big 3 bosch alt positive to battery

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russianblue

Member - Posts: 4

Joined: July 02, 2007

Location: United States

Posted: July 02, 2007 at 9:55 AM / IP Logged

Car is a 1988 BMW 5 series (heavily modified). With regard to the big three upgrade...

1) alternator positive to battery positive,

2) battery negative to chassis, and

3) engine ground to chassis

I have taken care of 2 and 3 above. Big improvement! My question is about #1.

My stock alternator positive wire is about 12 gauge in size at the most. Different than a lot of systems I've seen, this wire goes out directly from the alternator to the starter, and THEN from the starter to the battery.

I am assuming that I need to upgrade both of these wires, which isn't that big a deal other than it's going to involve a lot more labor. My question is whether this is a fairly common arrangement? Also, if any of you have done this upgrade on this particular BMW engine (M20) and known of anything specific to look out for?

I've posted a link HERE to the wiring diagram along with some pictures of the altnernator. The model is Bosch AL49X (90 Amps).

Also, I believe there is a bolt on upgrade from a late 80's 7-series that I can swap in for my existing alternator that puts out 140 amps. Have any of you e30 guys done this upgrade?

i am an idiot

Platinum - Posts: 13,667

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Joined: September 21, 2006

Location: Louisiana, United States

Posted: July 02, 2007 at 10:40 AM / IP Logged

I am not familiar with your particular car, but I would assume that the battery is located under the rear seat or in the trunk. If this is the case, I am sure they ran the charge wire to the starter because it was closer than the battery. I am sure the wire from the starter to the battery is plenty large enough to get the alternator current from the starter to the battery. Upgrading the 12 Ga. from the alternator to the starter should take care of it.

russianblue

Member - Posts: 4

Joined: July 02, 2007

Location: United States

Posted: July 02, 2007 at 11:05 AM / IP Logged

Interestingly enough, the battery is location is a typical front mount, under-hood on the driver's side. That's kinda one of the things throwing me off. The wire from the alt to starter goes back toward the firewall through the engine harness to the starter, and then the charge wire u-turns from there, runs back through the same harness up to the battery in the front. It seems quite illogical. It just lengthens the necessary path of the charge by about 5 feet. I am tempted to just remove the old charge wire and replace it with one directly to the battery (about 18 inches vs. 5 feet). However, I can't tell from the wiring diagram if the starter is purposefully put into the loop there for some other reason.

i am an idiot

Platinum - Posts: 13,667

Joined: September 21, 2006

Location: Louisiana, United States

Posted: July 02, 2007 at 12:00 PM / IP Logged

Silly Germans I can't think of any reason for the starter to be in the circuit. But why not leave that wire in tact and add the 18 inch wire.

Alpine Guy

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Joined: October 18, 2003

Location: Canada

Posted: July 02, 2007 at 12:45 PM / IP Logged

Leave the stock wire there, and add a 4 gauge wire fused at 150 amps between the alternator and battery directly. The little 12 gauge wire you see there is called a fuseable link, it's basically a fuse on it's own, it burns up internally if too much current passes through it.

2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.

russianblue

Member - Posts: 4

Joined: July 02, 2007

Location: United States

Posted: July 02, 2007 at 12:57 PM / IP Logged

Now, with regard to the 150 AMP fuse...is there any magic in choosing that number, or is it simply in a range where if something goes wrong, the fuse will blow before a complete meltdown? FWIW, My alternator is 90 amps

Sweet. Answers that question. Thanks a lot guys.

Should make things a LOT easier! Good call Mr. Idiot...on the Silly Germans. As much work as i have done on this car, I've uttered that same expression thousands of times - sometimes a lot louder than others.

Alpine Guy

Platinum - Posts: 2,478

Joined: October 18, 2003

Location: Canada

Posted: July 02, 2007 at 1:47 PM / IP Logged

The reason I say 150 amps is because thats the most current the 4 gauge wire can safely handle, you could fuse it less, say down to 100 amps, but there no point really.

2003 Chevy Avalanche,Eclipse CD7000,Morel Elate 5,Adire Extremis,Alpine PDX-4.150, 15" TC-3000, 2 Alpine PDX-1.1000, 470Amp HO Alt.

russianblue

Member - Posts: 4

Joined: July 02, 2007

Location: United States

Posted: July 02, 2007 at 2:01 PM / IP Logged

That makes sense. Works well too since all of my big 3 upgrade wiring is 4 gauge as well, so I already have the materials. I plan on upgrading the alternator soon to a bolt-on 140 amp model from a 7-series E32 evenutally. Might upgrade to 2 or 0 gauge at that point. Thanks again.

i am an idiot

Platinum - Posts: 13,667

Joined: September 21, 2006

Location: Louisiana, United States

Posted: July 02, 2007 at 4:02 PM / IP Logged

I don't think there will be any need to go any bigger than the 4 Ga. As Alpine Guy (Who I noticed reccomends Eclipse CD Players) said the 4 Ga. is rated at 150 amps. I am sure the alternator will not put out more than it is rated.

hustlin247

Copper - Posts: 79

Joined: June 08, 2004

Posted: July 03, 2007 at 2:02 PM / IP Logged

My '94 ford explorer does the same thing oddly enough. I've been planning to leave the the wires going from the alt to the starter and the starter to the batter and just ADD a wire going from the alternator the the battery.

'94 Ford Explorer / Kenwood KVT-815DVD / RF Power T1682C 6x8 (all doors) / RF Power T10001 / 12" Kicker L5 (x4) / Optima Yellow Top Battery

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