[BamaRidgeRunner]

Sorry for my first post to be a problem one, but here it is.

I recently purchased a 1987 900 4cy auto. I bought it because it has only 92K on it, and I know that guy that owned it. The car had been sitting for 3 years.
I am having a problem with the alternator not charging. I read previous post under the search engine, but found nothing to solve the problem.I read where the ground was possibly bad, so I ran 3 grounds off the alternator. 1 to the body, 1 to the engine, and 1 directly to the battery, none helped.
I installed a new battery when I bought the car. I took the alternator off and had it built by an alternator shop here in B'ham. I put it back on the car, still not charging. I went and bought a used alternator from the pull a part, had it checked before I put it in. The alternator showed good. I put it on the car, and same problem, not charging. The voltage starts of at 12.84 and steadily drops.
It will get down to 12v and then I just turn it off.
Any help would be appreciated as I am new to this whole backwards engine thing.
Thanks.
Eddie

 

[900t]

Maybe a wiring fault, eg crappy + cable, or...

 

[BamaRidgeRunner]

I used a volt/ohm meter to check the reading off the back of the alternator, it shows 12.8 at start, dropping down to an even 12v after 15-20 min.
The exciter wire shows constant voltage as well. I'm guessing that it's supposed to ?

 

[philbert]

when you turn the key to the "on" position (but car not running), does the battery idiot light in the dash light up?

 

[Matt88S]

Are you sure the battery is good?

 

[BamaRidgeRunner]

battery light does come on when switch is turned on.

I bought this battery for my Bronco, and put it in my son's 900 just to be sure it wasn't a battery problem. It is also a new battery. Holds a charge when hook a charger to it. WOuld a bad battery keep the alternator from charging ?

 

[BamaRidgeRunner]

Just got back from getting another new battery. The place I just bought the first one from replaced it at no cost. Charging at 13.94V . Thanks for the feedback.
I still don't understand how a bad battery would keep the alternator form charging, but not complaining.:cheesy:

 

[c900]

Just got back from getting another new battery. The place I just bought the first one from replaced it at no cost. Charging at 13.94V . Thanks for the feedback.

13.94 V is right in the ballpark! Optimal terminal voltage when charging should be between 13.5 and 14.5 volts, and when the engine is not running, the battery should sit at between 12.5 and 13.5 volts.

There are a few common things that can prevent batteries charging such as the ground wire connecting the alternator casing to the engine block (or to the chassis of the car) coming loose, or the voltage regulator itself failing (which is often the brushes wearing away though the regulators themselves to fail sometimes).

Good to hear you have a functional, new battery for the car though!

I still don't understand how a bad battery would keep the alternator form charging, but not complaining.:cheesy:

Depends what's wrong with the old battery. If it has a collapsed cell, one or more of the internal cells (each cell has a pair of plates) has developed a short circuit, and that reduces the battery's internal resistance and may mean the battery can't be properly charged.

If one of the plates has broken down too far (which can be caused by a number of things, one being the acid concentration going too high), trying to charge the battery can't reverse the chemical reaction enough to 'rebuild' the decayed plate so eventually the battery will develop a very high internal resistance and won't be chargable.

If the acid/water mix is not the right concentration, or the fluid level is too low, that will affect the battery's ability to store charge and may (esp. if the fluid level is too low) seriously reduce the 'cold-cranking' capacity of the battery.

The alternator will still work fine even if the battery is no good, but the voltage regulator will be sending current to a 'non-receptive' load, so even though the voltage regulator and the alternator itself are working properly, they can't charge the battery (or at least charge it completely).

The best way to look after a lead-acid battery is to make sure the fluid level is always correct so the plate pairs are fully covered by acid/water mix, and keep the battery relatively cool. That's a big ask with a classic 900 since the battery is right next to the hot side of the engine though it's really only a serious issue with turbo engines because the exhaust elbow out of the turbo is directly next to the battery (hence the fancy heatshields that Saab used).

Hopefully the new battery will be ok.

Craig.