16 replies to this topic

[Ajay]

Rover mini - 1993

Hi guys - I have an alternator issue - in that it isnt charging my battery. I've read loads about the issue and found that the bulb on my dash (which has never worked) is to blame. My question is.. can I bypass it on the dashboard, just to get me the 100 miles i need to go tomorrow?

Al

[dklawson]

I posted a reply earlier but for some reason it does not appear here... curious.

Please define what you mean by the warning light has never worked. Do you mean the bulb NEVER comes on or that it NEVER goes off?

If the bulb never comes on, the easiest fix is to first try replacing the bulb. There are other ways to power the circuit without replacing the bulb... but the work-arounds are more work than changing the bulb.

If the bulb never goes out, no... you can't bypass this. The warning light is telling you that you have a charging problem and you will not be able to bypass the warning light circuit to make the alternator work.

Regardless, start by checking the basics.
1) is the belt in place and is it tight?
2) are the wires to the alternator intact and plugged in securely?

[Ajay]

Please define what you mean by the warning light has never worked. Do you mean the bulb NEVER comes on or that it NEVER goes off?

NEVER comes on, hasn't since I had the car. I have tried replacing the bulb, no effect BUT on a multimeter, it reads 12v on the wire which is supposed to light the bulb up on the back of the dash.

Can I not route this 12v to it's destination without going via the bulb? - it's a 2 binacle if it makes a difference. I'm not bothered about a warning for now as I can get it fixed where I am traveling to.

[Guess-Works.com]

The ignition 'Red' bulb serves no purpose other than to indicate the alternator is not supplying a charge (power)....

It works on the principle of a potential difference between the feedline from the alternator ( basically the power the alternator is producing ) and the ignition live +ve side of the battery... Therefore when the alternator is delivering power then the PD between the alternator and live is 0, hence no light..

When the alternator is not working then the power from the alternator is 0 and the live is 12v, therefore it lights...

Which is why the light comes on ( or at least should do ) when the car is switched on at the ignition...

If your alternator is not charging the battery, then there could be one of two faults... either the alternator is not working or the battery is fubar. Simple way to check is when the engine is running the PD between live and earth should be between 13 and 14 volts, 13.8v is pretty usual. If it's lower than 13 then the alternator is at fault, if it's ok, then it's the battery....

Edited by GuessWorks.co.uk, 24 August 2008 - 06:54 AM.

[Ajay]

Simple way to check is when the engine is running the PD between live and earth should be between 13 and 14 volts, 13.8v is pretty usual. If it's lower than 13 then the alternator is at fault, if it's ok, then it's the battery....

ok, done that and it shows 11 constantly (no difference when running). BUT, i've read a lot of the replies on other threads, which lead me to assume that the alternator will not charge the battery atall IF the red bulb is not connected in the circuit. The battery is a nice 420cca battery which functions normally on my other car so I know it isn't that

If the voltage was 13-14, I wouldn't have the problem. The issue is, that it isn't charging, so to the original question (as a circuit needs to be completed before it will allow electricity to flow), can the bulb be safely bypassed and more importantly, how do I do it? or can the lot be bypassed on the alternator somewhere?

Edited by Ajay, 24 August 2008 - 07:17 AM.

[paulrockliffe]

Hello, I was going to post along these lines yesterday but I was on my way out, the red 'no charge' light is independent of the alternator operation. What I mean is, regadless of whether your light works or not, the alternator will carry on doing what it is or isn't doing.

If you think about it, BMC etc would have been slaughtered if the car stopped charging its battery because a bulb had blown.

[Dan]

Sorry but that's total garbage. If the warning lamp is not connected, is blown or the cables to it are faulty so that there is no continuity in the warning lamp circuit the alternator does not switch on. It's the same with all cars, not just BMC vehicles. That's why you have to look for at least two warning lamps that come on with the ignition and turn off when the engine is running, like you should have been taught when you learned to drive. The alternator field coils or switching circuitry (depending on type, self exciting or self switching) take their power from the warning lamp connection, if there is no current through the field coils it will do nothing at all. Please read the red box at the top of the page. What would be the point of the lamp if the alternator worked whether it lit up or not?

In order to bypass the lamp you have to provide an ignition switched supply to the small terminal in the alternator connection (with the brown/yellow cable) and it must carry more or less the same resistance as the bulb in the circuit does. So it's a little fiddly to do and as DK says, fixing the warning circuit will be a better and usually easier solution. Check for continuity on the brown/yellow cable from the alternator plug to the bulb holder. You can check for continuity from the alternator to the actual dash plug block too in order to eliminate possible problems in the printed circuit. The simplest way to bybass it would be to run an extra cable along the loom, through the bulkhead and splice it to the brown/yellow at both ends. That would mean the lamp still lit up properly but would be a little bodgey. Also check the other teminal in the bulb holder for 12v with ignition. The problem could be that the printed circuit is damp, try taking it off the clocks and leaving it somewhere warm to dry out nicely. If the problem is in the printed circuit then splicing the cable will be pointless.

[cambiker71]

Sorry but that's total garbage. If the warning lamp is not connected, is blown or the cables to it are faulty so that there is no continuity in the warning lamp circuit the alternator does not switch on. It's the same with all cars, not just BMC vehicles. That's why you have to look for at least two warning lamps that come on with the ignition and turn off when the engine is running, like you should have been taught when you learned to drive. The alternator field coils or switching circuitry (depending on type, self exciting or self switching) take their power from the warning lamp connection, if there is no current through the field coils it will do nothing at all. Please read the red box at the top of the page. What would be the point of the lamp if the alternator worked whether it lit up or not?

In order to bypass the lamp you have to provide an ignition switched supply to the small terminal in the alternator connection (with the brown/yellow cable) and it must carry more or less the same resistance as the bulb in the circuit does. So it's a little fiddly to do and as DK says, fixing the warning circuit will be a better and usually easier solution. Check for continuity on the brown/yellow cable from the alternator plug to the bulb holder. You can check for continuity from the alternator to the actual dash plug block too in order to eliminate possible problems in the printed circuit. The simplest way to bybass it would be to run an extra cable along the loom, through the bulkhead and splice it to the brown/yellow at both ends. That would mean the lamp still lit up properly but would be a little bodgey. Also check the other teminal in the bulb holder for 12v with ignition. The problem could be that the printed circuit is damp, try taking it off the clocks and leaving it somewhere warm to dry out nicely. If the problem is in the printed circuit then splicing the cable will be pointless.

It's not just me that knows this then, thanks for that

[dklawson]

On my first post I started writing an explanation of how to make a parallel warning lamp circuit but decided not to until I heard your reply to my questions. As I said earlier and Dan verified, fitting a second warning lamp to "bypass" this problem is not hard but it requires some creativity. However, to get you that 100+ mile drive you asked about, try this....

Go to an auto parts store. Buy some bulk wire close to 1mm^2 area... perhaps 6 feet of wire. Buy a replacement small lamp socket (the type that has two wires to the socket), buy a bulb for that socket, and buy a roll of electrical tape. Place the bulb in the socket and splice lengths of wires to the socket wires. Cover all the electrical joints with electrical tape. Make sure none of the lamp socket has exposed metal surfaces... wrap completely with tape if necessary. This step is important as you don't want any metal surfaces that may touch earth. You now have a test lamp you can use for other purposes later. Take your test lamp and connect it briefly across the battery terminals to make sure it works.

Return to the engine compartment. Find a switched +12v source. If you have a non-ballast ignition system (12V coil and points or an electronic ignition) connect one test lamp wire to the coil (+) terminal... the one coming from the ignition switch. Take the other test lamp wire and find a way to insert it into the alternator plug. You will be connecting this to the terminal with the brown/yellow wire. Tape these connections so they stay in place and so the test lamp cannot accidentally touch earth. Switch on the ignition to the run position. The lamp should turn on. Start the engine. If the alternator starts working the lamp should turn off. Repeat your battery voltage measurements. With the engine off you should find something close to 12V. With the alternator working you should find close to 14V.

[Ajay]

Hi all. I've done some testing and found that the bulb works, the brown/yellow wire is at 12v when ignition is on. The bulb comes on if i earth it... Hence i assume it works... But i cannot get it to go out. Try and follow me now... The brown yellow is connected to one side me the bulb... What wire is on the other side? The manual says it goes to the indicator... Down a green/red wire. I've done my best to trace it and it looks like it goes from the passengers side toward the back of the car. What i'm wondering is where this wire earths. Any ideas? As that could be the issue. I will be doing the test bulb idea when the shops open tomorrow. This is just an idea i had as people have suggested that a dirty earth could be a big problem

[Jupitus]

Hi all. I've done some testing and found that the bulb works, the brown/yellow wire is at 12v when ignition is on. The bulb comes on if i earth it... Hence i assume it works... But i cannot get it to go out. Try and follow me now... The brown yellow is connected to one side me the bulb... What wire is on the other side? The manual says it goes to the indicator... Down a green/red wire. I've done my best to trace it and it looks like it goes from the passengers side toward the back of the car. What i'm wondering is where this wire earths. Any ideas? As that could be the issue. I will be doing the test bulb idea when the shops open tomorrow. This is just an idea i had as people have suggested that a dirty earth could be a big problem

The wire on the opposite side should be a live feed from the fuse box with power only when the ignition is switched on... probably a white wire behind the dash...

[dklawson]

The current path for the warning light bulb is:
ignition switch, white wire (usually), lamp, brown/yellow wire (usually), alternator plug.
(I say "usually" because you NEVER know what a previous owner may have changed. )

The ground for the light is reached by passing THROUGH the field coils of the alternator as I tried to explain above. If you unplug the small brown/yellow wire from the alternator and use a jumper to short that wire to earth (when the ignition is on), the warning light should go on.

The brown/yellow wire should not go to the back of the car.

[Dan]

The brown/yellow is at +12v with the ingition on? Was that with the bulb connected to it or not? Remove the bulb and test the teminal from the brown/yellow, if this wire is at +12v with the ignition on and no circuit through the bulb then your alternator may well be toast or there is a short in the wiring. The brown/yellow should be a resistive path to ground until the alternator is spinning, at which point it will become 14.2 ish volts. The other terminal for the bulb should test as no connection with the ignition off and +12v with the ignition on.

[Ajay]

Bulb test which was written above has been done. I have no reaction from the bulb atall. It simply stays off Tried it back where it came from and it lit up fine so it wasn't that. Jigled the wires and made sure it was all connected... Still nothing. I'm assuming that means new alternator?

[dklawson]

Before you buy a new or reconditioned alternator, to back to my post above (post #9) where I describe wiring in an external warning light. If after wiring that indicator lamp in place you still do not get the proper light operation and 14+V out of the alternator... yes, consider replacing the alternator. Do the testing first as electrical parts are often not returnable.