No output from alternator

Hey guys. I’m having a weird issue that doesn’t make sense to me and has me at a complete loss. I joined the forum hoping to find some help as this place seems pretty active.

Anyways I have a 86 Suzuki samurai that I have just swapped a vw 1.9 turbo diesel into with a gm 3 wire 10si 110amp alternator.

Wire number 2 is looped back onto the 12v post and has 12v at all times
Wire number 1 is hooked into my dash light. Key on engine off it reads 0v with the dash light on, key on engine on it reads just below battery voltage and dash light is off.
The 12v post is hooked into my stock wiring connected to my brand new battery.

Today I had my battery and my alternator bench tested and they both came back functioning normally.

My issue is I’m never reading above battery voltage with the engine on (either from the post on the alternator or my battery). I grabbed a used alternator locally and I was having the same issue. However with that one if I pulled the positive lead from the battery with the engine running it would read 14.3v until I would hook it back to the battery where it would drop back to battery voltage. I was able to repeat that behaviour mutlitple times until I ended up killing the alternator. My other alternator is new and I don’t wish to damage it pulling the positive lead but I’m betting it’s functioning the same way.

What in my system could be causing this? The truck is pretty old and doesn’t have a ecm or much of any eletrical and I just can’t figure out what’s going on. Its like some kind of feedback is shutting down the alternator when it gets hooked up to the battery.

So far I’ve tried temporarily adding grounds with jumper cables
Measured 0.2ohm resistance between my ground cable and the alternator housing
Measured 0.0ohm resistance between my positive lead and the 12v post on alternator
I’ve also tried pulling all the relays and accessories I’ve added to the postive lead with no change.

Any tips you guys could give would be greatly appreciated. Camping weather is quickly approaching and I need my truck back on the road!

Temporarily connect a 10 ga wire from the alternator BAT terminal to the battery positive post and test the running voltage.

I tried hooking it up with just a jumper cable (all I had on hand) but I’m not sure I had a good connection. I’ll buy some better wires tomorrow and give that a try.

So I’m thinking if I run some new dedicated wires between my alternator and starter and battery it should get me up in running. The problem with that is I’m basically bypassing all of the stock wiring. I think the only fix for this issue is going to be hooking the factory wires up 1 at a time and finding the circuit that is causing the issue. There must be some kind of short somewhere on the truck causing the alternator to shut off when the battery is hooked up.

Just for hoots, turn the key to the RUN position (engine not running) and touch the alternator pulley with the tip of a screwdriver. You should feel a magnetic attraction.
If so, the alt. should be good to go and you need to check the alternator to battery circuit. Maybe a fusible link has given up.

The alternator was bench tested good today and it’s having the same issue as a used one I tried a few days ago so I highly doubt its the problem. But I’ll give your screwdriver trick a try.

I really think it’s some kind of feedback from my trucks wiring. Probably some kind of short somewhere causing it to cutout.

I’ll post updates tomorrow when I completely bypass my trucks wiring. My engine only needs 1 12v wire to run so it won’t be hard to have just the alternator, and battery running with the engine on it’s own circuit.

We used to just run the wire from the battery post to the positive terminal of the battery and go from there. That’s in addition to running the wire from terminal 2 of the plug back around to the post on the back of the alternator also. I can’t remember if we did anything with the wire from terminal 1 but that probably doesn’t make a difference.

Bypass all of your stock wiring for the charging, there is no problem with running straight to the battery permanently.

Try revving the engine to 1500 RPM’s to get the alternator to go into it’s initial charge.


From what you say about the No.1 wire it seems that it is working like it should be. Power from the ignition switch passes through the battery warning light and then to the alternator exciter circuit. It supplies power to the exciter inside the alternator and that is why it is important that the warning light is working. If the light burns out no current can flow to the exciter. This would make it seem the alternator is bad but the real trouble is within the wiring to it. The No.2 wire is for the battery sense lead of the regulator inside the alternator. So the alternator knows how much power is needed to keep the battery charged up to a normal level.

You have already found out about the dangers of removing the battery lead while the alternator is working. One other test you can do safely is to measure the voltage drop across the main output wire of the alternator. This test will allow to see how much voltage loss you have between the alternator output and the battery. Place the red probe of your voltmeter on the main alternator output stud and the black probe (COM) of the meter to the positive battery post. While the engine is running around 1,500RPM and with the blower on and headlights on HIGH, check the voltmeter reading. You should see no more than a +.35 volts on the meter. If you have a 2 volt meter scale use that scale. If the reading is higher than that then there is a connection problem on that wire. Trace the wire and find the bad connection. Keep in mind that the wire is always hot to the battery so remove the negative battery cable before working on that wire so no trouble happens if the hot wire touch’s ground. That wire is also fused for protection and is a good place to start if there is connection problem.

If the voltage reading shows a negative voltage instead of positive during the voltage test then the alternator isn’t charging the battery, it is discharging it and the alternator has a problem.

Measured 0.2ohm resistance between my ground cable and the alternator housing

Way too much. Get it down to 0.0. Bad connection somewhere.

Figured it out. The belt was slipping. The angle the belt grips the crank isn’t the best with this setup

when the alternator was trying to kick on it was acting like a brake and slipping on the crank, stopping it from revving up and spinning the alternator fast enough. I had to tighten the belt way more then I would like to get it to stop slipping. Its odd it wasn’t making any noises or giving any indication it was slipping. Looks like an upgrade to a serpentine system is in my near future. I feel like an idiot with this one.

Thanks for the help guys.

That’s not my belt setup btw, just a pic I found online. Mine is the same just with a v belt.

If the resistance to ground really is .2 ohms you should try cleaning the alternator body contact to the engine and the other ground leads between the battery, engine block and chassis. Your charging system will work better due to less voltage drop across the extra added resistance.

The slipping belt makes sense since it seemed like your alternator should be working going from what you stated previously about the leads to it and the voltages. Especially the exciter warning/ light lead.

Good for you for figuring it out. I agree w/your idea to reconfigure the belt system, the current belt routing leaves much to be desired. Is the belt pretty new? If not, maybe a new belt would let you back off on the tightening. Otherwise you risk premature bearing wear on the alternator and the other gadget on this loop, looks like the water pump I presume.

One idea, it looks like you have a mounting point between the crank pulley and the alternator, maybe an idler pulley could be configured there to pull the belt up against the crank pulley better. Best of luck.

In the picture there appears to be an unused second grove in the crankshaft and alternator pulleys. I would use the first grove for the water pump and the second for the alternator (two belts).

Typically the alternator is run off the crank with a serpentine belt and then the water pump and power steering pump (if applicable) are run on that v belt pulley. I might run the system this way. I already have the serpentine harmonic balancer so it wouldn’t be hard to change it over. I’m also playing with the idea of routing it the way I have it now with a serpentine belt (like it is in the pic)

This engine has a common problem with the keyway in the crank sprocket wearing out and eventually throwing the timing belt and causing damage to the valves and pistons. Its said to be caused by the low end torque of the engine fighting the torque of the alternator (coming in and off of the throttle). Some belt slippage is good as it minimises the risk of damage to the keyway. On the newer engine designs vw used a D shaped mating face and installed a clutched alternator pulley to fix the problem. I’m hoping the extra surface area of the serp belt will let me have the belt at a reasonable tension, while allowing some slippage. If it doesn’t work I can always switch to a dual belt setup. I plan on running quite a bit of boost (25lbs +/-) and making a decent amount of power ( somewhere around 150hp and 200ft lbs) so whatever I can do to keep the crank happy is going to pay off in the long run.

Resistance measurements that low with an ordinary ohmmeter are not reliable. It could be the probe contact resistance. Did you try repeating that measurement? Make sure the probes are making good contact.

I'm hoping the extra surface area of the serp belt will let me have the belt at a reasonable tension, while allowing some slippage.

In my experience, a flat (serpentine) belt has to be tighter than a v-belt with the same angle-of-wrap configuration.

BTW, the picture you posted is of a v-belt setup.

I’m wondering if the battery has a short in it. I had that happen, and it killed 2 alternators before we realized what was happening. Can you swap out the battery with another temporarily?

The picture of the belt setup shows that not much of the surface of the crank pulley is contacting the belt which makes it easier for the belt to slip. It seems to me you really need to have two belts.

My fiirst thought was a bad battery…with dead cells or worse inside… If you saw 14.3 without the batt… a healthy batt would allow that charge to enter. Just a thought…and easy to prove out.

We always seem to suspect the battery last…when it should be first. I recall kicking myself several times over things like this…when it was the battery all along