alternator (generator) problems

I have a couple of questions. I was driving home and all the lights on the gauge came and and the stereo stopped working as though the battery was going dead, i made it home but the battery was dead this morning. I notice the tachometer is no longer working and from other posts assume the alternator is shot. what should the voltage across the battery be when the engine is running, mine was 12.4 volts. and last question is will an alternator from a 95 disco work on my 96. thanks

Hi mate

the voltage should go up to about 14.summat volts when the engine is running.

"last question is will an alternator from a 95 disco work on my 96"

It all depends on your vin no .. In my disco parts manual there's over half a dozen change points and five pages on alternators. I suspect the alternator is the same but the pully may be different, other than that you will have to visually compare them.

Sounds like your 'nator is goosed - or needs new brushes maybe.

Anyway, to test it needs TWO simple tests, VOLTS (pressure) and AMPS (current).

switch engine off.
Clip voltmeter across battery.
read volts. If battery is OK you should read 13½ volts or so.

turn on lots of lights.
read a bit less volts, maybe 12 to 12½.

Start engine and increase revs to about 1500 - 2000 rpm
Read volts, and even with lights on it should be holding a full 13½ volts.

Now turn on every leccy device you have, back window heater, lights, heater, the lot. You might drop to 13 volts, but if you're down to 12 or under the 'nator is goosed or not nearly big enough to cope.

I guess brushes at best, or goosed.

CharlesY

ok, first thing i did was replace the battery because it was old anyways, then i drove to the store to get a new alternator and on the way home the tachometer started to work again, i measured the volts across the battery and it was above 13 so i assumed the alternator was working intermittently. i installed the new alternator and the tach doesn't work and the volts across the battery when the engine is running and lights on is just below 12 volts so now i have to assume the new alternator is knackered as well, is there something i am missing instead? i will take the old alternator in tomorrow morning and test it but i need to return the old alternator tomorrow to get my deposit back on it, i had to trade my old alternator for a new one and pay $340 bucks

CharlesY said:

Sounds like your 'nator is goosed - or needs new brushes maybe.

Anyway, to test it needs TWO simple tests, VOLTS (pressure) and AMPS (current).

switch engine off.
Clip voltmeter across battery.
read volts. If battery is OK you should read 13½ volts or so.

turn on lots of lights.
read a bit less volts, maybe 12 to 12½.

Start engine and increase revs to about 1500 - 2000 rpm
Read volts, and even with lights on it should be holding a full 13½ volts.

Now turn on every leccy device you have, back window heater, lights, heater, the lot. You might drop to 13 volts, but if you're down to 12 or under the 'nator is goosed or not nearly big enough to cope.

I guess brushes at best, or goosed.

CharlesY

Click to expand...

ffs check all yer wires for dodgy connections and make sure yer earth is ok

how do i check the earth? this sounds like it is getting beyond my skills, i think i will get the new alternator checked first as it is a recon unit

slob said:

ffs check all yer wires for dodgy connections and make sure yer earth is ok

Click to expand...

take the earth strap off and clean all the contact areas, both on the terminals and on the block/ body where it is secured. they also need to tight.

the same applies to the +ve wires, make sure they are clean and tight

i cleaned the connections to the battery and checked to make sure the ground connection was still connected tightly to the frame and it was, i took the truck to the shop where i bought the alternator and at first the guy thought there was a problem with the alternator. he checked the voltage then asked me if there were any loads on the battery and i told him the heater was on and i turned it off, the voltage went up slightly and he thinks the battery was low because i drove the truck to the shop with the new battery and old alternator and advised i put the battery on the trickle charger and charge it up a bit. my question is wouldn't the rev counter still work if the battery was low but the alternator was functioning? i think the increase in voltage across the battery was only because i turned off the heater. i am charging the battery now and will put it back in the truck tonight to see if it works but i'm at a loss to what the problem is unless the alternator is knackered, thanks for all your help so far guys, i appreciate it greatly

simon

slob said:

take the earth strap off and clean all the contact areas, both on the terminals and on the block/ body where it is secured. they also need to tight.

the same applies to the +ve wires, make sure they are clean and tight

Click to expand...

The rev counter is probably driven electrically by a phase-count in the alternator. That means there's one or more wires out of the alternator to send that signal.

It sounds to me like the alternator isn't properly wired.

Charles.

thanks for all your help guys, i'm going to take the alternator back tomorrow , i'll keep you posted with what the problem turns out to be

CharlesY said:

The rev counter is probably driven electrically by a phase-count in the alternator. That means there's one or more wires out of the alternator to send that signal.

It sounds to me like the alternator isn't properly wired.

Charles.

Click to expand...

put the battery on a trickle charger overnight and everything seems fine, rev counter is working and volts on the battery are normal, thanks for your help guys

No kidding ....... ?

Whaddya think Lord Slob?

CharlesY (Rupert, Retired)

back to square one, battery charges at idle speed but the belt might be slipping at higher rpm, belt is a little loose so now have to invest in new belt tensioner

simonk23 said:

back to square one, battery charges at idle speed but the belt might be slipping at higher rpm, belt is a little loose so now have to invest in new belt tensioner

Click to expand...

Just 'cos the red light goes out is no guarantee the battery is being charged.

You'll need to check further than that.

CHarlesY

shirley if the light goes out then the alt is pushing out more volts than the battery has in it, so it should be charging, maybe not as quick as yer'd like

slob said:

shirley if the light goes out then the alt is pushing out more volts than the battery has in it, so it should be charging, maybe not as quick as yer'd like

Click to expand...

Not nessissarli sad to say .... 'cos of the way 'nators are wired ...

You can have a VOLT meter in the system, and even if it says green section is OK, it may not be charging hardly at all.

The battery needs TWO things to get it to take a decent charge, and that's an alternator voltage significantly higher than the battery is at, AND a whole lot of amps at that voltage.

Alternators cannot charge a battery as fully as a good dynamo could.

A dynamo could hit high votages (we used to set them about 16 volts to get the lights bright) and that charged the battery right up and caused it to gas off, so we needed to top up the electrolyte every week.

Alternators are set to a maximum voltage (13.8v) that is supposed to be JUST UNDER the voltage that causes the battery to gas off, and so (a) you get a "maintenance free battery", and (b) the battery is NEVER quite fully charged.

So they fit slightly bigger capacity batteries to make up for it.

It is GOOD for your battery to discharge it well down once in a while then immediately charge it back up again using a decent mains charger.

It is DEATH to your battery to let it get profoundly flat, and to let it sit like that even for a short time. Keep them charged ...

Sorry to preach ... didn't mean it like that!

CharlesY

CharlesY said:

' it may not be charging hardly
CharlesY

Click to expand...

int that wot ah sed?