What would cause the ammeter to peg full +40v 100%, at idle, every so often, and the engine to lug, then return to normal after a few seconds?

Would this indicate a poor battery ground, or positive?

 

I would be very careful…..get someone to check to see if it is really doing this.
The potential for an electrical fire is not worth waiting to rule it out.
Check the voltage out of the alt which you can do with any volt meter.
To check the amperage you will have to use an inductive lead which goes around the
wire and not in line. If all is normal then you can start looking for ground and other wire shorts.

 

do you have an ammeter or volt meter? if the former, not sure what could temporarily pull that many amps while running short of a dead short which would likely smoke and blow a fuse/fusible link, or start a fire. If the latter, the alternator may be full fielding for some reason. short from the internal regulator to the case is one possibility, or something else going on with the alternator.

 

Need more info. How does the ammeter react after cranking to start. After turning on headlights with the engine off? You've got three possibilities: bad regulator, bad battery or bad connection. Start with the easy ones first. Clean (that is remove wires, remove corrosion and reassemble with electrical grease) connection at the alternator, horn relay, battery and ground. Then have the battery load tested. Then the regulator.

 

The voltage sensing wire from the alternator is intermittantly losing contact. That will cause the alternator to drive to full output and the ammeter will show heavy charging and your engine will lug down from the added loading.

The most likely source of this problem is poor contact at the plug in the alternator. Remove the two wire plug and check it completely for loose contacts/dirty contacts/frayed wires. Also check the terminals in the alternator. Remember that battery power is in that power lead, so disconnect the battery before doing these checks.

Once reconnected, operate the vehicle and see if the problem is fixed. If not, you may have a problem with the alternator internally.

 

Sorry about the delay all. I had to go out and buy myself a clamp meter per James011 suggestion, and what I've found really supports your theory 7T1vette.

By the way, this is a new install of a high-output 3-wire alternator. A chrome powermaster, 150 amp high-output unit. Here is what is happening with my testing:

Normal idle: Alternator is outputting +4 amps
Problem time: Alternator intermittently (every 60 seconds or so) outputs +70 amps, engine boggs, and ammeter pegs +40v.

I will check for loose contacts on the two wire lead now! Thank you for that!!

 

By the way, what *should* the alternator be doing as "normal" behavior? Should it have a constant output that never changes, or is it normal to have it "cycle" in this manner??

 

The kind of cycling you're seeing is not normal.

 

For "normal" operation, the alternator senses battery voltage; it then loads the alternator in direct [inverse] proportion to the level of the battery voltage. Low voltage, maximum amps output to help recharge the battery quickly; near battery voltage, little additional charge--just enough to prevent any 'drain' on the battery.

After starting the engine (which is a pretty good draw of current from the battery), the alternator will perform significant recharging (15-20 amps positive). That amount will decrease steadily until the current used to start the car is replenished. Once that is accomplished, the ammeter will read near "0" charge; it will just compensate for actual current usage at a constant rate (for whatever electical equipment is running). At idle, the ammeter may show negative amps, as the power used could be more than the idling alternator can keep up with. But, the battery will cover the loss until the engine gets to cruising speed (20 mph or higher).

When all is well, the ammeter will almost always read "0" amps during normal running. When it doesn't (either showing positive swing OR negative swing), there is likely to be a problem with the charging system.

 

Hmm ok thanks for that 7T1vette.

I've partially figured out the problem. There was an "auto-resetting" circuit breaker in place of the fuse link at the starter. It was a 20 amp breaker (only 260 watts at 13v) and it was tripping and resetting every minute or so. This is what was causing the "cycling." I've since replaced this with a 14 gauge fusible link wire, and I'm getting a constant 60 amp while idling.

Possibly, the battery is a little low and I haven't let it idle long enough to see if the alternator will cycle-down and charge less. Thats the next thing I'll try.

Thanks to everyone for all the help in this thread!

 

You really ought to look some more into that....I wouldn't expect to be pulling 60 amps continuously.

How are you reading those amps? On the installed gauge?

I would suggest charging your battery fully and then take a reading...

 

Ground, ground and ground.
Make sure all your ground connections are good. Specifically the starter to frame and the battery to frame. Since these car body is fiberglass a lot of components need a remote ground path for good circuit continuity. 60 amp draw at idle is WAY too much and as others have said if your charging circuit is working correctly you should always hover around 0 amp draw. Your battery could be suspect as well. Electrical gremlins can be a bitch and very dangerous to you and your wiring. Please be careful.
Not sure why you need a 150 amp alternator unless your running a huge stereo system. These cars only need 60 amps at the most.

 

Put your voltmeter on the output of the alternator. Anything over about 14.4V means you have an issue other than a low battery.

Also if the voltage rises over about 14.4V when you bring the engine rpm up then you also probably have another issue. That alternator should easily hold at a constant 14.4V even at idle especially if you don't have any big loads turned on (headlights, heater etc off).

edit - make sure you have battery voltage on terminal 2. By battery voltage I mean whatever voltage the battery or the output terminal of the alternator or the fuse block reads. The alternator uses that terminal to regulate the voltage so if the voltage on that terminal is low then the alternator will crank-up the output trying to reach the proper charging voltage.

 

Sorry if this seems like a dumb question.
Did the car have an external type Voltage Regulator before you swapped in the new Alt? I'm guessing not but covering my butt.
It sounded like you had an actual current draw based on your earlier statement of a tripping Circuit Breaker and that after putting a fusible link it went to constant draw.
I'd be curious to know what your voltmeter reads at the battery and the clamp meter reads on the Alt. output cable and the battery positive cable (if your Amp meter indicates polarity, the Alt. output cable will always be a positive number and the batt. Positive/Red change from depending on current flow direction). As a sanity check for your Amp. meter, clamp it on the Batt. Red cable and with car off have someone start the car while you watch the meter, it should show a large number + or - and then once running a slowly decreasing number (the sign will stay the same while charging but will be the opposite your first number). When the car is running most current is supplied by the Alt. so the current reading on the Red batt. cable should level off to a small number and batt. voltage should sit still. It is possible that your Alt. is full fielding due to an internal issue. (goggle full fielding alternator).