Alternator Wiring

Following my electrical engine fire, I'm recreating my Alternator wiring which along with the rest of the engine wiring (except the Hyfire CDi wiring which, being separate, survived) is now a charred lump of molten plastic and metal.

To try and prevent a re-occurance i'm fitting a battery cut-off switch to make it easier to disconnect the battery when working on the car and a 60 amp fuse inline with the alternator wiring which will hopefully prevent the loom from melting again should it get accidently shorted out. Having to go into the car to disconnect the battery whilst the engine bay was still sparking was not a pleasant experience....

I'm just wondering whether the american manufacturers looms are identical to that of my Chesil and have no default protection here?

Does anyone have any idea what the charge current actually is from a 51A alternator? (Is it 51A constant?) The 60 Amp fuse i've selected can cope with surges to 100 Amp before blowing. I'm wondering if a 40 Amp fuse would actually be sufficient and be more likely to blow in a fault condition.

Simon

Simon, go with the 40, if you draw 60 you have other troubles.
throw a couple of spares in the trunk.

excuse me "boot"

Simon:

The quick answer is that an alternator produces a variable current output, from trickle to full output (55 amps or thereabouts in your case, I believe). The more your battery is discharged when you start your engine, the more current the alternator will produce.

I'm not an alternator "expert", but I thought that they produced a variable current output which is dependent on the voltage feedback from the battery. The alternator typically puts out maximum voltage (like 14 volts) from about 800 RPMs and up. If the battery voltage fed back to the alternator is significantly lower than 12 volts (due to heavy use without charging) then it will be lower than the alternator output voltage and act like a "sink" or resistor, pulling the overall circuit voltage down. This tells the alternator to put out to increase the voltage and charge the battery quickly to get it's voltage level to match the alternator output voltage (Maybe that's why alternators sound like they're working harder after you jump-start a dead car?? - Bet'cha because the stator and rotor coils are fighting each other harder trying to produce more current).

As the discharged battery takes a charge and the battery voltage begins to rise past 12 volts (typically toward 13.8-14 volts) and the internal resistence decreases, then the higher voltage feedback signal to the alternator causes it to begin to cut back on current output until, when the battery voltage matches the alternator output voltage the alternator is either trickle charging (or, more likely powering the rest of the vehicle and the battery is loafing) or not charging at all, since the stator and rotor coils are neutralized and not producing any current.

Now: There are a bunch of sites out there that explain how an alternator works, but that's it in a nut-shell. (I know, more than you ever wanted to know)........If this helps, you can buy me a Guinness (or better still, a Murphy's Stout) when I ever get back to England.

By all means, put a fuse in-line with the alternator, and choose the size depending on how much you might expect the battery to be discharged (or how much it will need to be charged back up). Try running a 40 amp or maybe smaller (it will probably suffice for most normal discharge states), but have a 60 in your kit (or a small coin!) just in case.

Gordon

Just as a sort of what's he posting that comment for.

My alternator light started coming on dimly after the engine was started. This was only visable at night. Looked like a "ground loop" feedback thing. Given 20 mins or so, it would go out.

Found the output wire covering from my alternator had suffered some liqufaction (real hot). After a cold start, by the time the car would idle without help, the alternator case was hot. The regulator had gone south and was requesting full output all the time - somewhere in the 18 volt department. Really bright lights but not good at all for fire prevention.

Bill: Yup, that alternator light is in there for a reason! If it ever comes on above a slow idle you've got something to look into.

The light expects to see "no current" across it, which is sees when the alternator is charging properly. It gets 12 volts coming from the battery through the dash wiring, and is "grounded" through the stator (or field) winding and the tri-diode stack when nothing is spinning. When the alternator is spinning, the field winding is making a magnetic field against the rotor winding and generating alternating voltage and current (converted to pulsed DC by the tri-diode stack) and sends 12 volts back up the dash bulbs "ground" wire, effectively cancelling out the lit lamp (it goes out). If one of the three-stack of diodes or the internal regulator go South, the light will typically either come on weakly or will pulsate. Either way, the dash light is a give-away that something's wrong. Of course, you may have a weaker diode in the stack, not failed enough to make the lamp come on but you don't get a good charge either - seen that a few times, too.

Also, with the older VW generators that some folks are running in their Speedsters, it doesn't really start to make current until over 1,200 - 1,500 rpm, which is why some of you guys may get a generator dash light lit at idle below, say, 800 rpm or so. Most of the newer alternators start producing current above 400 rpm, so the dash lamp stays out at idle.

I hope that if any of you have converted from a generator to an alternator you have also by-passed the old, external voltage regulator wiring and removed the regulator. Alternators have regulators built in and don't need the external one.

The thing that tipped me off was that the light was on dimly at idle and got brighter as you increased engine speed. One would think that if the alt output were low, increasing the speed would increase the output and the light would extinguish. It appeared that the wire to the regulator was acting as an alternate path to the battery and it's charging. Them electrons is a goin' the wrong way! No moss growin on me - well, at least not much.

Thank you all for your feedback, especially Gordon with the details on how the current varies depending on the battery state; Good to know how this works in reality.

I received a package containing all the parts I decided I needed from a mail order supplier today so will be re-wiring this weekend - complete with that fuse AND cut off switch (that dumps the alternator current to a load resistor as it kills the engine and disconnects the battery). Lesson learned.

Simon:

As I understand alternators, the diode load output is wired directly to the battery. Thus,if a this wire shorts out at any point along its length, your primary source of short circuit energy (current) will be from the battery, not the alternator. Depending upon the short circuit impedance, it may be possible for 1000's of ampereres to flow instantly to the short and create one hell of a fire in the process.

If I were selecting a fuse for this circuit, I would not only want to know the max rated output of the alternator, but also the rating of the load wiring. For example, if you fuse for 40 amps, considering only the output of the alternator and the wire is rated for only 20 amps, the wire's insulation will eventually break down and short out, creating the disaster outlined above.

I am not familiar with fusing alternators, but once you select an appropriate fuse size, I suggest that you insure that the load wire is capable of handling this amount of current 100% of the time. Also, remember that wire amperage capacity is reduced as you elevate temperature (engine compartment), so be certain you use the derated worst case when determining the wire size needed.

Brian: Thought you were out on the boat! Haven't heard from you lately......

Yes, alternator output diodes are connected directly to the battery. What you described is exactly what happened (the electrical fire) and what he's trying to prevent with his new wiring scheme. We're all talking the same thing, here.

I think Simon is headed in the right direction, and you're right to consider the output line current-carrying capacity, especially in light of the battery amperage on the other end, and that's what he's trying to fuse, NOT the alternator, per se. To that end, the fuse should be in the line as close to the battery as possible to protect the entire length of the line. Then the question becomes, "what's the right size fuse to protect the wire AND carry max alternator current, and what's the right size wire to carry that max current?" which is your other point.

Rather than do a big engineering study of the situation, I suggest we rely on other automotive designers and then shot-gun it.

I looked at the alternator output on my F150 V-8 and it's wire is a little over 1/4 inch in diameter (OD of the insulation). My wife's 4-cyl Honda CRV has about the same size output wire, and so is the one I put on my Speedster. In contrast, my wife's 1967 Austin Healey 3000 has a generator output wire under 3/16 inch in diameter (Don't'cha just LOVE Lucas electrics?).

Assuming that Ford and Honda engineers know what they're doing (Sorry, Lucas), I would suggest installing a heavy duty, automotive quality (read that, "insulation suitable for engine compartment heat"), 1/4 inch or larger CONDUCTOR diameter, heavy insulated, fine-stranded copper wire (to make it flexible) from the alternator output to the fuse and mount the fuse as close to the battery as possible, if not ON the battery.

Running wire that big will mimic what the "big guys" are doing, AND the current-carrying capacity should be well over 55 amps. Plus, the wiring (and that around it) will be protected by that fuse (and I would then use a 50 amp fuse link). Don't forget to crimp a copper lug onto the alternator end of the wire (crimp AND solder would be better) and put a rubber boot or hood over it like the Fords and Hondas do to protect it from dirt and contact with alien metal (I couldn't find a new hood in a store so I got one in a salvage yard).

Do all that and you should avoid any electrical fires caused by the alternator output lead shorting to ground in the future (and your headlights may even get brighter!!)

Brian: You coming to Carlisle? I'll buy you a beer while we figure out when we can go visit Simon.

gn

Gordon:

I have a conflict and will not be able to attend, but my car will most likely be there! I told Henry at IM that he could put the car in the show if he needed it, so he is now looking into arranging transportation.

Regarding alternator wiring, I think you have done a great job of covering all the major bases. I am not a car guy (trying to learn), but I believe the alternator must also have a small gauge voltage sense wire running from the internal regulator directly to the battery terminal(to get a true measure of battery voltage that is not impacted by the current flow present in the load wire) I guess one could make the case for a small fuse in this circuit as well, for a "belts and suspenders" approach. However, if this wire does in fact exist and it is a very small gauge, the wire itself may act as the fuse.

Brian,

My alternator (all alternators?) has an internal regulator.

The only other wire from the alternator (other than the dangerous one!!) goes to the telltale light on the dash, which is connected to an ignition-switched fused supply.

Worst case scenario of a short on this wire in the engine bay is it switching on the warning light; the current flow is limited by the bulb in the path, not the cable or current source.

You made a good point about the load rating of the alternator charge lead. The 6mm2 conductor thin wall cable I have is rated to a constant 50A - the same as the alternator. I'm trying to not let paranoia get the better of me and worry whether the 50A cable will let go before the fuse.

Note in last weekends fire the conductor DID NOT melt away - only the plastic coating - and i had to disconnect the battery to stop the sparking igniting the fuel again...... and believe the OEM loom wire was rated below 50A.

Simon and Brian: That lead coming from the dash lamp is fused at its' source (or is SUPPOSED to be!!) and provides BOTH a driver indication of what's goin on, as well as the reference voltage needed by the regulator. If you were to short it to ground at the alternator it should light the lamp. If you short that same lead at the alternator to the alternator output lead nothing should happen (both have 12 volts on them).

Simon- yes, as far as I know all alternators have internal voltage regulators. Also, not surprised that the conductor remained intact - big wires can take a lot of abuse - not so the insulation!

Just as an aside, I have a moosey output wire on my Speedster - had something around the shop that's fine stranded copper and the conductor is about 5/16" with really thick insulation (and RED, too!) I think it was used as a 12V DC power bus feed on a computer somewhere along the line and handled something like 80+ amps continuous and over 100 amps at start-up surge - Guess that'll handle it!

Gordon

Simon: You seem to have a good handle on things, so I assume that you will take proper care to prevent chafing. Electrically, the wire can have the necessary ampacity and still fail due to mechanical wear through of the insulation. Wire ties, feedthrough grommets, etc.prevent this sort of failure. Being a boater, perhaps I am going overboard with this concern, but it is a very big deal when doing marine wiring.

This sounds like the perfect place to ask my question!
My alternator light is on, and stays on. I have tested the voltage at the alt output post and get just under 12v. If I take the lead off the reg and test the voltage there, I also get just under 12v. I am interpreting this to mean that the voltage I am reading is battery voltage, that the alternator is not outputing any voltage, and that the cause of the problem is therefore the alternator and not any of the wiring or anything else. Is my reasoning correct?

I then loosened the reg and confirmed that there is pressure on the brushes. I did not pull it completely to look at the brushes.

Anything else I can do to check it or get it working? Thanks

Hold on there, Babbalooey! Help me out, here!!

You wrote: " I then loosened the reg and confirmed that there is pressure on the brushes" ??????????? What does that mean? By "brushes" do you mean the regulator's internal relay arms? "Brushes", to me, are the pressed carbon contact wipers against the generator rotor, making electrical contact, so your statement doesn't yet make sense to me. Help me out, here!

Please answer these questions first:

1. What are you running, a generator with an external voltage regulator mounted somewhere (which I suspect), or an alternator (with built-in voltage regulator and nothing mounted externally)?

2. Did the dash lamp used to be generally "off" whilst driving and is now "on" or has it always been solidly "on", no matter what?

Help me with this and we'll go from there.........gn

It is an alternator with built in reg. I felt a spring pushing on something as I lifted the regulator, after removing the 3 screws, and thought that was the brushes.

yes. Normally the dash lights go out once the rpms get above about 2k. Now they just stay on.

In this picture you can see what is inside the Alternator, and to the right, the piece that comes out, and the two things I was calling brushes.

Ron: Your "just under 12V" measurement sounds like the battery is partially discharged, which would be expected if the alternator is not charging.. To verify there is no unexpected voltage drop in the circuit,it would be wise to actually measure the voltage direcly across the battery terminals and compare this to the voltage you are getting from the alternator B+ terminal to the alternator frame. Obviously, you need to do this test with the engine off/ignition on.

With respect to your measurement when you removed the "reg" lead(which I assume is D+),you were simply measuring battery voltage thru the warning light. (The high impedance of your meter is causing virtually all of the voltage drop in the circuit). This test shows only that you have continuity from the battery to the D+ input. With the "reg" lead connected, ignition on/engine off, you might want to verfify the voltage from the D+ to ground is close to 0 volts, thus showing that there is a virtual short from D+ through the field winding to ground and that full battery voltage is being applied across the warning lamp.

With my extremely limited knowledge of alternator specifics and the above checks showing nothing surprising,I would conclude the alternator is bad from what you have said! I understand there are tests to isolate the problem to diodes, regulator, etc., but I will yield to Gordon and others that have more knowledge of the specifics.

You know, Brian, for a guy with "extremely limited knowledge of alternator specifics" you've been right on the money so far!!

Yup, I agree with Brian's statements, especially the last paragraph - for that light to come on at all with an alternator usually means that one or more of the diodes has gone South so you'll need to get it rebuilt. Blowing up your second picture looks like the brushes are OK and there's no "smearing" of crud on the rotor, so I would rule them out (and brushes don't usually fail nearly as often as the diodes, anyway).

The better news is that they're reasonably common and most larger Metro areas will have one or two places rebuilding automotive electrical stuff, like starters and alternators. Price around first, though, as you might find it almost as cheap to get another remanufactured one like you have (maybe in Chrome!) rather than go the local rebuild route. Around here they'd get upwards of $75 US to rebuilt that one.

gn

If it doesn't "come to life" today I will pull it & get it dealt with. I was kind of hoping maybe there was a trick or easy fix. I guess not. Thanks guys.

Ron, you may want to check those ''brushes''. I had same probleem w light staying on . I found one of generator brushes was chipped. Replaced it and light went out.

I have the oposite problem, my alternator w/internal regulator over charges. I have asked around and get "have not been able to find a rebuilt or new Voltage regulator/ You could change out the alternator"

I always have a wet battery after a good drive. Any thoughts?

Dale, I think (?) that if you have a bad or weak ground in the primary electrical circuit (increased resistance between alternator and battery) the alternator will work harder (higher output) which may in turn cause heat and boiling of the electrolyte (wet battery). Or, a bad cell in the battery will also increase alternator output above normal levels and create the same situation.

Every so often I do something like this to make my brother Ron feel like he is smarter than me. I checked the output at the alternator today again, and it was almost 13v! So I went and disconnected the battery with the car running, and it stayed running. I assume VWs are the same as other cars and that if the car stays running the alternator is good. So now the question was, why is the light still on. Then I had a brainwave and remembered that my emergency brake handle is out of the car. Yup, the light was the emergency brake indicatory light! Oops! At least I realized before I pulled the alternator.

So now I have an alternator light that is NOT working and need to find out why. If I understand what is written above, I should have 12v at the regulator wire with the key 'on' until the car is started, or until the the rpms go up? Or will it not read anything if the bulb is dead? Ideas guys? Thanks.

Once you disconnected the battery and it was still running, did you try shutting off the ignition and then try starting it again without the battery connected?

Sorry, couldn't resist that one.... ;)

OK, so it wasn't the Alternator light that was on (I'm jealous - you have an e-brake-on light, too??)

Let's see if I have this straight, Ron (and I bet Ron O gets his students to help on the tough ones):

1. The alternator is putting out 13 volts - that's good. It's supposed to be in the 12.5 - 14 range so 13 volts is nice. If it keeps your car running after you pull the battery lead then (technically) it's providing enough power to run your ignition, but you don't know if it's capable of charging your battery. However! Since you don't have a dead battery, chances are it's charging OK, so don't worry about it.

2. In an earlier post, you said that you tested the voltage at the "reg. lead" which Brian and I assume is marked "D+" (the small wire on the alternator with a 1/4" fast-on connector). If you're getting 12V there, then chances are the dash lamp is OK, since you're reading voltage THROUGH it. Another way to test the dash lamp is to remove the D+ lead from the alternator and ground it - the dash lamp should light when the key is "on". If not, change the lamp, or check the wiring, or check the brushes - see earlier posts).

3. When the car is running with the battery connected the alternator light is out. That's good. It's supposed to be out if nothing is wrong, and it doesn't sound like anything is wrong. Your battery is charged enough to start the car (the most load you can give it), and if the dash lamp were burned out the alternator wouldn't be charging ('cause it gets its charge signal through the dash lamp, right?).

4. If you turn on the ignition and don't start the engine, the alternator dash lamp should illuminate, 'cause the alternator isn't generating a back voltage to put it out. Your Oil light should come on, too, but that's another story. This is kind-of a cheapo lamp test (and all you ex-military pilot guys know what lamp tests are!!). If it doesn't light with key on and engine stopped, then something's wrong - lamp burnt out, mis-wired, wrong or partially failed bulb (might light too weak to easily see, but still passing a signal to the alternator - BTW, this is a way to adjust the output of the alternator.....different lamps have different resistences and give different signals to the alternator circuit, but that's another story, too!

So, does this help or should I just keep quiet?

That was just the info I needed. I was able to test the connection & determine I had a bad connection. Found it & fixed it. Thanks.

Thanks George. I will check the grounds and battery.

Well, I've completed the rewire (with fuse and cut-off switch) and the electrics are working as they were before before the fire.

By combining the Hyfire loom with the car loom, the engine bay wiring is much tidier. Only problem now is the new layer of dust (extinquisher powder) that appears each time the engine is run....

WOW! Simon - nice job!
Nice BIG wiring for the Alt. output, too!

Any time you want to come over here to do some wiring I'll bet there'll be a bunch of people lined up for you!

So at least for a little while, people will be able to follow where you are going by the trailing dust cloud.............

gn