Alternator won't charge and reference wires are grounded

[snowball]

Hi all,

My 90 burb R1500 350 won't charge. battery always shows 12 V. I brought my alternator to be tested and it passed. I checked the reference wires by putting a multi meter and testing continuity and they're grounded.

Just wondering if there is a common place to look for the short or any advice.

Thanks!

[speedygonzales]

However, the "L" terminal of the alternator is powered from a fuse so if the fuse is blown, which is likely if the line is grounded. You're ok to measure it to ground.

In a 98/99, the wire is pink from the fuse panel to the gauges. This line then changes to brown from the gauge cluster to the "L" terminal of the alternator.

Check for your blown fuse. Mine is labeled "gauges". It's a 20AMP. Yours could be a 10 AMP.

You need to find where the "L" terminal wire on your vehicle goes and trace it back to see where it is grounded if it truly is grounded.

Start looking in places where it either turns a corner or where it passes through a piece of metal. You can also try removing the plug on the alternator and jump 12volts to the "L" terminal while the engine is running and see if you then get about 14volts across the battery.

Be sure the battery is charged before correcting the wiring problem so as to not pull too much from the alternator. They are designed to top off the battery not charge a deeply discharged one.

[snowball]

Thanks speedy,

No blown fuses.

I traced the wires down and it goes from the alternator to the top of the block and into a wiring loom. When I giggle the loom the connection to ground breaks, and I can get 14+v out of the alternator.

I need to figure out where the wires connect out of the wiring loom.

[95 S_Trucker]

I would ask a mod to move this to the 73-87 forum or the 73-91 blazer/suburban forum.

You'll get more help in the right section.

[snowball]

Sure. I don't see a way to do that.

[95 S_Trucker]

On the main forum page, the names at the far right of the sub forum are the moderators

Palf70Step is the moderator of the 88-98 GMT400 section

http://67-72chevytrucks.com/vboard/member.php?u=5259

And old Rusty C10 is the moderator of the 73-87/73-91 blazer/suburban section

http://67-72chevytrucks.com/vboard/member.php?u=5988

[speedygonzales]

Originally Posted by snowball

Thanks speedy,

No blown fuses.

I traced the wires down and it goes from the alternator to the top of the block and into a wiring loom. When I giggle the loom the connection to ground breaks, and I can get 14+v out of the alternator.

I need to figure out where the wires connect out of the wiring loom.

You're confusing being grounded with not having voltage. That is the only explanation for not having a blown fuse and the alternator going in and out when you wiggle the wires. That small wire is breaking connection not going to ground. Think about it, no manufacturer in their right mind would run a supply wire without having a fuse on it.

Put a meter on the wire going to the "L" terminal and watch it go from 12 to 0 and back.

Now trace where the wire is loosing connection. Chances are it's broke inside the insulation and you will be in for a deal trying to find it unless you have a "sniffer".

[snowball]

So I use a DMM and set it to continuity. I touch the 2 connectors and it beeps. I connect either one to the ground post and it beeps.

I could be mistaken but that would show it's ground.

The reference voltage could be low enough that it does not trip a fuse.

Also, I did notice that if I pull on the wire loom then my problem goes away. my guess is the wires go from the alternator to the loom through the firewall, where the grommet has worn down and is shorting the wires to the body.

Would anyone know where the wires go through the firewall? Do I need to pull the whole dash or is there an access point?

I'll just take a vacuum hose and split it as a makeshift grommet.

Also, when you say a sniffer, what tool are you referring to? brand name?

[snowball]

Would it be safe to say that big hole on the middle left is where the wire loom should go through?



How would I access it from the inside?

[Irish1941]

Thought I replied..opps
Anyway, the "L" wire goes thru the junction block on the lower right. It's goes along valve cover then under the booster but your problem won't be under the dash. The big round hole is for ECM harness.

Like Speedy said it's just breaking connection and not grounding out. If it was grounding to the body, the fuse would pop and entire dash would stop working. All fused on same circuit.
Not sure where you are grabbing it to make the alt kick in and out, but L wire is the excite wire and is on 12v keyed. It's goes right to your gauge or lamp on the dash. That why alt starts and stops when you wiggle it. Alt needs to see 12v signal to turn on. No signal=no charge.

Check the plug first and foremost. Wiring is brittle after all these yrs and they take a beating with alternator changes & heavy handed plug pulling, corrosion, loose crimp etc.
If it checks out good, then just move along the wire til you find the bad spot.
Has to be with 10" of where you are pulling on it.

[snowball]

Gentlemen,

I first want to say thank you and to let you know I'm not trying to start a pissing patch but rather to understand what is going on.

My understanding is that there is power goes from the battery to the ignition to the light to the 1/L connection.



Therefor, if the wire is grounding through the body or the block then the light would kick on.

This would be similar to the way the light works.

Please correct me if wrong.

Thank you

[speedygonzales]

You never check a wire to ground with an ohm meter when it's connected to supply voltage. Both ends in many cases should be disconnected. For example let's say you were checking the supply wire to a pcm to see if it were grounded. The supply side would have to be disconnected AND the side going to the PCM would also have to be disconnected. If it were still connected to the PCM, it's resistance would be shown when measured to ground.



Also it's not the voltage or lack there of that will keep a fuse from blowing. Current is what causes a fuse to blow.

If you don't want to spend the time tracking down the problem spot, you can always run a separate wire.

[Irish1941]

Pissing match? Nah just trying to help you out..
Anyway,
Where did you get the alternator tested? They do an AC ripple test?

Been dealing with the CS130/144 alternators for yrs and you'd be surprised what passed on bench to what failed in the truck. Heck, seen the charge post test good until it was tightened down and spun enough to break the charge connection.

I'm still leaning towards the plug and/or internal problem with the alternator.
Plug goes directly into the regulator. Solder gone bad or loose connection can do exactly what you say as you pull on the harness.
Very little to next to nothing can chaff the L wire. It's in the harness pretty good.

[snowball]

It was at autozone and I had one of their lifetime warranty. It passed and as a courtesy they gave me another one anyhow. So, not the alternator.

[swissarmychainsaw]

Originally Posted by snowball

Hi all,

My 90 burb R1500 350 won't charge. battery always shows 12 V. I brought my alternator to be tested and it passed. I checked the reference wires by putting a multi meter and testing continuity and they're grounded.

Just wondering if there is a common place to look for the short or any advice.

Thanks!

Assuming you don't have this fixed yet, can you describe what "won't charge" means here? I'm assuming your battery is flat and wont start the truck?
How old is the battery? Did you fully charge it before doing any of these tests?
It should be charged at 12.4-8 volts.

I find that they key to diagnosing problems is: 1. be clear on what you assume 2. do the easy stuff first

So you're assuming the battery is ok. Have you had that tested?

My 1991 burb "wouldn't start" but instead of saying that, I said "it has a bad battery". Which I replaced (probably not a bad idea), only to find out the problem persisted! It was a bad starter.

Make sure you cover the basics, then we can go back to wiggling wires!
:-)

[snowball]

Sorry for the delay, I missed any responses.

Yes. the battery goes flat and won't start. Battery is 6 months old. Once the battery is charged it will start up again. There is a light on the dash indicating the alternator is not charging. Volts are 12.1.

There are 3 wires that come out of the back of the alternator. 1 goes to a fusible link to the battery. the other 2 plug into the alternator. Removing the wires causes the voltage to go up to 12.8V.

If I take those 2 wires, plug them back, they lead to the top of the engine block and into a wiring loom. When I jiggle those wires, the light on the dash goes out and the voltage jumps up to 14.4.

[Ryan Hubbard]

Originally Posted by snowball

Sorry for the delay, I missed any responses.

Yes. the battery goes flat and won't start. Battery is 6 months old. Once the battery is charged it will start up again. There is a light on the dash indicating the alternator is not charging. Volts are 12.1.

There are 3 wires that come out of the back of the alternator. 1 goes to a fusible link to the battery. the other 2 plug into the alternator. Removing the wires causes the voltage to go up to 12.8V.

If I take those 2 wires, plug them back, they lead to the top of the engine block and into a wiring loom. When I jiggle those wires, the light on the dash goes out and the voltage jumps up to 14.4.

You don't have a ground, you have a loose connection. The two wires you are talking about is a 12 volt supply to the alternator to energize the charging circuit and the other wire is for the alternator to measure the "load " on the system to adjust output. It looks like the wire that supplies voltage to the alternator is the one cutting out and effectively "shutting off" the alternator.