revisiting alternator upgrades and rewiring
 

Hi,

Thinking about my options for alternators for my 82 300cd, and the options of using the 80 or 115A saab alternators. The reality is that especially for the 115A the system needs to be re-wired, in order to handle the extra current, but I think Im missing something.

From what I see on a w123, the electrical setup about the battery is as follows:
Negative to body. Positive to the starter - the "heavy" cable is for discharge purposes in reality.

Now, there is a single cable that goes from the starter to the terminal block. Hot heavy, but not 18ga either. There is also the other smaller wire to the starter which engages it.

So, in reality, the entire charging current from the alternator to the battery goes through that one small wire that connects the terminal block to the starter. I believe it is 6mm^2 area per my manual, making it a 10ga wire.

OK, now here is where my confusion grows... There are two other 6mm^2 wires that are the connection from the alternator to the inside of the car (dc distribution I suppose at the fuse panel), which go in the upper grommet next to the battery. The connection back out comes from the lower grommet next to the battery and to the terminal block.

So is it correct to say that the current to charge the battery takes the trip of 2x10ga wire into the car, back out to the terminal block, then via 1x10ga wire to the starter and then via the 25mm^2 (6 ga) battery cable to the positive terminal of the battery?

OK, so where do we want heavier wires then? Bigger ground and maybe an extra one to the alternator body is good, right? Bigger one to the starter aides in faster cranking. But the limiting step is still going to be the old, 10ga wire that makes the distribution inside the car, right? Isnt that dangerous? Even if you connect a nice heavy conductor from the alt to the terminal block, so it charges the battery via heavy cables in the route from alt to terminal to battery directly, there still is the chance for a high-current alternator to put excessive current through the old OE cables that connect there.

Additionally, if one re-wires the alternator, what do we do with the connection that originally connected to the alternator? Just wrap it in electrical tape? One of the wires is still needed, and the harness as a whole goes straight into the car.

It seems to me that if the alternator is re-wired, whether it goes straight to the battery or straight to the terminal block (or both), we end up feeding the DC bus inside the car backwards from what was OE (since we are no longer sending current up the three-spade alternator connector). Is this an issue?

Should we be concerned about safety of a high amperage alternator given that the wiring to all of the loads (except battery charging) would still be the original small gauge wires? Shouldnt fuses be put inline with the existing distribution wiring to protect from excessive current from the upgraded alternator, NOT just bigger wires in some sections?

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Sorry for the lengthy post.

I just had to replace the wires you are talking about...they were rotten and I was not getting enough voltage at the battery to recharge. The wires that go into the cabin go to the acc. If you remove wires from the middle post, you have no power.
Frontmost post is ground. Don't remember what the rearmost wires are for.
The acc was where my first problems became apparent. Weird electrical issues related to blower current and monovalve. Then I losr ability to recharge batt.

I had to cut the bad writing out and spice in fresh wires.
You are correct that they are 10g

you want to replace all those wires you are essentially going to replace a huge chunk of the harness. Yikes!

Can't help with the other questions.
GOOD LUCK!

Thanks!

The frontmost post is ground? I don't think so... Isnt it switched for the starter solenoid? If so, it may appear as a ground when the starter is not engaged, but it wouldnt be a ground.

Not sure if that is correct, but I didnt get the feeling that the front one is ground. Anyone care to verify?

Front meaning closest to the radiator....I think we are saying the same thing.

The one with 4 wires going in is the one that goes to the ACC, and also seems to be the main current supply for the car. The one with two heavier gauge wires must be the starter solenoid switch then?

The smaller, violet wire that connects @ frontmost post and comes from the 4 heavy wire bundle must be to prevent the ACC from engaging while the starter is drawing current?

ouch, no...
not ground!
front most wire is the power lead to the starter from the solenoid!
if it were ground shorting it to battery would melt it. and if you short that wire to positive post on the battery, the starter engages.

Not trying to be snarky, just curious here...

Why would you want to have a higher output alternator than MB originally spec'ed for this car?

Are you running some kind of high-draw electrical accessories that require additional amperage most of the time?

SteveM.

Newer batteries, depending upon chemistries, like up to 14.4v charge at certain points. Gm alternators for example put out a good deal higher voltage than these Bosch units.

I don't have a lot of extra load beyond the gps, phone charger, sometimes a laptop, but it is the fact that the operating voltages are so low on known good systems...

The original alternator does not put out adequate power at idle to run the original accessories, so the battery short-cycles with a small discharge, shortening its life.

IMO a proper alternator is one of the best inexpensive upgrades for these cars (through the mid '90s).

Just an adder here: I pulled a (IIRC) 120a Bosch from a Land Rover Discovery, a bolt-in swap in the '87 300D, use the 300D pulley and crimp on some ring terminals.

First, let's get distribution correct. I'd start by saying the wiring diagram is vague, and that you are the best judge as to how the vehicle is actually wired. There is a 25 mm^2 (~3ga) wire from the positive post of the battery to the main starter post, and (though the wiring diagram shows it differently) a 6mm^2 (~10ga) wire from this post to the terminal block (C105). The two 2.5mm^2 (~12 ga) wires should be going directly from the alternator to C105, but you say the alternator wires go into and out of the firewall first. These may go to/from the light switch or main fuse box terminals, but that is not how the wiring diagram shows it. Also connected to C105 is a 4mm^2 (~11ga) wire to the glow plug relay. So C105 should be connecting the alternator, main starter post, lights/fuse box and glow plugs all to the same post. The main point is that the alternator only charges the battery through the wire to the starter, the car is powered via the two wires to the fuse box. BTW, the two 2.5mm^2 wires are basically equivalent to a single 10ga wire, which is capable of sustaining a 55 amp load.
The alternator will only 'put out' what is required of it. The extra capacity of the alternator is there if needed. If you changed nothing else in your vehicle the new alternator would only put out the same amount of current as your old alternator would. With nothing turned on, your original alternator will only be putting out a few amps to charge the battery and any electrical devices (i.e., the gas gauge.) It won't be putting out 55 amps (where would it go?) If you add high current loads, such as a 400W stereo amp, zenon headlamps, heated seats, etc., then the new alternator would be able to provide the extra current to feed those devices without discharging the battery. And you would be wise to provide additional current carrying wires to support those devices.
In my case the alternator wires went directly to C105 - very short wires and I was able to replace the wires easily. I replaced the two 12ga wires with a single 6ga wire, mainly to save the original wiring harness. The other wire is for the sense or field circuit in the alternator and that too was terminated at C105 (separate bus from the other wires) so I replaced all three wires. If you can't remove the original wiring to the alternator and you want to increase the current capacity to the fuse box, I would use a wirenut and tape it up. Cut it where it won't dangle anywhere it could get damaged. As you state, one of those wires is the field wire, so make sure that is separate and is reconnected to the new alternator's field circuit.
No. The only real issue is having good connections and the proper amount of ampacity to the fuse box.
Again, no. All the original wiring could be used safely as long as there is no significant increase in current loads. As stated above, the alternator will only put out what is required. The main differences between the new alternator and your old alternator are that your output voltage at idle will be greater, and the amount of excess capacity that is available will be much greater. Whether or not that extra capacity is required is determined by the amount of accessories you need to power which would also drive rewiring decisions. For myself, I put in an AL129X (115A), but the only extra current load will be a pair of euro headlamps (halogens) so I will not be doing any additional rewiring. But if I or some future owner decides to add accessories, the alternator will be up to the task and additional wiring will be the only consideration.

Good Luck! Hope this helped.

I dont know if the 82 has the same gauge wire with the 85 but i would have gotten away with rewiring if i had used an 80A alternator. Probably just need to change the terminals on the end going to the 80A alternator. Not really familiar with the 80A, I used the 115A alt. from the saab. Had to replace the harness from the battery to the starter and alt. I now have three wire from the positive pole of the battery. 4gauge to the starter and 6gauge to the terminal block I used the original alt.wire from the saab. its metric( between 4 and 6 gauge). I dont have any wires from the alt to the terminal block.

OK, that verifies it. No wires from the alternator directly to the terminal block.

My one concern is fault current in the case of an inadvertent short on the old wire distribution. Sure, the alt will only put out what is asked of it, but if there is a short, it will put out more and then risk a fire if the wires to the fuse block arent upgraded.

My issue is that I am still <14V, around 13.7 at idle without load... That is lower than Id like...

The fuses protect the wires. Whether there is a 55A or 555A alternator, an 8A fuse will still blow when too much current goes through it, but the duration is so short the wires don't get damaged. Now for the un-fused portions of the wires, the 115A alt will not be the problem. The battery is connected to the same circuit and will provide way more than 115A when shorted directly to ground.

But remember, the battery is connected to the DC distribution within the car only through that one small wire coming off the starter to the terminal block. Since the alternator feeds the fuse box directly, and only on the way out of the car does it connect to the terminal, effectively there is a "forward" current to the battery via the terminal block and starter wire to the battery as long as the DC bus voltage is higher than the terminal voltage of the battery. That single wire that comes from the battery to the terminal block may in fact act as a "fuse" in case the battery serves to short to ground inside the car, as it is fairly small... or it may be a fire hazzard.

But intermittent and soft shorts inside of the distribution in the car could still allow a higher than normal current to flow in the wires not rated for it, being a hazzard. I concur that the alternator will only put out what it is asked for, and in that theoretical world, right, the wires need only be sized for demand, whether the alternator is 55 or 555A. However, there is the "what if' scenario where the alternator can put out more current, and in case for whatever reason it gets asked to do so, then we do have a problem. I get what you are saying, but still have to wonder if the way the car is set up is the safest inherently, let alone if we install more generation capability.

If you have the alternator connected to the fuse box, and the battery connected to via a different route to the fuse box, and something in the fuse box becomes grounded, the current will flow from whatever source is available through the wiring to ground. Now, I'm assuming that the your alternator is connected to the fuse block through the two 2.5mm^2 wires, and the fuse block is connected to the terminal box through two 2.5mm^2 wires, and the terminal is then connected to the starter through a 6mm^2 wire to the starter, and a 25mm^2 wire to the battery. The negative post of the battery and the alternator are both ground, so they are in actuality wired in parallel to the fuse block. So no matter where the faulted ground occurs, current will flow from both the battery and the alternator to the fault.

The current carrying capacity of the two 2.5mm^2 wires together is roughly equivalent to the single 6mm^2 wire. If you are really concerned with the amount of current flowing to the fuse box from the alternator, see if you can move the wires from the alternator to the terminal block, and wire the alternator directly to the terminal block. That way you have four 2.5mm^2 wires to the fuse block. If you are really, really concerned, stick a 100/80/?? fuse in-line.

Regardless of how you wire it, if you short an unfused wire to ground, you are going to have a fire hazard whether you have a larger alternator or not. In fact, a larger wire would likely be more of a fire hazard since a smaller wire will fail sooner, and the larger wire will continue conducting and heating longer. I know you're trying to be safe, but I would humbly suggest that you are overthinking this. If this were a legitimate issue, you would be hearing about people having to replace wiring in the unfused portion of the circuit more often, and underhood car fires from insufficient wiring. My guess is that there is more of a hazard from frayed wires arcing and sparking at low currents that don't draw enough current to blow a fuse or burn out the wire than there is from the unfused power feeds. As long as the connections are secure, and the wires are strapped down to prevent chafing, and the wires are located away from strong heat sources (e.g., the header) and moving objects (pulleys and belts), then there is no reason to worry about the larger alternator. The battery has always been able to provide more current than the original alternator and the current wiring has proven sufficient.