Testing The Shunt?

[Gary Lewis]

As many of you know, I don't think the shunt in our Bullnose trucks is up to the output of any 3G alternator, much less that of a 130A unit, so I'd like to test that theory.  And I have the leftover shunt from the harness I made for Big Bro laying on the worktable.

But I need a source of stable power and I need an adjustable load.  I can use Big Blue's electrical system as the power source but have been struggling with coming up with an adjustable load.  Fortunately Jim has just reminded me that I have a carbon pile battery tester, and while it didn't work when I first tried it maybe I can figure out what is wrong with it and get it going.

As for the process, I think hooking the shunt up in series with the battery tester and placing that combo across Big Blue's battery would be the starting point.  And I'll put my clamp-style ammeter around one of the leads plus wire a Bullnose ammeter across the shunt.  And then we'll start ramping up the load in 10A increments.  Right?

If so, that leaves deciding what will be the symptom that says that's the limit of the shunt.  I can think of several, like the insulation melts off or the thing starts smoking.  But what about taking temp readings of the shunt with my infrared thermometer?  Is there some point where we could say "that's too hot"?

Ideas?  Thoughts?  Suggestions?  

[viven44]

Could you remind us what the problem with oversizing the shunt is ? If I recall correctly from our recent discussion it would affect the 'swing' of the gauge itself but wanted to confirm.

As far as your testing, how are you planning to limit the current from the battery ? I don't know of any loads unfortunately at that current level.

Another thought is to use a circuit breaker in series as a safety in your experiment

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[Gary Lewis]

The shunt is just a smaller piece of wire that is designed to drop the right voltage across it at a given current through it that will send the ammeter to full scale. If you use a larger wire to replace the shunt then it will take more current to peg the meter.

While that seems reasonable at first blush you’d quickly find that then the ammeter doesn’t move much, if at all, with normal loads. For example, the 1G alternator on Dad’s truck failed me on a trip to visit him. But it was daytime and I had the windows down so there was no load other than the ignition. And the movement of the ammeter was so small that I didn’t catch it until the battery was so low the engine wouldn’t run.

As for the load, that will hopefully be the carbon pile battery tester. And it should be slowly adjustable from nothing up to over 100A.

[viven44]

Ok thanks. I glossed over "carbon pile battery tester" but had no idea that it was actually a "load tester" hence the "load". Looks like I need to learn a "load" more

One thing to keep in mind is if you have soldered connections on the shunt, the solder to metal interface will actually degrade before the metal itself as it is subject to electromigration forces. Solder also melts around 200C, intermetallics (Copper-tin intermetallics mainly) grow with heat aging and Joule heating, all of these will cause the solder to degrade faster that way.... we are talking probably a long-term reliability issue, which I don't think is part of your tests here. That is why on some of the high amp connections, crimping is actually preferred as it is a "thermocompression-like" metal-metal bond without any electromigration issues or joule heating effects.

[Gary Lewis]

My plan was just to test the shunt the way it is, but your comment made me think about that more and that may not work.

I say that because the harness has a relatively small wire feeding the shunt, meaning the Bk/O wire from the alternator, as shown below.  It may be as much of a limit as the shunt itself.  So maybe I need to take the splices, S202 & S203, apart and just test the shunt by itself.  And if I do then I'll probably crimp connectors to it.

[viven44]

Curious what has made you paranoid about this all of a sudden? Noticing anything on Big Blue ?

A metric to keep track of long term (or even in your hammer testing here with the battery load tester) is the resistance from point A to point B. You can decide where point A starts and point B ends and what all it includes. (because when electrical connections (Wires, connectors) start to fail the resistance starts increasing but it may be hard to track precisely it with the non-kelvin multimeters we typically possess.)

[Gary Lewis]

The only reason I'm thinking about this now is that I have the harness laying on the work table and Jim mentioned the carbon pile load tester.  I put 2 & 2 together and realized that I could fairly easily test my assumption that the shunt can't carry more than about 70A w/o burning its insulation or some other failure.

But I don't have the ability to directly measure the small resistances we are talking about here.  From my testing it takes about .100 volts across these ammeters to get them to swing to full scale.  Doing the math: R = E/I = .1/70 = .0014 ohm.  I cannot measure anywhere near that small of resistance.

But I can measure current and voltage so I can back into resistance, which I will do.

[viven44]

I guess 'paranoid' is not the correct word, as I just realized Big Blue might have an upgraded shunt already. You just want to validate a prior assumption that the stock shunt wasn't good enough for 3G....

OK I am very interested to see what comes of it... I think that stock shunt in that pic is pretty darn beefy and should be able to handle the 130A.

Oh... and fusible link J is also in the same current path as the shunt right ? It will guaranteed burn up before the shunt.

[Gary Lewis]

viven44 wrote

You just want to validate a prior assumption that the stock shunt wasn't good enough for 3G....

OK I am very interested to see what comes of it... I think that stock shunt in that pic is pretty darn beefy and should be able to handle the 130A.

Oh... and fusible link J is also in the same current path as the shunt right ? It will guaranteed burn up before the shunt.

Yes, I want to validate my assumption.  And yes, Fuse link J is normally between the shunt and the battery, and it isn't going to like 130A.

As for Big Blue, he doesn't have a shunt.

[85lebaront2]

Neither does Darth, he has a 200 amp fuse between the alternator and battery and has the 1996 instruments with the oil pressure gauge made functional and a volt meter.

[Gary Lewis]

Yep.  Big Blue has a 150A fuse and a voltmeter in place of the ammeter.  Some day I may put a larger alternator in and all I'll have to do is to replace the fuse.

By the way, as part of this test I plan to use the new hand throttle to map out the current output of the 130A 3G alternator vs engine RPM.  I'd like to know that so when I'm winching, which I've done plenty of times, I'll know how far to rev the engine up to get max output.

[ArdWrknTrk]

There are no solder connections in a harness.
All splices are pressure welded (copper on copper). 💡

[viven44]

Thats a good point. I learned a lot of best practices from reading the painless manual on how factory connections work.

I only solder the connections that I know are low amperage and the ones that I know need to be taken apart most likely.

When I do crimp, I use the "marine grade" ones that come with the built-in adhesive that make the connection completely waterproof. I love these as they are so intact once the connection is made and in fact impossible to undo. Anything in the engine bay definitely gets this.