Alternator & Starter Fuse Question

[Joel K]

Starting to design my electrical system and main connections to the battery and have a couple questions.

1)Do you need a fuse between the Starter Motor and the battery? I’m referring to the large positive cable that goes from the battery to the starter motor and not the positive wire that goes from the ignition to the starter solenoid. My starter is about two feet away from the battery, what wire gauge would you recommend? I have the VW starter and Graz transaxle.

2)Also, do you need a fuse between the alternator and the battery for the positive wire that charges the battery? My alternator is about 3-4 feet away from the battery, what wire gauge would you recommend? The alternator produces 170 Amps.

Always appreciate the help, thanks.

 

[JP Verweij]

No fuse & no fuse.
I would recommend a FIA cut of switch / masterswitch between battery positive & anything anything else.
170 amps alternator is sufficient enough to power EPS and a electronic heater system and still enough to light up the moon.

 

[Frank Catt]

I recommend a Competition cut out switch between the NEGATIVE battery terminal and earth , There are FIA approved kits which provide both internal and external instant electrical isolation, particularly if you are going to use the car for any track use.

 

[Howard Jones]

Ok, this is always the question when installing a master switch.

If we use the master to isolate (interrupt) the positive (+12V) when it is in the off position then we must also connect the alternator output (+12V) line to the same side of the master that the battery is connected to. Everything must be supplied +12V from the other "on" side of the master switch. This ensures that ALL the +12V sources are removed from everything when the master is selected "off". In this position, nothing gets supplied +12V and the alteration cannot supply systems you want to master cutoff even when the battery is out of the circuit.

If the alternator output is connected to the "on" side of the master switch and the engine is running the alternation will supply power and the engine will continue to run even with the battery +12V switched out of the circuit. The reason for this is the alternator is now supplying +12V and in a lot of cars many systems have independent grounds such as the ignition box being grounded to the chassis somewhere. So the car keeps running until the engine stops. This same problem applies to things like fuel pumps.

The other way (Franks) is to turn off the ground by running the ground lead through the master switch instead of the +12V. This will of course work Provided that ALL of the other system grounds are also switched off through the Master Switch. That means no independent grounds to chassis. All the individual gound paths must be combined at a ground-isolated central bus and then the bus must be supplied with its main systems gound through the Master switch. This will remove the ground path to the example ignition box and the engine will stop when you open "off" the master switch.

Most production car electrical harnesses require individual local grounds for things such as lights, fans, pumps, and ignition boxes and are hard to modify to the switched ground method so people use the switched +12V method instead.

The alternative is to use a master Switch that switches off a second set of auxiliary contacts that are used to switch off the alternator output at the same time the main power feed is switched off. This will work also.

I like the first way. Why? because if the car is all wadded up in a stunt then you can remove the +12V battery power from everything beyond the master switch by turning it off. Damage to the harness will not result in an unintended short-circuit because there is simply no power in the harness after the master is turned off. Also, should a ground interrupt system (Franks) be used with the local ground harness configuration (the other way) then a shorted pump motor only has the fuse to protect the short, and the power cannot be removed should the fuse protection fail.

This is a lot to say that either way is correct as long as the entire wiring plan takes into account the master switch wiring method.

To answer the original question: Fuseing either the alternator output or the main power supply line for the starter is not usually done for a variety of reasons. I think the main one is the fuse values would need to anticipate maximum current conditions in either circuit. This can be a highly variable value and it would be hard to select a fuse value that would protect the harness (that's what you want to protect) and not prematurely fail without reasonable cause. So no to both.

That's my story and I'm sticking to it.

 

[Joel K]

Ok, this is always the question when installing a master switch.

If we use the master to isolate (interrupt) the positive (+12V) when it is in the off position then we must also connect the alternator output (+12V) line to the same side of the master that the battery is connected to. Everything must be supplied +12V from the other "on" side of the master switch. This ensures that ALL the +12V sources are removed from everything when the master is selected "off". In this position, nothing gets supplied +12V and the alteration cannot supply systems you want to master cutoff even when the battery is out of the circuit.

If the alternator output is connected to the "on" side of the master switch and the engine is running the alternation will supply power and the engine will continue to run even with the battery +12V switched out of the circuit. The reason for this is the alternator is now supplying +12V and in a lot of cars many systems have independent grounds such as the ignition box being grounded to the chassis somewhere. So the car keeps running until the engine stops. This same problem applies to things like fuel pumps.

The other way (Franks) is to turn off the ground by running the ground lead through the master switch instead of the +12V. This will of course work Provided that ALL of the other system grounds are also switched off through the Master Switch. That means no independent grounds to chassis. All the individual gound paths must be combined at a ground-isolated central bus and then the bus must be supplied with its main systems gound through the Master switch. This will remove the ground path to the example ignition box and the engine will stop when you open "off" the master switch.

Most production car electrical harnesses require individual local grounds for things such as lights, fans, pumps, and ignition boxes and are hard to modify to the switched ground method so people use the switched +12V method instead.

The alternative is to use a master Switch that switches off a second set of auxiliary contacts that are used to switch off the alternator output at the same time the main power feed is switched off. This will work also.

I like the first way. Why? because if the car is all wadded up in a stunt then you can remove the +12V battery power from everything beyond the master switch by turning it off. Damage to the harness will not result in an unintended short-circuit because there is simply no power in the harness after the master is turned off. Also, should a ground interrupt system (Franks) be used with the local ground harness configuration (the other way) then a shorted pump motor only has the fuse to protect the short, and the power cannot be removed should the fuse protection fail.

This is a lot to say that either way is correct as long as the entire wiring plan takes into account the master switch wiring method.

To answer the original question: Fuseing either the alternator output or the main power supply line for the starter is not usually done for a variety of reasons. I think the main one is the fuse values would need to anticipate maximum current conditions in either circuit. This can be a highly variable value and it would be hard to select a fuse value that would protect the harness (that's what you want to protect) and not prematurely fail without reasonable cause. So no to both.

That's my story and I'm sticking to it.

Thanks guys, appreciate the info. Howard, always appreciate your detailed responses and help. I’ll need to read this a couple times to digest it all.

 

[Ken Roberts]

I would definitely recommend you fuse both the alternator charge wire and starter power wire. I did and most OEMs do as well. In the early days they used fusible links and then migrated to fuses as they became more compact. My alternator charge wire is 4GA and my starter motor is 2/0.

Safety is the biggest reason to fuse these two circuits as close to the battery as possible. In a serious accident they could dead short and cause the wires to overheat and burn resulting in a fire or be an ignition source for a fuel leak. Having the fuses as close to the battery will help prevent this. See page #53 and post #1054 of my build log for the latest style of convenient fuse connections that mount on the positive battery terminal (post style).

 

[Joel K]

I would definitely recommend you fuse both the alternator charge wire and starter power wire. I did and most OEMs do as well. In the early days they used fusible links and then migrated to fuses as they became more compact. My alternator charge wire is 4GA and my starter motor is 2/0.

Safety is the biggest reason to fuse these two circuits as close to the battery as possible. In a serious accident they could dead short and cause the wires to overheat and burn resulting in a fire or be an ignition source for a fuel leak. Having the fuses as close to the battery will help prevent this. See page #53 and post #1054 of my build log for the latest style of convenient fuse connections that mount on the positive battery terminal (post style).

Thanks Ken, I’ll take a look.

 

[Howard Jones]

Ken. I forgot about the fusible links. You're right about that. I stand corrected. What value fuse did you use for the starter main power protection circuit? Is this a slow-blown type fuse? There are circuit breakers intended for boat use that incorporate a master switch function. Might these be useful? In any case, I like circuit breakers instead of fuses.

As far as the alternator output line goes. Alternators (one wire type) will usually fail in such a manner that they stop producing output current, However, they could short internally in such a manner that creates a short circuit to ground since they are connected to the battery-positive side of the main power bus via their change output line.

I didn't do either in my SLC because I was trying to reduce the number of points of failure. In effect, I have protected the main power circuit by incorporating circuit breakers in each of the load circuits individually. No harm in doing both I suppose. So I guess I didn't stick to it very long.........ea.

 

[Fred W Ballinger]

I got one of these which interrupts the whole car earth and kills the motor.

Cartek GT Solid State Battery Isolator Kit from Merlin Motorsport

Cartek GT Solid State Battery Isolator switch or cut out switch. The Cartek Solid Sate battery isolator kits are a small reliable cut out switch for isolating batteries in competition cars. Buy your Cartek Battery Isolator Kit from Merlin Motorsport.

www.merlinmotorsport.co.uk

https://www.merlinmotorsport.co.uk/p/cartek-gt-solid-state-battery-isolator-kit-ck-bg-06

 

[Frank Catt]

Thats the kit I use !! Frank

 

[Joel Heinke]

I have a question that may be a little bit off topic but I think is relevant here. For cars using EFI that has some sort of "self learning capability", how much of the "learned information" will be lost if/when the power is cut off from the ECU?

Obviously, in a crash situation, reducing the risk of fire/explosion is more important than optimal engine function. But with a master power switch, it's easy to drop power to the whole car, say when storing the car over the winter, and the if the learned information is lost, the EFI system has to relearn stuff to re-optimize it's operation.

I have an EFI powered car with a master power switch. There's been times when I thought about turning the power off but didn't because of lingering doubts about loosing self learning information. It would be nice to know if my doubts are unfounded or have some validity. Again, not exactly on topic but a related question.

 

[Joel K]

I have a question that may be a little bit off topic but I think is relevant here. For cars using EFI that has some sort of "self learning capability", how much of the "learned information" will be lost if/when the power is cut off from the ECU?

Obviously, in a crash situation, reducing the risk of fire/explosion is more important than optimal engine function. But with a master power switch, it's easy to drop power to the whole car, say when storing the car over the winter, and the if the learned information is lost, the EFI system has to relearn stuff to re-optimize it's operation.

I have an EFI powered car with a master power switch. There's been times when I thought about turning the power off but didn't because of lingering doubts about loosing self learning information. It would be nice to know if my doubts are unfounded or have some validity. Again, not exactly on topic but a related question.

Joel,

That’s a very good question, I am planning to use a stock GM ECU and honestly don’t know what if any ramifications there will be by disconnecting power to it. I would think it is ECU specific and how it stores Training data.

I’m working on my battery management approach now and designing it that when I shut the car off and lock it, the only item that has power will be the Alarm system which draws about 15 milliamps. The AIM PDM and GM ECU will be powered off.

 

[Mark B.]

I'm using a GM (E67) ECU with a battery cutoff switch. As long as your tune is good you won't have any problems. On mine at least the only thing that gets 'learned' are long-term fuel trims for each bank (i.e. computer adjusting more or less fuel based on the closed-loop measurements of your O2 sensors). The Short-term trims adjust in real-time and over time the computer will adjust the long-term trims across the air-fuel map so the short-term trim doesn't' have to adjust as much. If your tune is way off, it could just run like crap for a few mins after you re-connect the battery until it has time to adjust -- particularly when cold before the ECU goes to closed-loop mode.

 

[Joel Heinke]

I'm using a GM (E67) ECU with a battery cutoff switch. As long as your tune is good you won't have any problems. On mine at least the only thing that gets 'learned' are long-term fuel trims for each bank (i.e. computer adjusting more or less fuel based on the closed-loop measurements of your O2 sensors). The Short-term trims adjust in real-time and over time the computer will adjust the long-term trims across the air-fuel map so the short-term trim doesn't' have to adjust as much. If your tune is way off, it could just run like crap for a few mins after you re-connect the battery until it has time to adjust -- particularly when cold before the ECU goes to closed-loop mode.

Mark: thanks for this information. The car I was referring to is powered by an LS engine and uses an ECU from a 2000 Camaro (not sure of the actual ECU model). I've had an exhaust leak upstream of the O2 sensors that lets in oxygen at 3,600 to 4,000 rpm range. This leak has been causing havoc with A/F ratio to say the least. As a short term work around, maybe I should flip the master power switch so the ECU looses this bad data and falls back to the base tune which was done custom on a dyno. Right answer is reposition the O2 sensor, but now I'm a bit smarter on the dynamics of what might happen when I do turn the power off.

 

[Joel K]

I'm using a GM (E67) ECU with a battery cutoff switch. As long as your tune is good you won't have any problems. On mine at least the only thing that gets 'learned' are long-term fuel trims for each bank (i.e. computer adjusting more or less fuel based on the closed-loop measurements of your O2 sensors). The Short-term trims adjust in real-time and over time the computer will adjust the long-term trims across the air-fuel map so the short-term trim doesn't' have to adjust as much. If your tune is way off, it could just run like crap for a few mins after you re-connect the battery until it has time to adjust -- particularly when cold before the ECU goes to closed-loop mode.

Thanks Mark, after reading this I double checked my harness install instructions. Sure enough the way it is wired the ECU stays powered up. The positive wire attaches to the starter post. I’ll check what Speartech recommends for the LT ECU. If needed I’ll keep it powered up, but curious how many milliamps it draws when not running.

 

[Joel K]

Just heard back from Speartech who made my harness and the LT engine behaves similar to an LS if you power down the PCM/ECU.

1. The ECM’s Battery power is for learned fuel trims functions. So this helps on subsequent restarts of the engine. You can cut battery power if you want. Just keep in mind it can lead to hard starts.

 

[Neil]

Just heard back from Speartech who made my harness and the LT engine behaves similar to an LS if you power down the PCM/ECU.

1. The ECM’s Battery power is for learned fuel trims functions. So this helps on subsequent restarts of the engine. You can cut battery power if you want. Just keep in mind it can lead to hard starts.

The 12V is probably a "keep- alive" power for its memory.

 

[Alex]

I've put a battery cut-off (Flaming River Big Switch) on all of my cars, except my daily driver, as I only drive them once a month, if that. Lamborghinis, vipers, mustangs, corvettes and all my homebuilt vehicles (carb'd and EFI).

I've never once had any issues in over 15 years - the only thing you should lose is long term fuel trims (and radio settings, if you have one).

If you can't disconnect the power, power back up and have it immediately fire up and idle/drive fine, your baseline tune is garbage.

 

[Steven Lobel]

I am using this:

Remote Master Disconnect Switch Kit

DescriptionControl your battery disconnect with a small, remotely mounted switch. The included SPST latching solenoid is rated at a continuous 110 amps, 750 amp surge.Use with our Lifeline memory retainer to keep your radio and EFI presets in place!Fit NotesSPST LATCHING, 2-5/16" battery studs...

americanautowire.com

ECU has direct wire from battery for memory.

 

[Tony Eupizi]

This is what I used.