2003 Disco battery light occasionally flashes while driving. Mostly right after start up, before the truck warms up. Also seems to be affecting the idle while it's happening.


I cannot remember what the voltages are supposed to be. What should the voltage read at the battery before and after start up. I'm assuming it's either the battery or the alternator. Any way to test the alt?


Any experiences with tis?


Thanks

 

Got the same thing on my 03 once in a while, I just dont know where to start because next day its gone. My truck is scared to the mechanic. I'll follow this post, I hope you solve yours.

 

I recommend getting the cheap battery/alternator tester from Harbor Freight. It is only 4.99 and I have had one for 10 or more years. You can check the voltage before startup, then at idle.
One common issue is the ground. There is a ground bar that is connected to the body, right behind the battery. The ground cable connects to it and then continues down and connects to the engine. I would remove the nuts, and wire brush the connections very well, then follow the cable to the engine and make sure it is nice and tight. I would also remove the cables and make sure they are nice and clean.

 

Mine flickered and after I found a fried plug wire and replaced the coils, flicker went away.

 

So here are the numbers form the alternator and battery.


Battery 12.9
Alt running 14.4
Battery running 14.3
Fuse box running 14.3


Full load - Lights, fan, high beams, wipers all on.


14.1 across everything.


Checked the voltage by putting positive lead on the alternator, and every ground connection I could find, including the strap. All were consistent with 14.3 - 14.1.


Same if testing at the positive battery terminal, or the fuse box.


I noticed that there are two posts on the back of the alternator. One is obviously the main power out. It leads to the battery terminal, and then the fuse box.


The second lead, from what I can tell from searching the forum, is the lead that causes the battery light to go on, and also "excites" the alternator and causes it to produce power.


I read the same voltage at that terminal as I do at the main terminal, although it is more difficult to get a reading. If I disconnect the wire that attaches to that lead, the battery light does not come on. If I drive the truck with it disconnected, the light does not come on. If I reconnect the lead, the battery light flashes as it has before while driving. The alternator puts out the same voltage at both terminals whether that lead is attached or not, at idle.


It may be important to note that the light does not come on at idle. It only seems to flash intermittently while driving, or in park revving the engine.


Reading through some other posts it appears as though the battery voltage routes through the dash light via under dash fuse 27. I checked that fuse, and it is okay. Also may be worthwhile to note that fuse 27 shows as cruise control and not the battery light. There are two fuses under the hood that show as battery light. Checked them both and they are fine.


Rigged up the voltmeter inside the truck with leads to the body ground and the positive battery cable. Drove the truck watching the dash, and the meter. Voltage was a consistent 14.2 - 14.6 depending on RPM. Never dropped below 14.2. Battery light flashed when I revved the motor in park, or when I accelerated. The light flashed regardless of the voltage.


It seems as though the alternator output voltage is sufficient and consistent. My guess is that there is an issue with light, and whatever activates it.


Any ideas on what I may be missing? Anyone with any experience with the system that monitors the voltage, and illuminates the battery light?


At this point I'm stumped, and don't want my wife to get stranded somewhere.

 

Wires and coil packs are all pretty new. The engine does not run rough, as if a wire or coil pack were bad.

 

your alternator is good. have you had the battery load tested?
the second wire off the back that excites the alternator also powers the tach,
so if you have a problem there it will usually effect the tach also

 

Probably just a a connection that needs to be cleaned and causing an intermittent indication fault.

Ops and diagram below.
......

 

When I had the plenum off, I also tidied up a few things. Electrical taped over places where the plastic conduit was gone, and at the alternator, the shrink wrap was hard plastic and cracked, so I cut that off and re-taped it. In the end, whether it was the ignition system or cleaning up wires, flicker was gone.

 

Alright, thanks for all of the input. I believe I have found the problem.


If I attach the voltmeter to the main power output terminal of the alternator, voltage is consistent at all RPM's. In park or drive. It slightly increases under load, but the voltmeter reads a consistent measurement between 14.2 and 14.6. It appears that the alternator is putting out appropriate voltage at the main terminal, and back to the battery and fuse block main.


I took the voltmeter, and I connected it to the smaller terminal, that routes back to the dash via the brown and yellow wire. At idle it reads fine, whether in gear or in park. Raising the RPM's causes the volts to drop out. I ran the voltmeter back into the truck, still connected to the body ground/negative cable and the smaller terminal.


I then drove the truck to see what would happen. The voltage completely drops in and out at that alternator terminal. As the voltage drops in and out, the voltmeter flashes to zero, and the battery light in the dash comes on. As the voltage flashes back up, the dash light goes out. The dash battery indicator light is flashing on and off in exact accordance with the voltage dropping in and out at that terminal.


If you can open the video below, you can see the voltmeter flash in and out.


IMG_8550_zpsbxosmseg.mp4 Video by gulfstream69xr7 | Photobucket


Next question.


It appears as though the alternator is producing plenty of voltage to the battery and the rest of the truck. I have had no problems with starting, battery charging, or running any of the lights, wipers or fans.


The issue seems to be the circuit that sends the signal back to the dash light losing voltage.


Are these two circuits independent of each other? Is it part of the voltage regulation system? Can I replace the voltage regulator component of the alternator, or does it make sense to just replace the whole unit?


Thanks again, this forum is a great source of information. It looks like I should be able to get this sorted.