85 4runner smoking alternators, please help!

dweatherman2000

Active Member
Joined
Apr 7, 2007
Location
candler, n.c. 28715
I have put on 3 alternators and the alts keep smoking out.I,m getting an ohm reading from the main hot fuse block next to the battery, to any ground from the motor to the body. Have everything turned off in the truck. ohmed from the battery wire that goes in to the fuse block to a ground on the motor and then pulled each fuse to see if the circuit would break and it did not. I hate electrical gremlins. Anybody have any idea what to check. Could it be a bad ignition switch, seems to work right. When i have the ohm meter hooked up and press on the brake peddle the reading goes down, but i have disconnected the wiring harness to the back from the ecu and still nothing. HELP PLEASE any ideas!!!! This just started been running fine. Also disconnected all stereo hot wires. Thanks for any help.
 
So your saying there's a short to ground from the battery to pdc? I'd check current flow from the battery with the key off and see what it's at. I'm guessing your trying to say you've got a parasitic draw that's causing alternators to die? Don't really have enough information to help.
Maybe the questions to ask are 1. When you say the alternator is smoking out do you mean it just quits or its literally burning up? 2. What brand alternator are you using?
3. Have you checked alternator current output? 4. What symptoms were present for the original alternator replacement?
 
Last edited:
Yea burning and smoke from the back of the alt. There has to be a dead short somewhere. The alternators i put on, they were used but all 3 and yes 3 all came off of running trucks with no problems. I have no fuses burnt can,t see any wires that look like they were hot or burnt.
 
Let's start with the basics - what's the charging voltage? A dead short would melt a wire, blow a fuse or kill the battery in minutes if it was that significant. Is yours internally regulated or externally?
 
You should have some resistance - it should not be open! Resistance doesn't really tell you much in this instance. If you had a short it would kill the battery. I'm in town and then headed to my little ones birthday party. If I get a second I'll pull a diagram and try to help. It sounds like you have an externally regulated alternator (not a Toyota expert but I think they switched In 85 from externally to internally) and maybe the regulator circuit is shorted to ground. Look at your alternator and write down the wire colors - record the voltage off each on with alternator unhooked and key on.
 
I think you are performing some incorrect tests or describing something wrong. I'm not sure how you are measuring resistance with an ohm meter if voltage is present in the circuit. Are you sure you aren't doing a voltage drop test?

I would think that 3 alternators in a row would not have internal shorts but then again crazier things have happened. Can you confirm that these alternators are internally regulated? I could swear that an 85' has a regulator up near the radiator somewhere but then again these things are 30 years old so I forget the specifics. If externally regulated, I would start there.
 
Skinny said:
I think you are performing some incorrect tests or describing something wrong. I'm not sure how you are measuring resistance with an ohm meter if voltage is present in the circuit. Are you sure you aren't doing a voltage drop test?

I would think that 3 alternators in a row would not have internal shorts but then again crazier things have happened. Can you confirm that these alternators are internally regulated? I could swear that an 85' has a regulator up near the radiator somewhere but then again these things are 30 years old so I forget the specifics. If externally regulated, I would start there.
Click to expand...
I think he's thinking that if he test resistance to to ground in the b+ circuit and it shows resistance that it's indicating a short therefore killing his alternator ( I've seen people try and test parasitic draws this way) . I think it's far more likely that it's full fielding the alternator - with a bad or weak battery an alternator would get mighty hot in full field!
When I get a second I'll pull the diagrams but it seems like in 85 there was a internally regulated and externally regulated?
 
That's a good check, the OP needs to know that the battery must be disconnected for this to test correct. Plus all fuses should be pulled because the short would be pre-fuse.

Wondering if the battery has a shorted cell causing it to full field. I think you are on to something though, full fielding is a definite possibility.
 
I,ll try a different battery, but it keeps a charge good never went dead. I just hate to buy a new alt and burn it up. my alt is internal round plug pre 85s square plug. I rebuilt the motor in this one and have put 30,000 on with no problems till this. Hate electrical gremlins. I disconnected the alt plug before the alt. When i ohm from the battery + side to the alt side ground im getting a signal. When i ohm from the alt plug ground back the the fuse block no signal.
 
I did notice for awhile that the wire from the battery to the fuse block would get warm, but it had been doing that for a long time and never any problems. That could have been a short starting but never any blown fuses or anything ever noticeable.
 
I assume that it is the alt ground wire in the plug. You have a main hot that is a separate wire in a case right next to the alt plug. Its the main hot from the alt, it looks like fuse block but the wires are nutted together, when i ohm from this hot wire and its pretty heavy guage, to a body ground or an engine ground i get a signal. So thats why i feel theres a dead short. This main wire connects to the rear of the alt and then the round plug.
 
dweatherman2000 said:
I,ll try a different battery, but it keeps a charge good never went dead. I just hate to buy a new alt and burn it up. my alt is internal round plug pre 85s square plug. I rebuilt the motor in this one and have put 30,000 on with no problems till this. Hate electrical gremlins. I disconnected the alt plug before the alt. When i ohm from the battery + side to the alt side ground im getting a signal. When i ohm from the alt plug ground back the the fuse block no signal.
Click to expand...
So with the diagram I have I see this - a red, 2 whites and a yellow. Is that what you have?
 
Get those colors and I'll tell you which wires should have what on them. Also which color wire is the big wire will be helpful.
 
dweatherman2000 said:
It is your basic runner not sr5. It,s got duel cases, sliders, 4 inch lift with 33,s. Detroit in the rear. You want just the plug wire colors.
Click to expand...
Yup and the terminal it goes into - just to make sure. Should have 2 whites, a red and a yellow. The alternator terminals should be b, s, ig, and L. Red wire should be b+ (12v) with ignition on, both white terminals should be b+ at all times. Yellow we need to check with key on, and with engine running.
Any short to ground in any of these circuits will blow a fuse! Disconnected the yellow, will have resistance to ground, pretty likely that the red will as well. Both whites are straight through to FL - 80A there should be no resistance to ground under most cases. The red goes through a 40 amp fuse on one end and a 15 amp prior to the alternator, the whites go through a 80 amp fuse and the yellow is also fused. I seriously doubt you have a significant enough short to cause your issue. You'd definitely blow a fuse first !
 
Thats what i would think, but no blown fuses. I,m almost positive those wires are what i have. I have the tech cd and have looked at the diagrams. It want run hardly at all with the bad alt. If i jump it off with the bad alt it still starts smoking the alt. The resistance tests i have done are with no battery.
 
Back
Top