Jump to content
Dave WM

Ammeter acting up

Recommended Posts

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 by Dave WM

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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.

 

 

Share this post


Link to post
Share on other sites

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.   LOL

  • Like 1

Share this post


Link to post
Share on other sites

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.

 

Share this post


Link to post
Share on other sites

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.   :beer:   Intermittent electrical issues suck.

Share this post


Link to post
Share on other sites

@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.

Share this post


Link to post
Share on other sites
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.   :beer:   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.

Share this post


Link to post
Share on other sites

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 by Dave WM

Share this post


Link to post
Share on other sites

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

:)

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites
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...

Share this post


Link to post
Share on other sites

so after all my 70 mile trips and all good, I start out on my 400 mile trip (800 round) about 200 miles it I notice my headlights getting much brighter (more white than yellow) while the ammeter is acting up again....

so we really do have a excess voltage causing excess charge current. Here is the symptoms (I had 200 miles of night time driving to panic about it, lots of observation going on).

there was a pattern, high charge for a couple seconds, between 2-3 seconds I would guess, followed by nearly as much of a discharge for about the same amount of time. Charge then Discharge, then A.OK for a variable interval of seconds to minutes..

My guess is the alternator was at times overcharging causing the VR to kick down the field winding current throwing alt into a discharge as soon as the intermittent overcharge would stop. How would alternator actually do this? again a guess if one or more of the diodes in the alt was failing open, then I presume the overall output would be less, the VR tries to compensate by increasing the field current at which point the intermittent open diode begins conducting again, now with excessive field current, suddenly the over charge, the VR sees this, cuts the current  and then the diode goes open again now with reduced field current therefore reducing alt output going into discharge mode. the actual cycle starting on over charge.

So the diode is mostly open but will momentary conduct throwing the charge cycle into the charge/discharge mode. Just a guess. I was able to get a reman alt on the road, swapped it and the problem ceased. I still get a little jumpiness at idle but its only a fraction (maybe a few amps tops) and only while sitting at idle a completely different situation. will go back and review the my connections as the swap was done in the field with not my best tools.

again the pattern was Normal reading (center) then a sudden charge for seconds of 30-40 amps, followed by discharge close the same amperage and time the return to center on the scale. pause for seconds to minutes to hour...random time between events, but each event lasting approx. 4-6 seconds (2-3 charge, 2-3 discharge).

due to circumstances beyond my control I was NOT able to have them test the alternator that I replaced. so will never have a definitive test to know if that was indeed the issue.

I know typically SS devices fail either open or shorted, but perhaps its not the diode but rather the connection of the diode? Its some kind of top hat looking diode soldered into a mount (multiple diodes that is).

 

 

 

 

 

 

 

 

 

Edited by Dave WM

Share this post


Link to post
Share on other sites

another data point, the OE Nissan green fuse link looks like the plastic heat shrink on the female spade terminals cooked itself off. Ordered some new ones. The wire is still intact but I suspect it got hot...

I have searched on intermittent over follow by under charge then return to normal in google, nothing came up. there was something on shorted windings but it was a blocked site for me.

Edited by Dave WM

Share this post


Link to post
Share on other sites

When I get a chance, I'll take another look at the wiring diagram and see if I can draw any conclusions.

I still don't think you were really pushing 60A into anything. That's welding current.  LOL

Share this post


Link to post
Share on other sites

I would suspect that the voltage regulator was messing up if you were seeing voltage spikes.

The symptoms you described would be along the lines of a loss of sensing (maybe from the VR overheating) causes current injection into the stator of the alternator, kicking up the voltage. However, shortly after that, sensing resumes, the system stops injecting current into the stator, dropping the voltage of the alternator (maybe too much).

Is your charging system stock, or do you have an internally regulated alternator?

Edit: I forgot to add that you could get something like this to get a voltmeter in your car: https://www.amazon.com/gp/product/B07VZFSY4SInsert other media

Edited by SteveJ

Share this post


Link to post
Share on other sites

I discounted the external VR as an issue (stock system) since I got the exact same behavior after switching to a backup new VR. I could accept a bad VR even a bad second VR but for the  identical pattern, "ok, then high charge then low charge then return to normal" exhibited by both.

So since I know its not a meter malfunction (much brighter head lights during the charge event) and the behavior is the same regardless of a different VR, that leaves me with 3 options

Bad battery (somehow influencing the charge/discharge cycle).

Wiring (I checked for good ground and good connections thinking the VR's may be sensing a low voltage) but the recurring pattern seems odd for that.

Alternator somehow confusing the VR's, this was the most likely although hard to figure the exact fault. My assumption is the alt was having an intermittent fail, be it loss of internal ground of the diodes, intermittent open of one or more of the diodes, winding faults of the stator or field. Again not exactly how but it seemed the next logical thing to replace.

After replacing the alt I made the 400 mile return leg with out a recurrence of the "charge/discharge/back to ok" cycle. there remains some very small events I noticed at idle but it was a very different event, no pattern and a much smaller scale (amps not 10's of amps flux), and more of a ammeter needle vibration than a sustained 2/3 seconds long event. this event is not constant either, just every now and then when I am sitting at  stop light the ammeter would vibrate a bit. I suspect something is still not 100% correct, I should have just rebuilt the OE part with some bearings and a new diode pack (I kept the OE alt from years ago when it shorted the diode, and I replace the alt back then, did not give it back as core). So the presumed bad alternator that I replaced on the trip was the autozone one with about 5+ years and about 25k miles on it. Maybe that's about what to expect with a reman...

I did order some new OE Nissan green fuse links, since the one in there looks like it got hot.

and after going out to take a ride around the block to test again, I have a FLAT TIRE, thanks to a well place nail, argh...

 

Edited by Dave WM

Share this post


Link to post
Share on other sites

Ah, since you seem to have eliminated the VR from potential causes, I would think it's the stator or something related to the stator. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.