Electrical gurus: How can my charging system be doing this?

[BradW]

I don't know the specs on your car so I will ask the following.
Is the engine standard? Standard turbo? Standard boost pressure? If it's been modded and is coming on boost harder than standard, then I would put my money on belt slip.
It is hard to explain but here goes.
When the alternator is charging it is harder to turn. When the engine comes on boost, the sudden increase in engine revs might cause the belt to "loose traction" on the alternator pulley. The alternator will still turn, just not quick enough (due to slippage) to put out proper charge. When the engine comes off load, the belt "regains traction" on the pulley and starts charging properly again. Hope this makes sense.
Just be careful not to overtighten the belt as it can put excess load on bearings and pulleys and cause something to fail. If you find you need the belt super tight to overcome the problem, then it might be better to put a slightly bigger pulley on the alternator. This will make it easier for the engine to drive the alternator and hopefully eliminate the slippage. It will also protect the alternator from being over reved if you are revving the engine alot higher than standard. The only down side will be the charge rate at idle may drop a bit, depending what size pulley you opt for.
Hope this helps.

[hemi twofifteen turbo]

if you had a wire that big shorting you'd know about it! Try putting an amp gauge (multimeter) inline with the main positive feed to the battery, and also one the alt feed. See if you get a big change in amps under load, and if so it's a short, if not, then something is going on with the reg part of the alternator. Being that u've used 3 diff alts, it has to be part of the car. From memory it's a 3 pin plug? Battery sense, charge light (fields) and B+ output? is battery sense it's own wire running all the way to the battery positive? Have you tried hooking a direct + from the battery to the charge light wire to make sure it's not losing power there? Also how is the plug to the alt, (maybe a bad contact in there?) (hooking the direct +'ve wires up will show that)

oh make sure when you do the +'ve test wires to have the other wires leading to the plug disconnected (ie don't just stick your +'ve probe wire in the back of the plug, cause if there is a fault on the wire (short etc ) it will still effecrt the direct +'ve as well

hope this helps, i'm very tired so i might have got my diagnostic logic a bit wrong.

[1JZ-Rolla]

BradW: Car is not quite stock, making 278rwkw at the moment with single turbo, bigger injectors, aftermarket ECU etc etc. It does come on boost very hard with the auto box & 2800rpm stall converter, you may be on the money with belt slip now that I think about it, but perhaps not the belt slipping around the alt pulley, but rather the crank pulley slipping inside the belt.

The belt is tensioned by the standard JZ spring loaded tensioner, so I can't adjust the pressure. I already have a slightly larger alternator pulley from a 1UZ alternator, so it is underdriven somewhat.

setup looks like this:

[1JZ-Rolla]

if you had a wire that big shorting you'd know about it! Try putting an amp gauge (multimeter) inline with the main positive feed to the battery, and also one the alt feed. See if you get a big change in amps under load, and if so it's a short, if not, then something is going on with the reg part of the alternator. Being that u've used 3 diff alts, it has to be part of the car. From memory it's a 3 pin plug? Battery sense, charge light (fields) and B+ output? is battery sense it's own wire running all the way to the battery positive? Have you tried hooking a direct + from the battery to the charge light wire to make sure it's not losing power there? Also how is the plug to the alt, (maybe a bad contact in there?) (hooking the direct +'ve wires up will show that)

oh make sure when you do the +'ve test wires to have the other wires leading to the plug disconnected (ie don't just stick your +'ve probe wire in the back of the plug, cause if there is a fault on the wire (short etc ) it will still effecrt the direct +'ve as well

hope this helps, i'm very tired so i might have got my diagnostic logic a bit wrong.

Yep, I reckon i'd just about have an engine bay fire to almost completely drop the charge from a 100amp alternator to a short circuit.

I think it might get a bit expensive to buy two current/amp gauges capable of up to 100 amps (not to mention the cranking current much higher) to check the battery vs alternator loads.

Plug to alternator fitting is secure, battery sense bridges directly to the positive (B) post on the alternator, the other two pins (12v ign and dash light) are factory wiring and have been tested. This is exactly as it was wired with the 4AGTE setup with no problems (also denso alternator)

Cheers
Phil

[DX20VT]

Have you looped the sensing wire ( usually white) out of the 3 pin plug onto the alt output terminal? Normaly they go back to the main alt fuse so they compensate for voltage drop.

Are you measuring the voltage at the alt? or at the batt?

All you can do is as said above, check the voltage on the sense and ign wires when the fault is occouring.

Are you sure you have the 3 wires the righ way around?
If you look into the plug so that the pin at the top is flat, and the two underneath are parrellel to each other,
top one is sensing and should be permanent 12V
bottom left is ign and should be 12V with the key on. 0V with the key off.
Bottom right is the Light and if you earth that one the dash light will come on, and go off if you remove the earth.


Stupid thing with the way the regulators work, the ign wire turns the internal circuitry on,
so if the IGN wire has no power or breaks etc etc, the warning light will not come on as it thinks the car is turned off.

[BradW]

BradW: you may be on the money with belt slip now that I think about it, but perhaps not the belt slipping around the alt pulley, but rather the crank pulley slipping inside the belt.

That sounds more likely. Not sure how you would overcome this with the spring loaded tensioner setup. Maybe a tensioner off another car with a stronger spring (???) or redesigning the belt setup to include an adjustable tensioner?

BTW, very nice, clean engine bay you have there. Look like a high quality set up .

[1JZ-Rolla]

DX20VT, Yep, the white sense wire is looped onto the alt outlet terminal.

I have measured voltage at both the alternator and battery, both read identical.

Checked the 3 plug wires tonight, the dash light and IGN pins are reverse to what you are describing. I assume you are referring to viewing the plug from the front, ie: terminal side, not wiring side?

The diagram on the alternator itself refers to looking at the plug socket on the back of the alternator yeah?

[DX20VT]

DX20VT, Yep, the white sense wire is looped onto the alt outlet terminal.

I have measured voltage at both the alternator and battery, both read identical.

Checked the 3 plug wires tonight, the dash light and IGN pins are reverse to what you are describing. I assume you are referring to viewing the plug from the front, ie: terminal side, not wiring side?

The diagram on the alternator itself refers to looking at the plug socket on the back of the alternator yeah?

If it does the fault free reving without driving, try hooking the the sensing wire to the battery rather than the alt output terminal and see what happens.

the diagram I described is looking into the alt plug itself, so will be looking into the wiring side of the plug not the pin side. If the diagram's on the alternator go off that, but should be as I described.

[1JZ-Rolla]

unfortunately the fault can't be replicated by free revving.

ok cool, then we are on the same page. My wiring is the same as you describe, which matches the ID sticker on the alternator itself.

[lexsmaz]

Sounds like there is some good feedback you have received, Adding to the belt slip theory how much grip are you getting from the belt, when you try & turn the alternator pulley / nut with a socket & ratchet set up ... Are you getting any heating up issues of the pulleys from slippage ...
Would like to see some photos of this car if possible would be an interesting looking beast & certainly fast ...
Regards ..... Rob ..

[thechuckster]

i have a similar thing with the RA40 where the tacho plummets to 0 under boost.

-the 12V supply for the tacho driver is from the injector 12V supply (same wire also powers my wideband controller)
-same 12V source (upstream of the main EFI relay and fuse) is the same power source for the dash and coil
-under boost during the day, the tacho drops to 0 when i nail it
-at night-time the tacho seems to stay at the pre-heavy-boost rpm until i let off the throttle.

i think its a two-part problem - lowZ injectors plus resistor pack being driven at significant % duty pull a lot of current? and poor earths up the front of the chassis (just fixed one in the lighting system yesterday).

1JZ-RollaP: do you get any flickering of the headlights/dashlights or noticable system voltage drop when you turn the indicators on?

[wilbo666]

DX20VT, Yep, the white sense wire is looped onto the alt outlet terminal.

The Sense wire should go directly to the battery, not the output of the alternator!

Reason:
When you are charging there will be large current flowing from the alternator to the battery. (Large load [boost], means large spark and large injector PW, means large current being drawn...)

This current will cause a voltage drop across the thick wire from the alt to the battery (V=IR), say 50A of current is being used (50A should be coming from alt), and AWG10 wire (~3.34ohms/km) then the voltage drop is 0.5V over 3M (I'm not sure how long your alt->battery wiring is!).

This will mean that the voltage at the alternator end will be 0.5V higher than at the battery.

So the sense pin on the alternator will really be 0.5V lower than it should be!
(calc)

The sense wire should be run directly to the battery. As the sense pin on the alt is high impedance it will take very little current and the voltage at the alt sense pin will be very very close to the voltage at the battery.


I'm not saying that this is the problem, 100%, but it is something that I would fix

Cheers
Wilbo

[The Witzl]

Wilbo is on the money.
Get the voltage sense wire from the 6-pin connector that the external voltage regulator used to plug into on the factory wiring. That's what i;ve always done when upgrading to internally regulated alternators.

[1JZ-Rolla]

The Sense wire should go directly to the battery, not the output of the alternator!

Reason:
When you are charging there will be large current flowing from the alternator to the battery. (Large load [boost], means large spark and large injector PW, means large current being drawn...)

This current will cause a voltage drop across the thick wire from the alt to the battery (V=IR), say 50A of current is being used (50A should be coming from alt), and AWG10 wire (~3.34ohms/km) then the voltage drop is 0.5V over 3M (I'm not sure how long your alt->battery wiring is!).

This will mean that the voltage at the alternator end will be 0.5V higher than at the battery.

So the sense pin on the alternator will really be 0.5V lower than it should be!
(calc)

The sense wire should be run directly to the battery. As the sense pin on the alt is high impedance it will take very little current and the voltage at the alt sense pin will be very very close to the voltage at the battery.


I'm not saying that this is the problem, 100%, but it is something that I would fix

Cheers
Wilbo

I understand the theory in this, however using the theoretical example above (which is actually quite close to my setup), the voltage difference of 0.5v between battery and alternator post should result in the battery voltage registering as 0.5 volt below ideal (as the regulator will think the voltage is actually 0.5v higher), correct?

If that is the case, my setup losing 2.5 volts (drops from 14.0 to 11.5) is pretty extreme, perhaps out of the scope of any potential voltage drop through the charging lead?

I am happy to give it a try at this stage though, its one more thing to eliminate.


thechuckster: No, I don't get any significant fluctuations with the indicators flashing or any other intermittent electrical load.

Cheers

[wilbo666]

I understand the theory in this, however using the theoretical example above (which is actually quite close to my setup), the voltage difference of 0.5v between battery and alternator post should result in the battery voltage registering as 0.5 volt below ideal (as the regulator will think the voltage is actually 0.5v higher), correct?

If that is the case, my setup losing 2.5 volts (drops from 14.0 to 11.5) is pretty extreme, perhaps out of the scope of any potential voltage drop through the charging lead?

I am happy to give it a try at this stage though, its one more thing to eliminate.

My example was pretty rough,

It also doesn't take into account things like connection resistance of the wires, connecting resistance of the battery terminal etc (and you can see that milliohms add up when we are talking 50A +)

To have a voltage drop of 2.5V @ 50A you need a resistance of 0.05Ohms (50milliohms!)!

You could try putting a multimeter from the alternator output to the battery +ve terminal and determining the voltage drop yourself if you are interested!

Cheers
Wilbo