Hey Team,

This is my first post on this forum, but I've been lurking for a while now. I really hope a few of you guys could come together and help me out.

Synopsis: I bought a 91' 4runner last week. When I went to look at it, there was a squeal on start up. Previous owner advised it was a belt. Had new OEM one to replace. I noticed that the belt also squealed when there was a load applied to the system. For example, when the turn signal was on, or the window was rolling down and I held the button down after it stopped, it would squeal (belt would slip). Also, every time a load was applied, the voltage would drop. With the car running, and the heating fan on + 4ways and highbeams, the voltage will drop all the way down to about 12V. The battery otherwise sits nicely at 12.5 when the truck isn't running.

What I did: I tightened the slack on the old belt, and it still squealed. Put the OEM unit on and it did the same. 4 days ago while the truck was parked, the alternator stopped squealing, and I noticed voltage was at a steady 12.4X. The alternator had stopped charging (it was a remanned unit). I went to the local yard and got a DENSO one for $70. No change, still the same voltage drop, and slipping belt. It seems like that under load, or even start up, the alternator gets so hard to turn, that the belt glazes right over it.

I grounded the battery and engine with AWG 4. I ran AWG 4 from the 80A fuse to the + battery terminal. I even ran a jump lead from the L of the alternator to the + of the battery, and while doing so, I jumped the mounting bolt of the alternator to the - of battery. Nothing solved the belt slip. The alternator sends 13.9X volts when I measure between ground and the L terminal of alternator. But the second a load is applied, the belt slips.

Please help me guys I am absolutely heartbroken and stressed beyond my mind. I bought a whole bunch of new parts from the states ready to go into the truck. I never thought this would be such a big problem.

I thought maybe the Sensing input to the alternator might be low, and causing it to work extra hard to keep voltage up. But in that case, wouldn't the battery be overcharging?

I will be adding photos and videos shortly. If there is anything, absolutely anything you can do to help me, please let me know. Any vids, any photos, anything at all.

Oh no, do not connect "L directly to battery. You could mess something up. It is meant to connect one side of a resistance to low voltage if there is a charging fault.

What you want to connect directly to the battery is the "IG" terminal. This will eliminate any (if any) problem with wiring preventing proper battery voltage from getting to alternator terminal IG. IG provides excitation current to allow alternator to generate voltage.

Here's link to charging system explained.

Cheers!

Adding to the thread here, how is the battery? Terminals clean and corrosion-free?

Voltage really sounds low here, the alternator should be putting out 13.5-15.5v (or thereabouts, check the manual for exact voltages), so either there's a major short somewhere when key is on and/or engine is running, or you have another bad alternator.

How did you check tension on the belt? Perhaps there's still not enough tension on the belt, allowing it to slip? Use this for proper tension:

Using a torqure wrench, place the socket on the alternator's front bolt holding the pulley on. Set torque wrench for the following number that applies to your belt:
New 3/8” belt, 11-13 ft/lbs; used 3/8” 7-9 ft/lbs
New ½” belt, 13-15 ft/lbs; used ½” 9-11 ft/lbs

Turning the pulley with the torque wrench in the same direction as the belt runs, tighten the belt down until you get no slipping of the belt until you hit the proper ft/lbs above.


Try checking the belt first for proper tension, then let's go from there...

thanks a tonne for your response. First time ever posting on a forum and had no clue if anyone would respond.

Turns out the belt wasn't tight enough. I ended up beefing up the whole electrical system in that process. I just didn't think the belt needed that much tension. I was rolling through some service manual information and I noticed a diagram of a pry bar and lots of force being necessary to tension the alternator. This gave me an idea that what I was doing wasn't enough.

I got some muscle into it and now its just fine. I still have a voltage drop, but it never drops below 13V. Oh well, ended up learning a tonne about electrical systems. My windows roll up and down faster. My bass is back in my stereo. Everything is better since the upgraded connections. Reading your post confirmed everything.

Work Done:
4 Gauge Battery - Terminal to Body Ground.
4 Gauge Engine to Body Ground.
4 Guage Alternator L pole/Positive to 80A fuse.
4 Guage 80A fuse to battery + Terminal

Not really sure if this is how to reply. I didn't see your reply earlier! I'm really happy you replied! I was reading your posts on the "Chronic Alternator Problem" thread, and wasn't sure if you were still active here.

I made a mistake in the Lettering. What I actually did was connect a 4guage from B (or the line that eventually goes to the positive pole of the battery) terminal to the 80A fuse to the fuse box. I left the old white wire there, as well. I thought since the flow will take the path of least resistance, my additional connection would solve the problem of a possible short in the other wire.

Let me know if I need to make a correction!

Not absolutely sure if you need to make a correction (without seeing the schematic). However, I think you do.

On older trucks, the "white" wire between battery positive and fuse box is actually a fusible link - a wire that will melt when too much current flows through it. It is an extra protection for the wiring. Works like a fuse but, while a fuse blows as son as there is an overload, the Fusible Link gives any overload a chance to go back to normal before blowing.

It is not safe to replace a FL with a thicker wire - There is a reason for the thickness of the FL. It should melt before an overload burns wires.

You need to find schematic or FSM on your particular truck, and verify what I said. (unfortunately I have a 22RE).

I remember reading this portion on your other post! I just made the change and put the slow blow back in. Thank you kindly for your support!