Does your radio have an AM band? If so, go off station, and start the car. Do you hear a whistling or buzz thru the speakers? If so, you most likely have a bad diode in the alternator.

 

Thanks I'll give that a try. Andy

 

I have a new alternator.

That's where I'd focus my attention first.

 

Sound like 1 phase of the alternator is out.

 

Or not making full contact until it warms up, not likely to get any better with time.

 

How do I test the alternator to confirm that it is the cause? It is a Bosch unit and I have a warranty but I presume I will need to prove it to them.

Thanks, Andy

 

We used to do it with a scope. But I forgot how. Haven't done it in 40+ years. I belive you need to start the car then disconnect the battery from the alternator. Or start it with the alternator disconnected than momentarily connect it to get the alternator going. Then look at the output on the scope. It should not look like a standard 3 phase AC signal. It should look like an 3 phase signal with the bottom half cur off. Just a series of humps. 2 humps and a space means 1 phase is out. 1 hump and 2 spaces means 2 phases out. 1 sign wave in the mix would mean a diode is shorted.

 

Pull it out and have it tested at Azone or another place. You could also put a voltmeter on the output to see if any awkward movement shows up.

 

Voltage jumping around like that pretty much has to be the alternator. More likely a bad connection or faulty regulator in my opinion, rather than a bad diode.

 

Interesting - the voltage regulator is external on my alternator/car so I will look to see if I need a new one of those. Andy

 

I would. The order I try to follow is Alt, Reg, Connections. If you have an ANALOG VOM, check voltage between alternator and voltage regulator. Look for a "jumping" of the needle on the VOM. A digital usually will NOT show this, unless you can set a high/low to show you the range of the voltage. If this checks out, just replace the regulator, cheap peace of mind.

 

I'd be scared to run the car with the battery disconnected, it's the only thing that damps down excessive voltages, a healthy alternator can produce some dangerous spikes that can damage things like ECU's when the battery is disconnected, who knows what a sketchy one could do?.

I too recall a scope test, but can't remember how it went either, you could pre loosen the D+ stud before starting and get it fired up and charging, then disconnect the big wire to the car and read voltage on the stud on the alternator. Just watch out where you let the big wire drop, it's still live from the battery.

 

From long ago at AAI, you NEVER want to run with the Alternator disconnected! A scope will pick up the waves in a DC Voltage, and the analog VOM will do close to the same thing. Just test the power lead between the alternator and the voltage regulator and look for spike or movement on the needle, depending on what you use.

 

From long ago at AAI, you NEVER want to run with the Alternator disconnected!

Funnily enough, that is what I thought might be a good test of the alternator!

Just curious. What harm could occur if you run the engine for short while with alternator disconnected? Back in day, that used to happen when a fan belt broke

If lights don't flicker then you would know the alternator or regulator is the problem.

 

Here is a quick link What happens if you run an automotive alternator with no load on it? . This IS what he said, but also what I remember from school MANY years ago! AAI is Arizona Automotive Institute, a school in the Phoenix area I went to in 1972/3. As I stated, you CAN see if there is a problem WITH the alternator connected just by looking for a wiggle on an analog VOM or the waveform on a scope.

 

Well no doubt that is correct. But you could remove the belt to do test.

That's of course what happened when a belt broke. Last time that happened to me was on w210. With serpentine belt a lot more stopped turning!

 

True, but without the belt, you can't test the alternator! If it's not turning, the only things you can test for is a bad diode or shorted/open winding. You can do that test on a bench with a VOM/DVOM.

 

Yes, but if the flickering stopped it would pretty well put all the blame on the alternator.