Wierd alternator troubles, now its not charging...

[6erWebb]

hhhh just noticed the electrical only section, would someone mind moving this please . sorry

I got this truck about two months ago. It is a 77 c10 shortbed, originally had a 6cyl in it with a column shift but has been swapped to a 94 lt1 with a carb and hei, and a th350 auto.

The voltage gauge always freaked me out. Upon start up the gauge would read it wasnt charging and then when the rpms got above 18 or so it would jump up. Lights would get brighter, it really wasnt chargin before.

I swapped it for another alt I had lying around but it made no difference.

Well tonight I go to bring the truck home from about 3 miles away, the entire way home it never started charging. Just sat at about 10 volts...


Any suggestions? Ive already checked for loose wires.

Any help is appreciated, -Webb

[vtwinsport]

these alternators have a voltage regulator that is internal to the alternator. That could be the issue even if the alt was swapped. Was the second alt in known working order? if not both could be dead.
Try going to o'reilly or one of the part stores, they can test your charging system for free while the truck is running. Takes about 5 minutes.
Leave it running and don't go at night cause the battery will die running the lights and you'll need a tow or a backup battery.

A bad ground can connection on the battery post, or frame ground from battery can interrupt the charging too.
Alternator output might be alright, battery could be toast. Got a second battery to test the voltage use?

[87chevy.com]

now are you saying 10 volts according to the factory volt gauge? or an volt meter? the facotry gauge might not be accurate enough to use it for diagnosis of the electrical system..

[6erWebb]

Ill cheack the grounds tomorrow, the gauge is an autometer gauge

[dwcsr]

If your running under drive pulleys this will happen

[geezer#99]

40 years ago (yup I be OLD) an altenator guru showed me how to check an altenator to see if it's charging. On the back of the altenator there's a plug which covers the centre shaft of the altenator. Dead centre. Use a flat blade screwdriver and touch the plug with the blade. If it's charging the plug will be magnitized and the blade will stick to it. Just touch the plug and nothing else. This has always worked for me.
Of course now a new modern guru will pipe up and say this is hogwash. But that will just be his opinion. LOL!!!

[6erWebb]

As for underdrive pullys is there a way for me to tell if I have them? because I have no idea.

Il also be trying the screwdriver trick

[geezer#99]

Just be careful with the screwdriver. You don't want to short any thing out.
If the altenator pulley is quite large then it might be underdriven.

[dwcsr]

Originally Posted by geezer#99

If the altenator pulley is quite large then it might be underdriven.

Under drives are smaller pulleys. about 4 - 5 inches across

[VetteVet]

Definition from wikipedia

An Underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such as an alternator pulley) that is designed to drive a vehicle's accessories at a slower rate than stock. Underdrive in general means the input rate of rotation in a system is greater than the output rate of rotation. However, an underdrive pulley is considered to be an accessory pulley that is larger than the stock pulley or a crankshaft pulley that is smaller than the stock crankshaft pulley, even though the accessory may not necessarily be underdriven with respect to the crankshaft pulley.

check the voltage on the 2 wires in the plug on top of the alternator. They should both be near battery voltage. with the key on.

[raycow]

Originally Posted by geezer#99

Of course now a new modern guru will pipe up and say this is hogwash. But that will just be his opinion. LOL!!!

I must be at least as old as you, so I'll say semi-hogwash. That test will show if the rotor (field) is getting power and is therefore magnetized, but it will not indicate if there is a fault in the stator or diodes, which is where the output current comes from. In other words, the rotor can be magnetized and you still won't be getting any charge.

Ray

[VetteVet]

Well since I'm an old fart too, I think that entitles me to my .02. Raycow is correct that field current will create a magnetic field and not necessarily charge the battery. Easy enough to test that, just turn on the ignition and field current comes from the battery through the alternator light or whatever feeds the exciter circuit to the alternator. Lay the screwdriver across the bearing cap and check for magnetism. Check for magnetic strength also. IOW how strong is the attraction?

Now the diode trio also provides field current to the rotor field
http://www.alternatorparts.com/under...lternators.htm
so if we fire up the engine and spin the alternator we should get field current from the diode trio which should strengthen the magnetic attraction of the alternator to the screwdriver. If it's the same then no charging but isrf it is much greater then the alternator is charging. The sensing wire will have some affect on this because alternator output is controlled by state of charge of the battery and system draw.
In my opinion if the attraction is much greater with the alternator spinning then the alternator is charging, The diode trio is OK and that other old fart is right too.

[fixit-p]

Whats the alt from the 77 (SI alt) or the 94 (CS alt)? They would be wired different, can you post a pic? Also you mentioned that the voltmeter is an auto meter I assume its after market under the dash type, is the dash originally a gauge dash or idiot light dash? If idiot light , what does the light do?

[geezer#99]

Thank you to both you old pharts. I guess I forgot to mention the magnetic strength thing too. And I've learned a couple things too.