Junkers88

I used to do this on older vehicles to make sure the alt is charging porperly.

Well just as a test before I start putting the meter (yes I borrowed a meter and I'll post the numbers that I get) onto things to find out where the juice from the battery is going I pulled the + cable off the battery and the rig stumbled and died in a few seconds. Does this mean that the alt is going out or is this typical of rigs that have ECU's and such?

Thanks bunches!!

86Toyrunna

iv always heard NOT to do that.. if the alternator isnt bad already it can blow out some of the diodes that are in it.. never actually tryed it.. for sure bust our the meter and get some numbers.

abecedarian

the problem with things now is that the electrical engineers have figured out that volts divided by amps = current. They use this as a way to get around having large wires run around- pushing more volts means less amps so a smaller wire can be used.
So take a minute to think about it... what size wire runs from your alternator to your battery? 10 gauge, maybe? ... and what gauge wire runs from the battery to the starter or the primary engine ground? ... 4 or maybe 2 gauge?
How does that work?

So given that... would you suddenly want the 18+ volts the alternator can generate pushed into your 12 volt system? That's like taking a 120 volt light from your house and expecting it to work with 180 volts.

Now you know why you shouldn't disconnect the battery: the battery acts as a buffer between the charging system and the rest of the vehicle. IF you're going to do that, turn your headlights on high as well as every other thing possible to create a load for the voltage spike that will happen when you remove the battery.

But if you happen to remove the battery from the circuit and things just die (you'd think the alternator would make enough power to keep everything running... since it's charging your battery too, right?) and nothing blows out... chances are, fair to midland, that the alternator is on it's way out, or at least the regulator is.

Junkers88

Hey thanks again Abe. I'll quit messing around and put the meter on the alt/battery etc this weekend and put the numbers up. Since the truck and 4runner have the same motor can I use the truck numbers as a baseline so I'm not flooding the forums with dumb questions? Or is there going to be other factors involved due to the years being different?

In case I didn't say this before I'm a wiring ding bat. I can't even really wire in my own radio and the only wire stuff I've ever done was a complete rewire of a 72 hot rod for a friend but that was a new aftermarket harness and had instructions.

abecedarian

All in all, yeah, you can compare the well running truck to the 4Runner, as far as checking volts at the battery and alternator and wiring issues. There's very little difference between the charge system between an 86 and 91. The biggest difference is carb or non-carb (which 91 generally didn't have).

abecedarian

This might help you: http://autoshop101.com/forms/h8.pdf

scuba

Do you have the charging light...Ie...Brake and Battery lights in the dash...?
If you see those 2 lights then your alternator is taking a crap on you...

abecedarian

I sort of thought we've already passed that point.

scuba

Which Point..? First sentence I dont see in this thread.
Second one sorta..

abecedarian

This:

GSGALLANT

I hope that was a typo...

Should say: volts divided by resistance = current.

amps are a unit of current.

Junkers88

There is a charging light/brake light in the dash but none of the dash lights work yet. If I'd have been able to see those two lights come on then I'd have known the alt was bad since the truck did the same thing last year.

Thanks though.

InternetRoadkill

Never tried it, but I suspect disconnecting the battery even with a healthy alternator is going to cause problems for the FI. It's not the volts, but rather the noise from the alternator.

scuba

Ahh, I see, Well Good luck with her buddy !!!

scope103

I've never actually taken the alternator apart to verify this, but most alternators have "external field excitation" so that you need the battery for the alternator to produce any juice. In a Toyota alternator, there is an internal IC regulator that is powered from the IGN line, and I think that line will go to zero when you disconnect the battery, shutting off the regulator.

In the old days, series wound (or is it shunt wound?) generators could keep producing output without a battery once you got them running and producing output. But I don't think an internal-regulator alternator will.

But I could be wrong.

I'm not sure why abecedarian expects 18v; the regulator should keep it at no more than 14.1 volts (if it worked at all; I would expect it to head instantly to zero). But there could always be a voltage spike if you do something as drastic as disconnect the battery while running, and the electronics will look askance at that!

1989toy4wd

Yeah dont try this on a newer vehicle...cause the voltage is regulated through the ECM!! So the 18+ volts will be going to the ECM. Imagine that repair bill!!

GSGALLANT

I believe you are correct.

... and I also recommend never disconnecting the battery from a running vehicle. There are even big bold warnings stating that in most newer vehicle FSM's.

fireman1559

one my old ford my 1997 when i was trying to sell it the battery was dead so i took it out and when ppl came to look at it i just put jumper cables on the terminals that hooked to the battery and the truck would start and once it started i pulled the jumper cables off and the truck ran fine i even drove it around for about 15 mins with out a battery and never had a problem

Ed H.

Was it a jumper box that you left connected or just cables that you used to start it, I used to use a jumper box on cars with bad alternators when I worked for a tow company I just left it connected and followed them short distances so they would not have to wait for a tow truck since I only drove a service truck. All of a vehicles electricity is supplied from the battery the alternator just keeps it charged, the alternator does not supply enough electricity to run everything on it's own.

GSGALLANT

Wrong... Think about it... If that was the case, you wouldn't be able to drive long distances without stopping to charge your battery with an external charger. The alternator supplies the running load for the vehicle, and also provides the current to keep the battery charged. The battery is reserve power, and is there to start your vehicle. It also acts as an electrical buffer to prevent voltage spikes. With most newer Toyota alternators, the battery voltage is also required to provide the excitation power to the voltage regulator so that the alternator will actually produce electricity.