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Discussion Starter #1
Hi All,

My alternator went out so I had my mechanic replace it with a Delco 3-wire. But, I'm noticing that the alternator doesn't seem to be working when engine is at idle. I have to give it about 300 RPM more to get it to kick in (I see the voltage jump when I do this with lights and fans on).

I also noticed that now the charging light will not light when the key is on (or any other time).

So, I checked the wiring and noticed they installed a 500 Ohm resistor in series with the field/lamp wire just before the terminal on the alternator. This seems way too high and possibly unnecessary. Is this the reason the alternator needs a few more RPM to get it to kick on?

Can I just connect this directly to the lamp wire w/o resistor?

Thanks!
 

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Hector:

I would guess that the 500 ohm resistor is related to your problem, but I'm not familiar with Delco alternators, so I would be hesitant to say "oh just take out the resistor". I can say that on a Bosch alternator you just connect that terminal directly to the light, with no resistor. Why not contact the shop that did the installation and ask why they used a resistor and why they sized it at 500 ohms?

Many alternators don't begin working until they either: 1) receive voltage through the terminal that goes to the indicator light or 2) are spun up to a significant rpm. It sounds like option #1 isn't working on your car, probably because of that resistor. The fact that the charge indicator light isn't working also points to insufficient current flowing through the 500 ohm resistor.
 

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Thanks Jay. I agree I'll need to ask them...I finally had some time to work on the car over the weekend, so wanted to attack this as soon as possible. After looking at the wiring diagrams and scouring the interwebs for info on Delco wiring, I came to the conclusion that removing the resistor (which was in series with the lamp) should do no harm. Putting a resistor or diode in parallel with the lamp would make way more sense since it seems dumb that if the lamp goes out it would affect the alternator operation by removing the field current.

So, I removed the resistor and the charge light worked again (at least it goes on when key turns to start anyway). The voltage output seemed stronger, but once again with fans and lights on, the voltage sagged at idle below charging voltage (a little higher than before, but not like it should be). If I increased the RPM it went up to 13+ volts. I observed my idle was 500 RPM on the tach, which is low compared to the manual's spec of 750-850. So, I think I have three things to figure out:

1. Is my car idling at 500 or not? If so, I need to get it to 750...if I do that then the alternator will be fine at idle. No idea how to do this with Motronic though. Will check idle speed shortly...

2. If Idle is OK, then can I increase alternator RPM with a smaller pulley? Not sure what is available, but will check if I get to this point.

3. Rewire the fans to be powered through a relay since its possible they are sagging voltage too much at idle due to the current path through the ignition switch (with fans off, voltage seems acceptable at ilde).

Option 3 seems like a good idea regardless so I may just do that anyways.

Any other thoughts or experiences appreciated....

Thanks,

Hector
 

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1. Is my car idling at 500 or not?....No idea how to do this with Motronic though.
I know nothing about Motronic, so you are on your own with #1

2. If Idle is OK, then can I increase alternator RPM with a smaller pulley?
That's a good idea. How does the diameter of the pulley on the Delco alternator compare with the diameter of the original pulley? If it is larger then yes, your new alternator is turning slower than the old. The Delco and Bosch alternators probably have different RPM : voltage characteristics, but it would be interesting to know how different their drive pulleys are.

3. Rewire the fans to be powered through a relay since its possible they are sagging voltage too much at idle due to the current path through the ignition switch
No, that won't help. The resistance of the ignition switch impedes current flow through the fan circuit: for a fixed voltage, more resistance results in less current. Adding a relay will certainly make the fans run faster, and extend the life of the ignition switch, but it will also allow more current to flow.
 

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Discussion Starter #5
Thanks Jay. I see what you mean on point #3. I had thought similarly but talked myself out of it because I wanted some solution to try :wacko: I'll put that on the backburner for now.
 

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I've done a bunch of stuff on this, so thought I'd update this thread and hopefully it helps someone else seeing the same issue. Sorry for the long post, but didn't want to skip anything.

1. I removed the 500 Ohm resistor and noticed a slight increase in voltage when running above idle, but no real difference at idle (which is my main issue) - still got a big voltage dip with fans and lights on at idle. I measured the voltage dip as 12.8 V with fans+lights; 12.3 V with fans+lights+ac. Without fans/lights/ac the voltage was about 14.3 V - so the battery gets charged as long as the fans don't go on which is impossible in lots of traffic.

2. I measured the RPM at the crank and at the alternator and found that at idle my crank was at 800 RPM and my alternator was at 1500 RPM. BTW, I used this inexpensive laser to make these measurements - it worked great!

So, I went to the Delco CS-130 specs page and found that it was probably making 40 amps. The amperage calculator at Quality Power says I'd need about 78 amps, which is probably on the high side, so figure around 60 amps.

3. So, I needed to raise the alternator RPM to around 2000 to get around 60 amps . Question is how small of a pulley would I need to do this. Given the ratio of RPMs between crank and alternator and knowing that my current pulley size was 2.625", you can figure this out as:

New Pulley Size = Old Pulley RPM / New Pulley RPM X Pulley Size
= 1500/2000 * 2.625 = 1.97" ~ 2"

You can calculate the size of crank pulley by using the RPMs and alt pulley size:
Crank pulley = ALt RPM / Crank RPM X Alt Pulley Size
= 1500/800 X 2.625 = 4.9"

Now we can check to see if the new pulley ratio give us the desired RPM :
Crank Pulley size / Alter Pulley Size * Crank RPM
= 4.9 / 2 * 800 = 1960 RPM ... close enough

4. It was tough to find a 2" pulley - Quality Power said they had one, but they only had 2.2" in stock. I went ahead and tried that. One problem with small pulleys is that the belt might slip, so 2.2" seemed like a good compromise.

5. I installed the new pulley (used an impact wrench to remove / install the nut while holding the pulley) and remeasured the RPMs. Alternator RPM at idle was now 1800 (with engine at 800). So, not quite at my target of 2000 but hopefully enough.

6. Got the engine hot and now measured 14.5 V with nothing on; 14.45 V with fan only; 14.2 V with fan+lights on.
Success! Now I can't see any voltage dip when engine goes to idle on my dash voltmeter.

7. Drove it around some traffic and it no longer had the problems of eventually discharging the battery and sputtering to the side of the road. I'll followup as I drive it around some more.
 

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Discussion Starter #7
Picture of the old and new pulleys.

Picture of bracket used to adapt the CS130 to the existing Alfa alternator bracket.

A few reasons for going with the Delco CS130 was that it is a 3-wire alternator which means it senses voltage and will adjust the regulator to keep voltage up (I put the sense wire on the firewall distribution terminal). Its also cheap and easy to find in the US and puts out around 100A.
 

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