Hi all,

1991 Toyota Pickup 22re 4x4

Battery/Brake light started coming on intermittently and then stayed on one day. Was having problems with intermittent no start (start with jump). ~12v across battery terminals when car was running. Removed alternator and had it bench tested (at O'reillys) and it was bad. Ordered reman alternator through Toyota and installed, still no charge on battery. 17.5 V from alternator charge post to ground. All voltages and continuity going in and out of alternator were as expected otherwise. Got Toyota to replace alternator and still having same issue.

I'm now on my second reman alternator from Toyota and still getting 17.5 V from charge terminal to ground but nothing is showing up on battery (11.94 V on battery while car is on). The battery briefly flashes 15.8 V when I first touch the leads but it goes to 11.94 in a split second. Here are some voltages / wires i tested:

Red wire (on alternator) = 11.94 V. Supposed to be constant 12V and excite alternator to produce voltage.
White smaller gauge wire (on alternator) = 12.24 while car off. Battery voltage as expected.
Yellow wire = 0.7V from charge fuse to negative lead on battery terminal. This goes low when alternator voltage is out of range. This also is what turns on battery/brake light.
Charge terminal (thick gauge white wire) = 17.5V from charge terminal to negative post on battery. Continuity from charge terminal to positive post on battery while ign on or car is running.
Charge post to ground (with charge wire disconnected): 72.3k resistance as expected
Ign, Charge, and 80a fuse all checked and working.

A couple things that might be unrelated but stick out in my mind. One day I was stuck in a parking lot and my car wasn't starting so I turned the key and held it hoping the starter would kick in and I noticed smoke coming from near the ignition. Car started after a few more tries without a jump. Power to my blower motor / ac went out the same day my charge system lit up on the dash.

Let me know if you have any more ideas for things I can diagnose. Thanks in advance!

4bang

 

I'm also going to add that I got the newest alternator bench tested at an advance autoparts before putting it in and it supposedly "passed." Based on my testing and everything else i've read it seems like the internal voltage regulator is shot but I also find getting two bad alternators in a row from the dealer kind of unlikely

 

The charge wire (thick white or yellow wire) goes straight from the "B" terminal of the alternator to the 80A fuse then to the battery. If you're getting 17v at the B terminal, but nothing (12v) at the battery, what does that tell you?

As a quick and dirty, what voltage to ground do you get on the B terminal with engine-off? You should just get battery voltage, and I expect you'll get basically 0v.

Check the 80A fuses. If they're good, check the length of the charge wire for a break.

The smaller red wire is, as you say, for field excitation. It's not regulated to be at 12v; it's just whatever the battery can push through the ENGINE fuse. The (smaller) white wire is the Sense wire; it tells the alternator what's going on AT the battery. Since your sense wire is telling the alternator that "nothin'" is happening at the battery, the alternator keeps futilely raising its output voltage (in your case, up to 17v, which is way too much to charge the battery if that made it to the battery). The (small) yellow wire goes to ground when the alternator is not producing output. Measuring its voltage to ground is not too helpful; if you care you could measure the voltage from battery+ to "L".

 

Hey Scope, thanks for the quick reply. What you explained all makes sense to me but I had a few follow-up questions just to nail down my understanding of the charge system. I tested the 80A fuse by testing the continuity between the two metal tabs at the bottom and there was no issue. I also tested continuity of the charge wire from the B-terminal to the 80A fuse as well as from the B-terminal to the positive of the battery. These both checked out so I assumed that meant there shouldn't be a break in the charge wire. I will check the B terminal to ground with the engine off when I get home. I guess my main question is how should I go about locating the break in the wire? When I measure from B terminal to positive pole of battery with the car on, I get around 5 volts which I assume equals the voltage drop between the two. Is it possible there is a short or faulty ground somewhere?

 

Sorry, a lot of this doesn't make sense to me. If the charge wire is continuous (resistance < 1 ohm or so), how can you have 17v at one end but only ~12v at the other end? You could try pulling out the 80amp fuse and check for continuity to the B terminal. Maybe the fuse is good, but not making good contact in the socket? Maybe your charge wire is good, but has a bad connection to the ring terminal at the end, so not connected at the alternator?

Divide and conquer. Divide up the connection from the B terminal to the battery and find where the voltage suddenly changes.

 

Alright, it seems as if I've narrowed down my voltage drop to somewhere between the B-wire connector and where it meets on the driver side wall. I'm getting a voltage drop of 5V between the positive lead on the battery and the wires leading to the connector but only 90mV between the battery and the other end of the B-wire where it meets on the driver-side wall before running along the front of the car by the radiator. It seems like the easiest option would be to replace this cable but the part is discontinued by Toyota. Does anyone have a recommended replacement?

 

Replaced the cable and fixed the issue. I was still getting continuity occasionally through the cable but it was probably fried / broken somewhere and not able to handle the output of the alternator.