Hi!

I decided to pop by here to ask a question about my 1990 940. Basically the alternator isn't charging the battery. Without thinking to much about it I went ahead and swapped the battery and alternator for new ones, however this didn't solve my problem. I know it's possible to buy faulty alternators but I'm 100% sure both the old and new alternator work (tested them in a friends Volvo). Here's what happened when the problem first occurred:

I woke up to drive a friend to an appointment, it had been a cold night, but the car started up just fine. We drove about 500 meters before the car started coughing as if it was out of fuel and then it just died. When we tried starting it up again it would either turn the engine over once very slowly or just make a click... so definitely a dead battery. We measured with a voltmeter and it measured just 11,84 volts. I charged the battery and the car ran fine on that for a while. While the car was running I constantly measured to make sure and only once out of five times did the battery measure 13,5+ volts. Otherwise it would be around 12v until it eventually dropped to around 10-11 and died. In other words it seemed as if the alternator charged the battery sometimes, but now it doesn't charge it at all anymore. As I mentioned I switched the battery and the alternator and tested the old ones. Seems they're fine, and the new ones as well. Terminals are clean on the battery, everything is tight and clean by the alternator and from the alternator to the start engine. However, it seems like the power my alternator produces doesn't get to the battery. I've put a voltmeter from the power output on the alternator to ground, and it measures 13-14v when running idle, however the battery isn't getting anything. The alternator belt is tight as well. There has to be a connection problem, bad wire or bad ground somewhere, that's my guess, but there are people here that know way more than me so I wanna hear what you guys have to say. Another thing is that when I turn the key to the ON position, all warning lights come on apart from the battery light... it either doesn't come on at all or it just flickers faintly. I should also mention that all fuses are good in the fusebox in the car (isn't this the only fusebox?).

Thanks in advance for all answers! Any help on what it is or how to isolate the problem easily is appreciated!

- Mikael

 

check the voltage from the alternator BODY to the battery - terminal when the engine is running, I bet there's a ground problem. also check from the alternator body to the engine block grounds near the fuel rail. if there's voltage from alternator to engine block, then its the ground wire on the alternator. if there's voltage from the engine grounds to the battery - terminal, than its probably the battery ground cable, or maybe the engine block ground cable. on all these ground-to-ground tests, near 0 is good, anything over a few 10ths of a volt + or - indicates a bad ground between the two test points.

 

Sadly I'm on holidays for a week so I don't have access to the car before a week has passed by, but thank you so much for that, I'll definitely check that out when I get the chance! Let's say it isn't a bad grounding, is there any other option? Just okay to have a plan B even though it really seems like a bad ground.

 

the alternator/battery light in the instrument panel is a critical part of the circuit. one side of that light si connected to the ignition switch (on in II/Run), and the other side is connected to the D+ terminal on the alternator. the light should come on solid when the ignition is switched off, and go out when the engine is running if everything is normal. when this light is on, its providing 'kickstart' current to the alternator to provide an initial magnetic field.

after verifying the grounds are solid, try this...

turn the key off. check the voltage from engine ground to the D+ terminal (small wire) on hte back of the alternator, it should be essentially 0 volts. turn the ignition on to II/Run (but don't start the car), and the D+ terminal should be close to +12V relative to ground (it will be somewhat less than full battery voltage because the bulb is in the circuit).

you said the light is flickering, but this could easily be bad grounds.

 

Okay thank you so much!