So I am having this problem for a while now on my 1990 Favorit, when I start up the car the battery light stays on and the battery is not charging (approx 12v at the battery), as soon as I rev the engine up above like 3000rpm the light goes off and the battery starts charging (14v). I have just replaced the alternator with a new one and the problem is the same, any ideas? Grounds look fine.

Check the blue wire. (They're usually blue, the excitation wire)

Took off the blue wire, rigged up another wire on that terminal and touched the battery 12v wire that goes on the alternator and it works, guess there is a short in the blue wire since the car also started pretty rough until the car started charging, doesn't happen if I don't hook up the blue wire. Going to run a separate new switched 12v to the alternator and problem solved then.

You need a diode in that feed I believe, so better to repair the original wire which may be broken, as opposed to shorted.  Not sure how it routes in a Favorit but someone like @RicardoM may be able to assist with that. 

Edited 18 January, 20224 yr by Wino

Before trying anything more harder, you can try changing only the end part of the blue wire and its terminal.

 

Corrosion usually affects the cable starting from its tip. Rest of the cable inside the wrapped loom may be OK.

Cut the blue wire 2cm from its end. Strip and check. If the copper inside is black or green, repeat the process until you see clear copper. Extend the cable by soldering same gauge cable to its original length. Insulate using shrink tube and/or insulator tape. End the tip using a new crimp terminal.

Still having problems? That means there is something else in other parts of the circuit.

8 hours ago, R_Blue said:

insulator tape.

Shrink tube is very good choice but with silicone inside is excellent for permanately job.

Some insulation tape pvc are cheap quality and after a while got sticky and unfold by themselves.

Adhensive Cloth tape as Tesa 51608 is fine for cables in dry conditions.

31 minutes ago, D.FYLAKTOS said:

Shrink tube

AKA "Macaroni"

17 hours ago, R_Blue said:

Macaroni

You remember good

On 21/01/2022 at 00:08, R_Blue said:

AKA "Macaroni" 

 Ha,ha yes we have many of them in various types. 

I peeled back the insulation on the blue wire and the wires are great, I noticed when the wire is unplugged and ignition on it has 12v, as soon as I plug it onto the alternator it only has 1 volt?

On 17/01/2022 at 18:25, chargerbandit said:

I have just replaced the alternator with a new one and the problem is the same, any ideas?

Tell us about the status of the battery. Age, make, model, etc.

Battery is 1 year old, 55Ah and it had this problem since it was new... in an ideal situation I would try another battery when I get my hands on it.

Testing the alternator

 

Test requirements !!

♦ V-belt or ribbed V-belt tension O.K.
♦ Battery O.K.
♦ Wiring at alternator and earth cable between engine, body and battery must be intact, tight and not be oxidized.

 

Are you absolutely positive that ALL the requirements are met?

 

4 hours ago, chargerbandit said:

in an ideal situation I would try another battery when I get my hands on it.

Borrow a known good one from a neighbor/friend ?

14 hours ago, RicardoM said:

Borrow a known good one from a neighbor/friend ?

Always check that the known good battery from a neighbour or friend is actually good.

 

In my own experience more often than not I have been told by neighbours and friends that their battery in use or the spare battery is good when it is not and at least needs charging.

 

With any electrical or starting problems a good battery in good state of charge will help but if not then it can hinder the diagnose, repair and conformation of repair.

 

9 hours ago, nta16 said:

Always check that the known good battery from a neighbour or friend is actually good.

By known good battery I mean one from a car having the electrical system in working condition.