Alternator warning light at speed

[rjkoop]

Went for my 1st nighttime drive last night. Everything ran fine but I noticed that the alternator warning light comes on (although very dimly) when accelerating or at cruising speed (+30mph or so). The light works fine otherwise... ie. The light comes on bright when I turn the ignition to the on position, turns off once the car starts and at low RPMs I don't see it. I don't think I noticed this before because I never drove at night.

Should I worry?

[GeorgeT]

Check the alternator drive belt for slippage.

[bradartigue]

Check the belt but this is often the case with a bad voltage regulator, it stops doing its job when the alternator is producing the most output. Test the output (you may have to have someone pressing the accelerator). If it is bad you can change it out - it screws onto the back of the late style alternators. If it isn't bad then check your engine to body grounds.

[rjkoop]

Thanks for the ideas. I'll check the belt 1st and then check the alternator output at higher RPMs. This video is great for checking the alternator, battery and voltage regulator.

https://www.youtube.com/watch?v=LGB6ZEjGm7Q

How did people work on these cars before these forums and youtube?

[spider2081]

This video is great for checking the alternator, battery and voltage regulator.

The video is helpful but has some issues. First in the beginning of the video where he is connecting the voltmeter black wire to the battery negative terminal the battery cable terminal is loose on the battery. He states be sure connections are clean and tight before testing and he is testing with a loose terminal.
Also alternators produce alternating current that is then converted to direct current inside the alternator by a 3 pairs of diodes. It is possible for one of the diode pairs to fail and the alternator will still produce proper output DC voltage under light loads. There is a test where you measure the alternating current (put voltmeter on AC volts) when that test is made, the AC component of an alternators output should not exceed 0.1 volt AC. In other words the alternators output should be measured for more than 13.7 volts DC out under full load and less than 0.1 volts AC under full load. It is best to perform this test at the alternators output post.

Here is a video for the AC test. This video states 0.5 volts as defective I have always considered 0.1 but that is not as important as doing the test.

https://video.search.yahoo.com/search/v ... ction=view

[rjkoop]

Belt is tight (no slipping even at higher RPMs).

Did some initial tests.

Car off - 12.75v DC at battery
Car on - 13.8v DC at battery
Car on and REVed to about 3000rpm - 13.8v DC at battery
Car on - 0.045 AC at battery

So I think I'm good for at least these tests.

[bradartigue]

Probably a ground.
Definitely a ground.

[spider2081]

There is a 7 pin connector behind the instrument cluster (I believe it is the tachometer connector) that has the battery warning light wires in it. I would be curious to know what voltage you measure on the pins that have the light blue/black wires on both sides of the connector and also there is pin with a black wire on one side and black violet wire on the other I would like to know what the voltage measures on that pin also. Measure both to ground.
There is battery voltage applied to the light when the ignition switch is in the start and run positions. The other side of the light connects to the alternator field circuit inside the alternator. When the alternator is not spinning it does not produce a voltage on that wire and the wire becomes a ground through the field winding in the alternator for the light to light. When the alternator is spinning fast enough the alternator produces a field voltage and the light then has voltage on both sides of the bulb and it is not longer lit.
The alternator ground is made through the mounting hardware of the alternator. All of them should be clean and tight. Many newer cars have a ground wire from the alternator case to the cars chassis to insure a good ground. One of the 2 screws that holds the voltage regulator/brush set to the alternator makes the regulator ground to the alternator I would make sure both of them are tight also.

[rjkoop]

There is a 7 pin connector behind the instrument cluster (I believe it is the tachometer connector) that has the battery warning light wires in it. I would be curious to know what voltage you measure on the pins that have the light blue/black wires on both sides of the connector and also there is pin with a black wire on one side and black violet wire on the other I would like to know what the voltage measures on that pin also. Measure both to ground.
There is battery voltage applied to the light when the ignition switch is in the start and run positions. The other side of the light connects to the alternator field circuit inside the alternator. When the alternator is not spinning it does not produce a voltage on that wire and the wire becomes a ground through the field winding in the alternator for the light to light. When the alternator is spinning fast enough the alternator produces a field voltage and the light then has voltage on both sides of the bulb and it is not longer lit.
The alternator ground is made through the mounting hardware of the alternator. All of them should be clean and tight. Many newer cars have a ground wire from the alternator case to the cars chassis to insure a good ground. One of the 2 screws that holds the voltage regulator/brush set to the alternator makes the regulator ground to the alternator I would make sure both of them are tight also.

Great info. Thanks. I think the dim red light means that the charging can't keep up with the load when the lights are on.

I'm going to check the alternator wires and mounting bracket more closely and ensure good ground. I'll try to get 14.5V DC when car is on and then ensure it doesn't drop below 13.5V DC under load.

[rjkoop]

Removed, cleaned up and sprayed alternator wires. Also removed alternator mounting bracket, removed the paint from the metal contact points (doh!) and reinstalled. I'll also check engine/head/bell housing grounds and ensure good contact.

I'll test today and hopefully get better results. I think good results would be...

12.6V DC - car not on
14.5V -> 14.7V DC - car on with minimal electrical load
13.5V DC - car on under electrical load (ie. lights, DEF fan)

[rjkoop]

Still have a very dim red light when the lights are on. So I think I'll add the ground from alternator to chassis.

Where is the main ground for the engine block supposed to be? I know about the transmission big ground wire and the head ground wires. Just not sure where the engine block -> chassis ground is.

If I have the DEF fan on and I turn on the lights the DEF fan speed goes down. With a well grounded everywhere car should I expect that turning the lights on wouldn't affect the DEF fan speed?

[baltobernie]

My guess is that you simply don't have enough output from your alternator. Take your car to one of the "big box" auto parts stores, and have them check it.

[TX82FIAT]

Alternator sounds good and you verified belt connection. I'd revisit your grounds. Also, the headlights are wired with everything else running through the ignition. A lot of folks install a headlight relay and create new grounding points in the process.

[rjkoop]

Tried an extra ground strap from alternator to ground post. This didn't increase the charging voltage. I can only get the DC voltage to 13.7V with car running. This is with lights, DEF fan and stereo on loud. Should I worry?

I noticed that the DEF fan speed lowers when I turn the lights on. Is that normal?

[TX82FIAT]

Yes, I think 13.7 with the motor on is fine. Your alternator should have a voltage regulator that is maintaining the proper voltage given RPM so it will vary a little but settle in the 13.7 range. I think above 13 is good for the limited draw. I suspect you have some resistance within the circuits at connections. Think of the connections in terms of corrosion restricting the flow of current to the components. It's like putting a kink in a hose that has several connections, the water source has the pressure needed but some restriction along the way is only letting a trickle through. The water would much rather go to places where it has less resistance.
Check grounds and connections. In particular, look at the connections at the ignition switch.