Alternator test

[rjbaren]

Hello guys, 
I always seem to have the dumbest questions but if you don't know you have to ask.  One of the lugs in my alternator on my 73 TCS broke, and the ammeter was reading negative, so I took it back to the rebuilder and he corrected it.  Now the gauge sits mostly at the zero in the middle.  Today I took a drive, and everything was normal, so I decided to turn on my auxiliary radiator fan with my manual switch and the gauge went to the positive side.  That seems backward to me.
I called the rebuilder and he said to put a multimeter on the battery.  Unfortunately, I don't have a digital multimeter so I can't tell if it is charging or not.  It looked like nothing changed from off to idling.  Can I reverse the two wires at the back of the alternator and see if that makes a difference or will I blow out something?  I guess I better buy a digital multimeter, but I thought I should ask before changing the wires around and ruin something.

[jbcollier]

Swapping the wires at the back of the ammeter won't hurt anything.  Multimeters can be cheap but really cheap ones are not that accurate.  I've seen some up to a full volt off.

[rjbaren]

Before I swap the wires at the back of the ammeter, I would like to swap the wires at the back of the alternator.  Those are the ones I removed when I removed the alternator to take it back to the rebuilder and possibly put back in reversed order.
I am referring to the brown/white wire going to the ammeter and the brown/yellow wire going to the charge warning light (which is working) and then on to the tachometer.

[Bryan Boyle]

Going positive means that the alternator is charging against the draw from the battery and holding against the current draw from the fan.  Unless it's more than a needle with, I'd not worry about it.  The ammeter is NEVER going to just sit at zero; the time to worry is when it's constantly discharging, which means that either your alternator is shot, you're overdrawing against the capacity of the charging/battery system, or you've a dead short somewhere past the ammeter and the car will soon smell differently.

[jbcollier]

STOP

Please don't swap wires at the alternator willy nilly.  Read the manual and the wiring diagram and hook each wire to its correct terminal.  If you would like assistance, please let us know exact details about your car and alternator.  It looks like a TCS, right?  Is this a federal car (two Strombergs)?  What is the serial number?  What brand of alternator?  Can we get a picture of the back of the alt?  What number, size (thick or thin) and colour are the wires going to the alt?  Is everything stock as far as you know? Or?

[Bryan Boyle]

STOP

***DELETIA***

Best advice I've seen all day!  Put things to stock.  Switching wires without knowing what they do/where they go is always a Bad Thing.  The stock setup with OEM bits works.  Get to there first.  THEN if there's a problem (and we know what JB asked for), further troubleshooting is more likely to solve the issue (if there is one...).

[rjbaren]

OK, 1st of all I used a digital voltmeter, and the alternator is putting out over 14 volts when I rev it up.  The guy at the alternator shop said it should be right but...if I do have it hooked up backwards, the battery will drain overnight.  I use a battery cutoff so I left it connected and I will find out tomorrow.  He also said, if it is wrong it will ruin the diode trio over time. 

My car is a 73 TCS with a Delco alternator, without a molded plug.  My serial number is 73053525R so I was told it was built May of 73, unit number 3525 and R for Federal.

The two wires I am unsure about are the Brown/yellow which goes to the charge light and then the tachometer and the Brown/white which goes to the ammeter.

Everything looks to be stock.  Someone had cut and spliced one of the wires with a crimp connector and some white wire, so I bought the correct size and color wire and spliced and soldered it further back and put some harness tape over my soldered splice and now it really looks stock.

[jbcollier]

Is this what the back of your alternator looks like?

[jbcollier]

Just a couple of points.  The NY wire is a ground path for the charging system warning light.  When you turn the key on, the green wire fuse is powered.  This fuse also powers a number of other things but they are not relevant in this situation.  With the alternator not turning, it provides a ground and the charge light comes on.  Once the alternator starts turning and producing power, it also feeds system voltage back into the NY wire shutting off the light.  The tachometer, and other items on the green wire circuit, have no effect on the charging system.

Switching the NP and NY could/would fry the diode trio feeding the field IIRC.  Don't do it.

Switching the NW and ground will short the NW (back to the battery through the ammeter) to ground, frying the rectifying diodes, and, hopefully, toasting the ammeter before the car catches on fire.  Don't do that either.

[jbcollier]

13.2v to low 15v is fine.  I like to see it just above 14v and not into the 15v range.

"Now the gauge sits mostly at the zero in the middle."

That is perfectly normal.  The gauge will go up slightly after starting so the alternator can recharge the battery from its labours turning the starter motor.  Then it will settle with the needle very close to the mid point.


"Today I took a drive, and everything was normal, so I decided to turn on my auxiliary radiator fan with my manual switch and the gauge went to the positive side.  That seems backward to me."

These cars do not have a manual override for the rad fan.  How is your bypass wired in?

The ammeter measures current flow into and out of the battery excluding the starter solenoid.  If the alternator can't keep up and the system is using some battery power -- say when your lights and wipers are on at an idle -- then ammeter will deflect to the negative side.  If the battery requires "filling up with power", the alternator will flow current into the battery until it is "full" again and the ammeter deflects to the positive side.

Back to your ammeter deflecting to the + side when you switched on the rad fan.  How large was the deflection?  Just a wee bit?  Or did it move quite a bit?

[rjbaren]

Ok, with the key on & engine off the needle goes a little negative, if I turn the fan on without the engine running the gauge does not really move.  I removed the dimmer switch on the dash lights and connected them to the headlight switch.   Then I added my fan override switch where the dimmer for the dash lights used to be.  The aux. fan will turn on at around 95 or 100 degrees.  I put a BMW  2002 temp switch in a threaded fitting in the radiator to replace whatever Lotus had in there.  I didn't like the bulb type switch; I thought it could pop out.

With the engine running and the auxiliary fan on, the needle will go about halfway across the positive side of the scale.  Without the auxiliary fan on the needle is pretty close to the zero, maybe a slight bit on the positive side of zero.  A while back the needle went way negative and that's when I checked the wires at the back of the alternator and one of the push-on connectors was lose.  I realized the wire was no longer making contact inside the alternator, so I removed it and took it the alternator shop and the piece inside the alternator was resoldered to accept the push on connector for the wire.  I also just found my drawing of where the wires were before I removed the alternator, and I did put them back correctly.  I was afraid I had it backwards and that was why it was reading in the positive rather than the negative when the auxiliary fan was turned on.
I don't remember who, but someone here did a fan relay and override switch and also two relays for the headlights and an additional fuse box.  I copied what I saw and the schematic that was also provided which has the headlight relays and the auxiliary fan relay and switch.   

[jbcollier]

Just a clarifying point, it's not an auxiliary fan if it is the only fan...unless you have two rad fans?

If you took power for your new relays off the battery, when the relays energize, the ammeter will move to the positive side.  This is also why there is no indication on the ammeter if you turn on the lights or the fan while the ignition is off.  If you want the ammeter to work properly, then you take your relay power off the back of the ammeter, ignition switch, or fuse box.  Right now the alternator is feeding through the ammeter to supply your relays.  Thus your "false" positive.

This is why diagnosing "over-the-phone" takes a good deal of time and lots questions.  We can't see that you have modified your car.  Something that would be obvious if we were there.  I appreciate your prompt and detailed answers.  It made things a lot easier.