Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Replacing an alternator

Hi guys.
Okay, now I’m going to access your collectoive knowledge and experience.

My alternator is whirring, confirmed by me with a mechanic’s stethescope. So I’ve ordered one and will change it.
Typical protocol is to disconnect the battery + lead (using the backup 12VDC bit to retain the memories).

But last night I was wandering through my circuitry schematics (light reading…I was actually preparing to install fog lights) and I looked at the alternator circuits (attached). As can be seen, the system is negatively grounded, and the solenoid to the IG1 relay, which enables the positive circuit to the alternator, is no energized when the ignition cylinder (key) is OFF. The circuit to the IG2 10A fuse is also routed through the ignitions switch. The positive circuits go through the ignition switch (one), to the battery positive plate (two) and through the contacts of the IG1 relay. Three of the positicve circuits disconnect by unplugging the 3-pin plug, the others aren’t enabled with the key OFF.

Whi made me wonder. If I unplug the plug on the side of the alternator, all positive-side circuits open. At that point, only the case ground is connected. It seems to me that by unplugging the alternator, it should be perfectly safe to dismount and remove the alternator even withe the battery fully connected to keep my memories alive.

Gentlemen, any comments?

That link doesn’t seem to work.

Did I see it correctly- what about the B wire that comes directly off the battery?
Battery through 120A fuse to 2A (B) then directly to alternator. Those are usually a lug on stud connection but maybe different on this car…

You can buy a small device to plug into a 12V DC outlet to maintain your memories.

Unplugging the alternator with the battery connected could be OK, but why take a chance?

Ask the counterman at your parts store if they will give you another alternator if you install it with the battery connected? If he says yes, then you are only out the labor and extra trip to parts store if you fry a diode.

First, let me repost the link.

TT, in this case the lug circuit goes through open contact sof the IG1 relay. Perhaps the L,B,& S circuits are in the 4-pin connector.

Uncle T, I thank you for the suggestion. I’m all set with that, but got to wondering after lookin at the circuit if it was even really necessary. Once the 4-pin plug is unplugged from the alternator, there’s no possible circuit from which to create a spark or a pulse of any kind unless I should somehow allow the pins on the plug to contact a ground surface…which would, of course, create an arc and a dead short. In short, if I pull the plug and cap it while I work, there should be no need to disconnect the battery.

I’m open to comments, especally now that the link should be good. I have an enquiring mind.

still not working for me, either in FireFox or Safari. error 404 not found.

Bummer. I’m not a computer guy, but I just tried to open it and it won’t open for me either. I’m going to play with it at this end and see what I can do.

I can’t seem to get it to work. It’s a different area in the Toyota documents area than I generally try to attach, and pperhaps there’s some sort of security function involved. I’ve looked at the properties, secuity, etc., but to no avail.

Try the above link and go into Wiring Diagrams, to page 2 of the very last link in the list.

It works if you cut and paste the whole link, including the (Part%202).pdf

The wiring diagram shows two connectors. Connector “B” is a stud and eyelet connector, always hot.

I have left the battery connected when there is enough room to remove the cable from the alternator without bumping the wrench into ground. After removing the cable from the alt. slide the rubber boot over the eyelet and cover it with tape.

You can do it, but that B lead will be hot and if it touches ground anywhere, there will be arcs and sparks and blown fuses. If you do decide to do this, I’d suggest that you have something that you can cover the terminal end of the wire immediately to insulate it, and then be very careful.

Hooking up that lead to the alternator while hot is no different than hooking it up cold and then reconnecting the battery.

Your wisdom is appreciated. If I do try it, and I might, I’ll surrround the work area with a rubber mat befor estarting and be sure I have rubber tubing to isolate the lug while working. There doesn’t appear to be any risk to electronics with a bit of care. The ECU circuit is isolated by the Ig1 relay open contacts.

I realize this stuff can surprise a man. I may just chicken out, apply a drycell 12VDC source in parellel with the battery to maintain my memories, and disconnect the negative battery cable.

I’ll let you know when I get to the job.

Any mechanic worth his salt knows before working on any part of the electrcal system on a vehicle always: DISCONNECTS THE BATTERY FIRST!



Thats true tester but when Toyota pays 24 minutes labor to diagnose and replace an alternator sometimes we cut corners. At home I always disconnect the battery before working on the car.

BTW with any airbag equipped vehicle the first step in the instuctions is to disconnect the battery, so why would you connect an alternate power source?

TT, in this case the lug circuit goes through open contact sof the IG1 relay. Perhaps the L,B,& S circuits are in the 4-pin connector.

The B circuit I referenced does not appear to go through a connector. Looks like Nevada sees it that way too.

I work on house wiring live a lot of times but I always disconnect the battery in my cars when I’m going to have a loose BAT+ line dangling. Go figure.

I’ll bet you don’t have an extra 120 amp fuse.

Don’t want to lead you astray but I’ve never disconnected the battery to change an alternator. You have to be careful not to let the wires touch ground though and usually put a short piece of rubber hose over the connection. Otherwise got to use that memory keep alive device.

Me, I’d jot down the radio stations you’ve programmed, etc, then \ just disconnect the battery (negative side first) and be done with it until the new alternator is installed. After everything was verified working, I’d reprogram the memories. It’s certainly possible to install an alternator with the battery connected, but there’s a risk of spaks and damaging the new alternator diodes should a mishap occur.

Call me crazy call me insane call me what you will but late for dinner. I my self in 30years on playing shade tree mechanic never have disconnected the battery when disconnecting the Alt.i just always made sure to cover the wire if its not part of a quick connect with something to keep it from grounding out.

You guys have made a lot of excellent points, and provided a lot of good insight.

Tester, your coment is well founded. I would never recommend that anyone else do it this way (although it’s apparently more common than I would have thought), nor would I do it on anyone else’s vehicle, but I happened to be perusing the circuits in preparation for installing fog lights, knew I had to replace my alternator, and wondered “what if?”. So I took a peek at the schematic. I enjoy such light reading, and often just sit and study manuals and textbooks. Novels to ma are a boring waste of time.

It’ll be a few weeks, as I just got scheduled for cataract surgery on Dec 10th, the other eye to be done 4 weeks later of all goes well. But I’ll follow up when I get there.