

Yeah, intermittent electrical issues are the worst. And at some point, you just have to call it.

So about the battery being a big load... The battery can be a big load if it's not fully charged. But if you're driving the vehicle regularly and the battery is fully charged, it's not much of a load at all. Like a capacitor that is fully charged to the supply voltage. It's essentially an open circuit. The battery does the same thing. I've measured the quiescent current into a fully charged battery (a float charger sitting at about 14V) at maybe a hundred milli-amps.

And about starting the car... "But I crank the starter and draw 40A out of the battery for a short period of time." The catch is you just can't put it all back in as quickly as you took it out. The battery just won't accept the energy that fast.

But in any event, I hope you fixed the issue and it doesn't come back.      Intermittent electrical issues suck.

[SteveJ]

@Dave WM I came across this in a recent search on Amazon. It could serve as a way to confirm whether there's an issue with the shunt or charging system. https://www.amazon.com/dp/B01DDQM6Z4

You could mount it on the battery cable and run the cord inside the cabin. It's not for precision measurement, but it would definitely pick up if the battery is discharging from a bad charging system.

[Dave WM]

10 hours ago, Captain Obvious said:

Yeah, intermittent electrical issues are the worst. And at some point, you just have to call it.

So about the battery being a big load... The battery can be a big load if it's not fully charged. But if you're driving the vehicle regularly and the battery is fully charged, it's not much of a load at all. Like a capacitor that is fully charged to the supply voltage. It's essentially an open circuit. The battery does the same thing. I've measured the quiescent current into a fully charged battery (a float charger sitting at about 14V) at maybe a hundred milli-amps.

And about starting the car... "But I crank the starter and draw 40A out of the battery for a short period of time." The catch is you just can't put it all back in as quickly as you took it out. The battery just won't accept the energy that fast.

But in any event, I hope you fixed the issue and it doesn't come back.      Intermittent electrical issues suck.

where I was going with the load comment was if I was indeed flowing 60 amps into the battery, I don't think the voltage would spike to the point of electrical destruction. That is once you go beyond 14v THEN it would become a big load. Not sure if that is correct but it was why I assume a voltmeter test while driving would still work as there would be a large increase in voltage (but not so much as to fry things) if there was an actual 60 amp charge going on and not just an indicator problem.

IF indeed I did have a 60 dump into the battery I can say for sure the ECU did not self-destruct, I don't know the max voltage it would take for that to happen.

[Dave WM]

1 hour ago, SteveJ said:

@Dave WM I came across this in a recent search on Amazon. It could serve as a way to confirm whether there's an issue with the shunt or charging system. https://www.amazon.com/dp/B01DDQM6Z4

You could mount it on the battery cable and run the cord inside the cabin. It's not for precision measurement, but it would definitely pick up if the battery is discharging from a bad charging system.

Yes! I like it. I was going to hook up a old school ammeter that I have laying around but it was only 30amps, would have to figure a good point to test. This is much Cleaner. I guess the best place to test would be right on the battery cable itself.

I wonder if extending the wires long enough to get it in the cabin would be an issue? The wires on the test that is, not the battery cable although that would work :)

 

Edited January 6, 2021 by Dave WM

[Dave WM]

I ended up buying a DC clamp on probe, has a 6ft lead that I can connect the a dmm. Will do a video when I get it of course

:)

[Captain Obvious]

I just doubt you were really stuffing 60A into the battery. If the battery only takes 100mA at 14.25 volts when fully charged (on my HF float charger), then I don't want to know how high the voltage would need to be in order to make it eat 60A.

So getting off in the academic weeds... If you knew what the resistance of the shunt is. And you knew what the resistance of the ammeter coil is. You could calculate the current ratios. In other words, you could calculate "If there is 'X' flowing through the shunt, the meter will read 'Y'  ". Armed with that info, you could back calculate what the actual system current was when your gauge was reading 60A?

In other words, if the gauge current is supposed to be 1/100 of what the shunt current is, and the gauge was reading 60A, then the real system current was actually 600mA. And then if your clamp on gauge reads about the same, it'll confirm that's what happened? Maybe? Possibly?

Like I said... Off in the weeds. The DC current clamp sounds cool. Maybe it'll shed some light. My prediction? It won't do anything wrong while you have the clamp installed. So just keep the clamp installed on the car.  Haha!

[Dave WM]

1 hour ago, Captain Obvious said:

. My prediction? It won't do anything wrong while you have the clamp installed. So just keep the clamp installed on the car.  Haha!

Exactly, my guess as well, after 150+ miles not a hint of a problem...