How to switch alternator On and Off

[SmellyCat]

I have this 94 Volvo 940 and I put a switch on the alt exciter wire. With the switch off, start car. Alt is off. no charging.....I switch on the Alt and the motor stumbles and I get 14 volts. Turn off the alt and it continues to charge. Un hook the exciter wire and the Alt keeps charging. How do I make the Alt stop charging? SC

[brucepick]

Yeah, the "exciter" wire is just that - there to "excite" the alternator. Without it, many alternators will start running once you reach a certain rpm or after a minute or two.

Anyway, in my Civic, I used a switch at the alternator's output wire. I used to drive Volvo 240's; I think they had the alt output cable routed to the starter's positive terminal, from where another cable goes to the battery positive. So that works as if it were one wire from alt to battery, just with a "tap" at the starter.

With this switch you can't turn the alt on and off from driver's seat while driving. It goes under the hood. But it's rugged and does the job at what I thought reasonable cost.

So, build it this way - - -

Disconnect the battery ground for your protection. I found out the hard way, you can't work on these heavy positive cables without creating a short with a loud POP.

Disconnect the existing heavy positive cable at the alternator. Run a new 4 gauge cable from the alternator towards the battery positive terminal. 6 ga (smaller) would likely do but most people will tell you use 4 ga. It's a bit of a run to the battery so heavier is better.



Get a battery disconnect switch like pic above, which you will mount on the battery positive terminal. They're made for the neg terminal which is a slightly smaller diam post. So you'll need to file out the inner diameter some for it to fit your fatter positive post. About $6-10 in stores.



These have a screw knob that you only need to unscrew a quarter turn for the switch to be "off". Tighten it and it's "on" again. Get a small nylon bushing or spacer from hardware store, put it under the screw knob for driving with alt cut out. That way your screw knob is secure and the switch won't come apart from vibration. Remove spacer and tighten knob to activate the alt. Remember it was made as a DISCONNECT so they assume you're not driving when it's disconnected. You WILL be driving with it disconnected. So I say, tighten the screw for security, and put a nylon bushing under screw knob so it's "off" when tightened.

See pic (my Civic). I have two cables connected to the clamp-tightening screw, and the alt cable connected to the switched screw terminal.

All the regular stuff that's already hooked to your battery goes to the main bolt that tightens the switch's clamp. Your new alternator wire goes to the screw location on the other end of the switch. When switch is off, alt is disabled. When knob is tightened (without your nylon spacer in between) the alt is in the circuit.

I think you wrote about using a 2nd battery; I didn't discuss that here. Here I only talked about how to disable the alt with that switch.

[SmellyCat]

Thx Brucepic, I am learning a lot. That makes sense with the battery tab. I am sure that with the exiter wire off , this Volvo does no charging. Thats because I ran out on juice with it off. And I drove it all over the place. With my exciter switch open. No charging and if I get in a pinch, I can flick it on and not be stranded. You said my two battery rig was not ideal and my plan is to leave the 2nd battery in connected and try running on the marine battery 100 percent.. Sc

[brucepick]

Sounds good to me. Good luck with it!
Make sure you have a voltmeter set up where you can see it.
You don't want to drive + drain the battery to where something quits working.

If you have to, buy some cheapo item that plugs into your cigarette lighter with a cord. Cut the doodad off the cord, now you have a plug that goes into the cigarette lighter. Hook that up to a voltmeter ($10-15 from a big box store) and you can see what your voltage is. Or maybe you have the time to find a nice Volvo voltmeter from a junkyard and install it.

A mechanic friend of mine told me that some components like computers get damaged when voltage drops low, something like 10V. I don't know if he's right on that, but I definitely don't recommend letting voltage sag a whole lot.

[bobo333]

ive done quite a bit og work with electrical systems in cars being the owner of a car audio shop and in my experience you dont really want the voltage dropping too far below 11.5v, this is where you will reduce the life of most lead acid batteries if you stay under it, if you have deep cycle batteries you can probably go down to 11 or 10.8v before you risk shutting down computers and stuff but this is probably pushing it, if i put a switch on my alt i would never let it go below 11v

[paulgato]

Hi Brucepick,

I'm looking again at how to disable, or switch on and off, an alternator, and I saw your post (here quoted) from 2012.

My current alternator just happened to self-excite at 4000rpm, so with the exciter wire disconnected the alt would not operate during normal driving - especially low-rev eco-driving! - but could be brought into action with a little blip of the throttle if voltage ever got too low. So I never needed a switch.

However, that alternator just recently decided to die, and when I replace it, the replacement may self-excite at 1000rpm, or not at all, and there's no practical way of predicting that. No one knows, including the people who sell alternators, how an individual specimen, or even a particular model, will behave in that regard, as it's outside the normal design spec. (I know - I tried last time!)

So, I just saw your old post from 2012 here and wondered if yours might be a more foolpoof method. It looks a good, simple method, and would certainly stop charging voltage getting into the car, but how safe is it? Can the switch be used while the engine is running? This is an issue because the only time I'd want to switch the alt ON is when the voltage has got so low that I may not be able to start the engine again. (Diesel.)

Did you measure what voltage the alternator was outputting with that switch in the OFF position? The voltage on the alternator side of that switch?

I would never need to DISconnect the switch with the engine running.

I've been driving around with a dead alternator (just disconnected at the exciter wire) for a month or two now, because I have the luxury of an auxiliary battery and a 25A dc/dc converter supplying 14.0v, and that is well sufficient for daily commuting and work, etc. but I do get nervous on longer journeys. I must get the alternator replaced before winter.

So, can you advise at all?

Originally Posted by brucepick

Yeah, the "exciter" wire is just that - there to "excite" the alternator. Without it, many alternators will start running once you reach a certain rpm or after a minute or two.

Anyway, in my Civic, I used a switch at the alternator's output wire. I used to drive Volvo 240's; I think they had the alt output cable routed to the starter's positive terminal, from where another cable goes to the battery positive. So that works as if it were one wire from alt to battery, just with a "tap" at the starter.

With this switch you can't turn the alt on and off from driver's seat while driving. It goes under the hood. But it's rugged and does the job at what I thought reasonable cost.

So, build it this way - - -

Disconnect the battery ground for your protection. I found out the hard way, you can't work on these heavy positive cables without creating a short with a loud POP.

Disconnect the existing heavy positive cable at the alternator. Run a new 4 gauge cable from the alternator towards the battery positive terminal. 6 ga (smaller) would likely do but most people will tell you use 4 ga. It's a bit of a run to the battery so heavier is better.



Get a battery disconnect switch like pic above, which you will mount on the battery positive terminal. They're made for the neg terminal which is a slightly smaller diam post. So you'll need to file out the inner diameter some for it to fit your fatter positive post. About $6-10 in stores.



These have a screw knob that you only need to unscrew a quarter turn for the switch to be "off". Tighten it and it's "on" again. Get a small nylon bushing or spacer from hardware store, put it under the screw knob for driving with alt cut out. That way your screw knob is secure and the switch won't come apart from vibration. Remove spacer and tighten knob to activate the alt. Remember it was made as a DISCONNECT so they assume you're not driving when it's disconnected. You WILL be driving with it disconnected. So I say, tighten the screw for security, and put a nylon bushing under screw knob so it's "off" when tightened.

See pic (my Civic). I have two cables connected to the clamp-tightening screw, and the alt cable connected to the switched screw terminal.

All the regular stuff that's already hooked to your battery goes to the main bolt that tightens the switch's clamp. Your new alternator wire goes to the screw location on the other end of the switch. When switch is off, alt is disabled. When knob is tightened (without your nylon spacer in between) the alt is in the circuit.

I think you wrote about using a 2nd battery; I didn't discuss that here. Here I only talked about how to disable the alt with that switch.

[paulgato]

...Alternatively, would a relay work? And would I be able to switch that on and off remotely without risking damage?

A 200A relay (rated for 120A continuous) could be bolted more-or-less directly to the alternator's B+ terminal and would cost about the same as running a new cable from alternator to battery and fitting a manual disconnect switch.

The use of a relay there, provided it can be used safely, also gives the potential to switch the alternator on and off automatically in response to events like braking or slowing down on engine with zero fuel being delivered. Or even if I had to operate the relay manually using a switch in the cabin, it would still be very useful to be able to switch the alternator output on during long hill descents.

[teoman]

It is a signal wire so i doubt you would need such a high capacity relay (unless you want to open and close the output of the alternator).

There was a circuit design in a recentish thread that let you change the signal wire and have the alternator off or charge at a higher voltage. You could use such schemes to have the alternator charging wile you apply brakes.

[paulgato]

Originally Posted by teoman

It is a signal wire so i doubt you would need such a high capacity relay (unless you want to open and close the output of the alternator).

There was a circuit design in a recentish thread that let you change the signal wire and have the alternator off or charge at a higher voltage. You could use such schemes to have the alternator charging wile you apply brakes.

Yes, Brucepick's solution was to interrupt the main output wire with a mechanical switch, hence the 200A relay. They're only about £15.

Not sure what you mean by the 'signal wire'. There's a couple of signal wires (i.e. that don't carry much current.) There's the IGNition wire that switches the alternator on in response to the ignition switch being switched on, and the battery voltage sense wire, which monitors the voltage at the battery terminals, so as to better regulate alternator output.

Perhaps the "recent thread" you mention concerned using the voltage sense wire and varying the voltage on that, going back to the alternator? Can you find that thread? I had thought it might be possible to put a small (AA?) battery in line with that wire, to increase the voltage above the point where teh alternatoir would decide it didn't need to put out any current itself. If that worked then rigging that up to brake lights or fuel cut-off event (?) would be quite straightforward.

[oldtamiyaphile]

Is it safe to simply pull the high amp fuse?