84 conv rabbit battery dies when engine is on

Hey, I have an 84 convertible rabbit and when I was rewiring the dash and now the battery won't hold a charge when it's on. When the cars off the battery keeps 12.6 but when I turn the car on it drops quickly until it dies.

So I figured out its not the lights, it's just something in the car. Maybe the alternator not working? Basically I jump start the car and the moment I take the cables off the voltage drops .01 a second until it gets to around ten then dies. I tried taking out most relays that are not crucial to engine running and the fuses, cleaned up the wiring and the ground to the battery. Could this be an alternator issue? I was messing with dash wiring up until now which makes me think it's something to do with that. The battery stays at the same levels when the engines not running, it only drops when it's on.

Check all the earths first, then have a check of the battery and the alternator.

Might be that with the extra load the alternator struggles and then the electrics to the engine fail. Should have 14v at the battery when the engines running and the alternator is charging.

Ian

borednow said

Check all the earths first, then have a check of the battery and the alternator.

Might be that with the extra load the alternator struggles and then the electrics to the engine fail. Should have 14v at the battery when the engines running and the alternator is charging.

Ian

Hey! Thanks for the reply. I think I spoke too soon, I just updated my post. When I charge the car it runs at 14 then the moment I take off the charger the voltage drops quickly until the car dies then stays still. So when the cars on it drops. Cleaned up the ground and battery wiring, not sure what would make it drop so fast though? It's not any relays or fuses, could it just be the alternator has a bad connection or isn't working?

Edit, checked all the connections and cleaned up wiring from alternator to battery and still nothing. Battery keeps a charge until its running. I'm unsure it's the alternator issues sense it was running great until my dash wiring project, could a short drain the battery that quickly?

Yup, either the battery is dead or the alternator's dead.

Check the connections to the alternator. There should be a live an earth and a blue exciter wire.

Ian

Well the charger is supplying the 12V to the system, so you are looking at a bad alternator or battery.

Replacing the main ground from the Battery to Frame and frame to engine is a good thing to do on these older  rides as the original was single stranded braid unshielded and that corrosion can get between the strands.

Second is that you need to verify that your battery is good, as it isn't holding a charge.  A fully charged battery should be 12.5-13.5V to ground.  

Then you need to verify that the itty-blue wire on the alternator is intact as that supplies 12V to from the cluster to the Coils of the Alternator to start the field so the alternator can produce a charge.  That is why it is red or lit because it is being biased to turn on from the battery…Once the alternator is producing more voltage than the battery, it turns off, as the alternator back biases the diode that powers the lamp turning it off.

You can rig up a temp exciter by using a Bulb and a sire directly from the Battery to the Exciter.  (look up hot rod exciter circuit).  

Lastly I always run a wire from the alternator case to the  engine as the only grounds that the alternator get are from the Bracket bolt and the pinch bolt with tend to rust.  I am a strong advocate of keeping those clean.

The Pinch bolt as well as the upper bracket bolt are Steel going in to aluminum and the only contact points on all the brackets are the areas of the alternator that come in contact with the bracket.  Since they are steel against aluminum and that starts electrolysis from differential metal corrosion, add current and the process starts.. a small dab of Never-seize or di-electric grease can slow the effect down..but it is always there.

Get the Battery Tested, as well as the alternator to eliminate those and simplify your trouble shooting.

Then you know if they are good or bad.  if they are bad then replacing them may solve your issues,  if they are good then you are at a wiring issue usually the exciter circuit or battery cables.  

Briano1234 said

Then you need to verify that the itty-blue wire on the alternator is intact as that supplies 12V to from the cluster to the Coils of the Alternator to start the field so the alternator can produce a charge.

Firstly, thank you! Secondly, does that blue wire need to be connected to the instrument cluster for the alternator to work? Maybe I'm confused by what you're saying, I thought it was just for the light on the dash.

Thank you both for all your help. My stupid amature self did NOT realize that blue WIre was connected to the accesary panel until physically tracing it today as the last wire and it ending up empty in my hand… I was rebuilding that panel. Plugged it in and charges perfectly… Thank you for all the help !! Always something simple ain't it.

It is usually something stupid and simple. Me usually          

Ian