[DeTomaso] Alternator found

[John Buckman]

David,

> Sorry if this had been hashed over many times before...


>>>No problem.  It's good to review from time to time.

> LET'S SEE IF I HAVE THIS CORRECT...

 
> 1) Run a 4 or 2 AWG wire on the 9' run from the Alt to the Fuse block.   This is the most likely wire to overheat as it will carry the full Alt output in the times when both the battery is low and the consumers are on full blast.


>>>Any increase in wire size helps.  
 
> 2) Live with the 8 AWG from the fuse block to the Ammeter but clean the terminals, put copper and star washers on and tighten up well.  Per SOBill's note and the wiring diagrams, this circuit only sees the current going to and from the battery.


>>>Just remember that with a low battery and very few consumer on, most of the alt's current will be going back to the battery via that 8 AWG wire.
 
> Assuming a dead battery and all consumers on, the Alt is working at full tilt, sending (for sake of argument) 150A down the wire to the fuse block.   From there the current splits with part of it going off to run consumers (through numerous wires) and part going through the ammeter to try and charge the battery.  Only part of the current is going through the ammeter circuit and can probably live with 8AWG.

>>>Yes (see previous comment).
 
> 3) Given that the starter circuit does not go through the ammeter, we dont have to worry about it.

>>>Correct.
 
> 4) This still leaves 2 potential fire hazard cases.  The first is when your battery is dead and your alternator is trying to push whatever the battery will take through the ammeter circuit.  I'm not enough of a battery charging expert to know, but I am guessing this could be hundreds of amps, probably about the same as the battery is capable of delivering.

>>>The alt won't produce more than its rated current (+/- a small percent), and the battery (when low) will take all that it can get. 

> Note, that it doesnt matter if your consumers are on - they are on a seperate circuit, but it could be fatal if you are trying to boost someone else from your dead battery as the alternator is now trying to both charge your battery and the other guys, all through the ammeter circuit.   Soution here: Never boost with the  Alt on (engine running) .

>>>If the engine is running on the donor car (and its alt is working), then the donor car's alt will be trying to charge the dead battery.  When the car (with the dead battery) is started, both alts will be trying to charge the dead battery.  
 
> 2nd hazard case, (which could happen with either a hi output or low output alternator).   Your alternator is not producing at all, and you have all consumers on.  You bought a nice hi output battery and it is now trying to power all the consumers through the Ammeter circuit because the Alt is doing nothing.   It you added a lot of consumers (fans, etc) this current need might exceed the Ammeter wiring.

>>>Correct.  If possible, turn off some of the consumers.

>  Seems like these last two cases are fairly remote, but one would have to be aware of them. 

>>>Correct, but they do happen.

John






On Sep 13, 2012, at 9:37 AM, David D Fisher wrote:

> John,
>  
> I echo Chucks comments... good info.
>  
> Sorry if this had been hashed over many times before...
>  
>  
>  
> I guess if one was to design the wiring upgrade based on a pareto of likely failures you might do the following (for a later car)...
>  
> LET'S SEE IF I HAVE THIS CORRECT...
>  
> 1) Run a 4 or 2 AWG wire on the 9' run from the Alt to the Fuse block.   This is the most likely wire to overheat as it will carry the full Alt output in the times when both the battery is low and the consumers are on full blast.  
>  
> 2) Live with the 8 AWG from the fuse block to the Ammeter but clean the terminals, put copper and star washers on and tighten up well.  Per SOBill's note and the wiring diagrams, this circuit only sees the current going to and from the battery.
>  
>   Assuming a dead battery and all consumers on, the Alt is working at full tilt, sending (for sake of argument) 150A down the wire to the fuse block.   From there the current splits with part of it going off to run consumers (through numerous wires) and part going through the ammeter to try and charge the battery.  Only part of the current is going through the ammeter circuit and can probably live with 8AWG.
>  
> 3) Given that the starter circuit does not go through the ammeter, we dont have to worry about it.
>  
> 4) This still leaves 2 potential fire hazard cases.  The first is when your battery is dead and your alternator is trying to push whatever the battery will take through the ammeter circuit.  I'm not enough of a battery charging expert to know, but I am guessing this could be hundreds of amps, probably about the same as the battery is capable of delivering. Note, that it doesnt matter if your consumers are on - they are on a seperate circuit, but it could be fatal if you are trying to boost someone else from your dead battery as the alternator is now trying to both charge your battery and the other guys, all through the ammeter circuit.   Soution here: Never boost with the  Alt on (engine running) .
>  
>   2nd hazard case, (which could happen with either a hi output or low output alternator).   Your alternator is not producing at all, and you have all consumers on.  You bought a nice hi output battery and it is now trying to power all the consumers through the Ammeter circuit because the Alt is doing nothing.   It you added a lot of consumers (fans, etc) this current need might exceed the Ammeter wiring.  Of course this isnt going to last long as your battery will soon be dead.   Seems like these last two cases are fairly remote, but one would have to be aware of them. 
>  
>  
> Thoughts?
>  
> David
> 
> From: John Buckman <jb841 at cox.net>
> To: detomaso at realbig.com 
> Sent: Thursday, September 13, 2012 8:39 AM
> Subject: [DeTomaso] Alternator found
> 
> Anyone care to guess how long the run is from the alternator through the ammeter and to the battery?
> 
> 
> >>>Early cars:  ~25'  Alt to ammeter to starter soleno
> >>>Later cars:  ~20'  Alt to ammeter to battery.
> 
> >>>The tap (splice) for the fuse panel, ignition switch, and headlight switch is ~9' from the alternator.
> 
> >>>The original ammeter wires (at least on the early cars) were 8 AWG.
> 
> >>>One must remember that, when sizing wires, heat generation / dissipation can be a major concern. 
> The more current (amps) that passes through a given size wire, the more heat is generated, and the more
> heat that is generated, the more the resistance (in that wire) increases, and as the wire's resistance increases,
> so does the voltage drop.
> 
> Selecting a wire with a higher insulation temp rating is always a good idea, but NOT a good idea if you are
> selecting that wire size based upon a given current (amps) at that (high) temp.  Wires in bundles have a much
> slower heat dissipation rate than a wire that is run by itself.  
> 
> Tefzel (aircraft) wire is an excellent choice, although expensive.  It can withstand 200* C (392* F).
> 
> John
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