My alternator doesn't charge at idle. Charging begins at 1200 rpm. Is this normal?
I "think" that my alternator used to charge the battery at least somewhat at an idle when the truck was newer. In stop-go driving in Phoenix (freeeway = parking lot) in 120 degree heat, the battery discharges pretty badly.
Is this another sign of a sick alternator?
Thanks,

 

NO! A good alternator will put out 13.7V +/- .5V. First check output with fully charged battery, then test voltage drop on + and - cables. This will tell you the condition of the cables and connections. If all is good then you might have a problem with the alternator. If you get a good reading at 90 and and a bad one at 120 then you may have a bad alternator. I had a Boch alternator,in a car, read good when first started but after test driving for 30min. the voltage out put droped to 12.2V. with a load on it; headlights, A/C,and fan motor. It needed a new voltage regulater. Later I found a service bulletin, 'IN hot and/or humid weather voltage could drop due to defective regulater'.

 

I'm getting 12.2 volts at idle, which is the battery voltage with the key off, and up to about 1200 rpm. After that speed the voltage is 13.4-13.8. Its the same situation load or no load.
Also, its stable over time.

 

It sounds like you may have resistance/poor connection in a cable. At idle the alternator is not putting out the amps needed to over come the resistance.You might want to run atemp ground wire from the alternator to the battery an then check the out put right at the B+ post on the back of the alternator, it's the big post at 6 o'clock. If cables and connections are good then pull the alternator off and have it tested at an electrical shop.

 

Okay, auto-electric shop installed new brushes, checked batteries...all okay.
Alternator is a Denso 136 amp.
But, its still not charging properly.
I have 12.7 volts with the key off. With engine running at idle it drops down to 12.4 volts and only rises higher when I rev the engine to about 950 rpm.
With headlights on, drops to 12.1 volts. Looks like no charge at idle at all.
Once the engine speed is up over 1000 rpm, the charge is about normal 13.6 volts or so..actually, that seems slightly low to me, should be 14.1 ideally.
I'm thinking the regulator may be sick.
Where is the voltage regulator on this truck anyway? Anyone know?

thanks for the help.

 

It's been a long time since I worked on 12Vers. Does the AC and tach work at idle?

I seem to remeber having a truck that wouldn't charge at idle and I had to adjust the crank position sensor but it was several years ago so I could be wrong.

The voltage regulator is in the PCM.

 

Yes, the A/C and tach are working fine at all rpm.
Another symptom I just discovered is that the alternator gets pretty warm in about 1 minute of idle. That is peculiar since it shows zero amps output with a clamp-on DC ammeter. Diode?

So far no bad ground connections found. Thanks guys, for the good input.

 

I disconnected the two small 7mm connections on the back of the alternator coming from the PCM.
Then I ran wires from the battery directly to those terminals, I assume these are the field. Anyhow, it drew some current from the battery, but the charge remained at zero, no output at all, there is no output at all at idle. But rev it up, and voltage goes up to about 14 volts or so...way too low.
So, assume we're looking at a diode or winding short in the alternator?
Back to the electric shop Tuesday.
Can't believe he didn't check it when he replaced the brushes.

Geez, I'll have to become a mechanic yet to keep this thing running!
It was so hot out there in the sun, each time I needed to take a reading with the voltmeter I had to bring it inside to cool it off, as the LCD display turns all black in that heat. Now, time for something cool .

Okay, how can I test for a bad diode in circuit? I am guessing since its a 3 phase bridge arrangement I should be able to check with a scope on the output. The pattern should be nice and even for each half-cycle. Thats my theory anyhow...any comments?

 

IBmobile: you stated:
"If you get a good reading at 90 and and a bad one at 120 then you may have a bad alternator..."

What are 90 and 120, temperatures or what?

I have tried wiring output of alternator direct to battery, both ground and "hot" and still no charge at idle.
Another symptom I affirmed today is that if I have the CB radio turned on there is a whine and hash coming thru, while driving at normal engine speeds, which is also audible in the AM radio, not normal, this truck has always been a very quiet electrically. This points to diodes in the alternator again? Since the brushes are new they should not be arcing now.

 

Sounds like a toasted diode.

 

Does sound like an open diode or an open stator winding.

ECM regulates alternator output, based in part on the temperature of the driver-side battery - there's a sensor in the tray, under the battery.

Be very cautious when feeding battery voltage into the alternator terminals.