MG Sports Cars

I have a 1979 MGB with a Chevy V6 60 degree, 2.8l from an 83 S10. The alternator is the stock one from the S10. It has a terminal connected to a cable that runs from the battery, and a small, two-prong plastic connector. The wires from the connector are brown and red; the red one loops back to the terminal, and the brown one is connected to a 12 volt source on the fuse box that is only on when the ignition is on.

Here is my problem: If I have the brown wire connected, as described above, it starts off fine (13.5 volts at idle), but rises to 15v+ as I rev the engine. This seems way too high for this battery. Usually, the voltage regulator controls all of this, but I believe this alternator has an integrated VR, and I think I am just wiring this incorrectly. Any ideas? I am about to give up after a weekend spent playing with it, and just take it in.

Thanks,
Ron

Are you using a meter with a digital read out or an older analog meter? 14.7 is a typical charging voltage. 15 may just be an inaccuracy in the volt meter?

Yes Rob is correct. Normal charging system voltage ranges between 13.6 to 15.2 volts, depending on the manufacturer, ambient temp and RPM.
If it starts climbing past 15.2 volts, then I would check for voltage drops in the ground circuits. Sometimes high resistance in the grounds between the battery, frame, and engine can cause an overcharge condition.

Bill

Thank you both for the reply. I am using a high end digital voltimeter, so I feel pretty good about the reading. I will check for high impedance and see what I find. I just feel like I do not have the feedback correct, so I can't tell how the altenator would know that the battery is fully charged and to cut back on current. Does that make any sense?

(lack of) Progress Update: I checked through the wiring and did not find anything odd, or any grounded circuits. I then found this site, and wired it per the instructions (http://www.teglerizer.com/alternator/wiring.htm). It still did not fix the problem until I connected the black (sense wire) from the alternator to the brown/yellow wire from the MG (the idiot light) and then connected it to a 12-volt, ignition on source. Then it charged as expected (although the idiot light is now on whenever the key is on). Charging got up to 14.5v at speed, and everything else seemed to work. I declared success and went to refuel.

At the gas station, turning off the key did not turn off the engine, until I switched off the lights. Then it turned off. .

I am guessing I have a short somewhere that I missed. Any other ideas?

Ron,
Try unplugging the sense wire when the engine is running with the key off.
If that stops the engine then you just have a small feedback problem that can be easily fixed with a diode in line to the idiot light.
Cheers
Fred

I will give that a try tonight Fred. Thanks.

Ron,
I just reread your post.
The black sense wire should go to the idiot light only, no 12volt power feed.
That's probably whats making the car run on.
Cheers
Fred

Fred:

Without a 12 volt feed, the alternator is not charging at all.

Ron

Hey Ron,
Maybe we should start at the beginning just to make sure that nothing got missed.
It sounds like you have a Delco SI style alternator.
These are an inexpensive, robust, plentiful, easy to work on alternator with an integrated voltage regulator.
They are available in a variety of amperages and basically two case sizes.
As long as you have the room available they are an excellent choice.
There are three terminals that need to be connected to the vehicle.
The first is the large studded terminal marked BAT.
This is connected directly to the battery + terminal.
The wire needs to be large enough to handle the full output of the alternator that you are using.
I would consider 8 gauge wire to be a minimum.
It also needs to be fused according to the wire size.
The BAT. terminal would normally be connected to the large brown wire or wires of your MG but these aren't really large enough so run a dedicated cable to the battery.
The next is the left spade terminal generally marked "R" or "1"
This goes to the idiot light only.
This is the brown with yellow wire in your MG wiring harness.
Check to make sure that the warning light system works properly by grounding the brown/yellow wire to the engine.
Turn the ignition on and the idiot light should illuminate. Unground the wire and it needs to go out.
If this doesn't happen, it needs to be repaired before you continue.
The last connection is the most important and just about everyone does it wrong!
The right hand spade connector marked "F" or "2" is the voltage sensing connection.
This connection needs to go to the central power wiring junction of your vehicle.
Not the BAT. terminal of the alternator.
Your alternator gets all of it's charging info through this connection and it can't get that at the alternator terminal.
A huge # of charging systems under perform because of this miss connection.
Two easy sources in the MG are at the ign switch supply or at the starter + term.
Fortunately that brown wire that you aren't using is a perfect candidate for this connection, so use it.
Make sure that you have a good ground from the battery to the engine and from the engine to the body.
Missing ground cables to the engine cause a surprising number of problems.
Start up your car. Everything should be working fine. Check charging at the battery not the alternator.
You should have approx. 14.2 volts at 1500 engine rpm.
Turn on your lights wipers and heater fan, you should be able to maintain the same 14.2 volts @ 1500 rpm.
Lots of guys worry about the amperage output.
But if you can maintain the voltage with all the loads on.
Then you must have enough amperage.
If you have an electronic ign system you may run into a run on situation with the key off.
This is due to the alternator backfeeding the ign through the idiot light wiring.
A simple diode spliced into the brown/yellow wire will cure this.
Hope that"s a help

Cheers
Fred

One more thing, though it probably does not apply to the Delco. That sense connection on some alternators, especially from Japanese cars, is designed to connect to switched power (the main power relay) and doesn't have any internal provision to prevent current flow when parked. It can run down the battery on a direct connection.

Also since copper wire has resistance it drops a small amount of voltage for every foot of length. With the battery in the back that is worth remembering. The closer you are to the alternator the higher the voltage will be. Ideally the sense and voltmeter connections would be on a dedicated wire direct to the battery with a relay in line to disconnect when power is off. Or as Fred advises, with a separate charge line to the battery it can come from the ignition switch and still be isolated just fine. If the charge line just goes to the main cable at the starter like most, a separate sense line is a good idea.

I run both a voltmeter and ammeter. One shows pressure, the other shows flow. This gives a more complete picture of electrical system health and allows you to instantly pinpoint problems such as a bad battery or failed diodes. For instance, on a trip to Townsend a few years back my voltage was fine but amperage was way up, pegging the 50A ammeter. Obviously a problem with the battery but I'd never have known it with just a voltmeter. Upon arriving I pulled the Optima battery out and it was so hot I don't know why it didn't melt or explode. It had a shorted cell and it had to sit on the grass for a couple hours before it was cool enough to handle safely. That's the most dramatic example but I can't count the number of times the dual gages alerted me to trouble enough in advance that I could avoid serious inconvenience.

JB



Edited 1 time(s). Last edit at 08/04/2011 11:56AM by BlownMGB-V8.

Thank you all for the help, and for the detailed instructions (Fred, those were great instructions). They worked like a charm. My call is charging well, the idiot light works as expected and my radio does not fade out when I have the lights on and put my foot on the brakes. :-)

Still have a small issue where my water temp gauge (after market) only works when the lights are on, but that is a simple crossed wire and I can handle that repair.

Really appreciate the help.

Ron