The Stovebolt Site Forums The Shop Area The Electrical Bay Voltage problem

There is a problem I've been having for a while with my truck. It is a '53 with a 292 six out of a '66. It has the GM 3-wire alternator, the one with a wire for ignition and two red wires that come together into one. The problem I'm having is keeping a steady voltage. The biggest culprit would be the electric fans. Since the 292 didn't leave enough room for a regular fan, I had to wire up some electric ones. It is a dual fan that I salvaged from a 2000-something Dodge truck. Just wired up directly with a switch on the dash. Whether I have one or both fans connected, turning them on causes a huge drop in voltage, then settles for barely above 12v. When off, it is above 14v. I also noticed it fluctuates a little with the turn signal on.

Am I in desperate need of a different electric fan? Is there a better way to wire it up? Is the internal voltage regulator shot? Can an external regulator be added to an alternator that has an internal one? Or is everything actually just fine?

(Recently tried to add an external regulator, but I was given a Chrysler/Dodge two pin and couldn't figure out how to wire it into the GM alternator wiring.)

Have you checked the current output of the alternator? One bad diode will drop the current capacity of the stator to less than one third of its rated capacity, without affecting the alternator's ability to produce normal voltage. I've seen "55 amp" alternators with an open circuited diode in the rectifier bridge produce less than 15 amps max- - - -runs the vehicle OK in the daytime, but slowly discharges the battery with headlights or a heater fan running. Have someone who knows how to use an ammeter check the alternator for max output.
Jerry

You need a larger output alternator. Those fans draw a lot of current.

George

You've received two excellent answers, Alex. Please let me add my personal note: First, I'd concentrate more on amperage than voltage. For decades I've told people to think of voltage as the size of your arm and amperage as the strength of your arm. I also have 2 electric fans, HEI ignition and I have a 100 amp alternator...or that is to say it is rated at 100 amp. The HEI ignition uses around 20 amps briefly at startup and after that settles back to around 7 amps. I haven't had the alternator checked for actual maximum output. I expect if it is a true 100 amp unit it will be ~80-90 amps given the pulley ratio I have and the rpm range at which my engine normally runs. One fan is a Spal puller fan (17") and one is a Spal pusher fan (16") which operates only when the A/C is on. The 17" fan is thermostatically controlled so it only runs when needed. When both fans are running I'll lose about 50 amps according to Spal's tech data. While that is happening, anything like the headlights, turn signal lights, etc will be seen as a reduction on my ammeter gauge (in the dash cluster). Hope this is helpful.

Given Alex's comment there is another thing that MAY help his situation. The T2 (sense) wire, the smaller of these "two red wires that come together", should ideally be connected back in the electrical system under the dash somewhere (that is how GM designed it) instead of to the output lug on the alternator. That way it will sense "system" voltage instead of alternator output voltage. A good place to connect it might be on either terminal of the ammeter or the bat terminal of the ignition switch.

This isn't going to SOLVE Alex's problem, but it should make it better than his charging system which is operating as a 1 wire alternator now.

THANK you! I just did that (to the ammeter), and it made a huge difference! Now it goes back above 14v when the fans are turned on.

Thank you to everyone for your help.

Since electrons move at approximately the speed of light (186,000 miles per second)- - - -tell me how connecting a wire in a different position on a vehicle is going to make a measurable difference in how the alternator performs. "Inquiring minds want to know!"
LOL!
Jerry

Likely poor connections in the electrical system. Drop a quarter volt here, a quarter volt there and eventually they add up. But I didn't feel like explaining how to use a voltmeter to look for small drops at each connection.


As I said above "that's how GM did it". They wouldn't have wasted a few cents (in 1972) on 6 feet of wire for a sense line under the dash. And they could have eliminated the T2 connector and just internally connected T2 to the output lug. Wait! They did that at one point. It was called a one wire alternator.

It isn't timing issue, it is a voltage drop issue. Hooked together at rhe alternator, the feedback is tied to the voltage at the alternator, but as you pass through various connections and wires that all have some resistance, the voltage drops a little bit with each one.

When you add a high current load, the small resistances suddenly cause a much larger voltage drop. So tying the feedback into the post-drop circuit allows it to manage the overal system voltage better.

Here is some interesting read on this http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml

So,that sense wire should be connected at the same point where the wiring feeds the loads. Maybe the starter stud where the main battery cable comes from the battery? Maybe use this point to feed the loads and the sense voltage? If the sense wire is on the alternator hot stud, maybe feeding some loads from that point would also act like that.

You know what else helps? Being more careful with old electrical stuff.
First, there was a bare wire to the coil and repeatedly shorting out.
Next, the key ignition was loose and hot. Likely because I failed to realize I should have changed it out from 6v to 12v.
I fixed those, but I really should plan to replace all old wiring in the future.

The switch doesn't care what voltage the system runs. However, loose connections are always "hot" connections- - - -looseness produces resistance, and overcoming resistance causes heat. A 6V ignition switch actually has bigger, heavier contacts than a 12V, because it takes twice the current to produce the same wattage as a 12V accessory- - - -"voltage X amperage = Wattage". 6V wiring is always heavier gauge than 12V, also.
Jerry