Lights dim, then bright - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

techman1963 · 04-08-2012, 19:37

Got a '99 and '00 both doing it. When driving at night you can see the head lights as well as the dash lights dim slightly, the go back bright again. The battery light will come on and flash usually as well. Battery voltage will stay in the 14 vdc range when the engine is running. Have not yet had the codes checked yet, which is my next step. Just thought it odd that the '99 was doing it and then when I bought the '00 it is doing the same thing. TIA.

djzx3 · 04-08-2012, 20:00

Fixed your title for you.

And I would double check the voltage reading that your getting, get a volt meter and just double check it.

**R.S.LOGAN** · 04-09-2012, 01:34

A low/weak battery wouldn't help , get it load tested at same as checking all earths . Also check alternator output with electrical system loaded .

techman1963 · 04-09-2012, 05:59

Thanks, guys. I have load tested the batteries in both cars and alternator output has been checked with lights full on and A/C and blower motor on full speed. Ran engine speed up to 3 grand and the voltage will go up to 15-15.2, then taper off back down to 14-14.5 vdc. The '99 is kind of a mute point since it took a hit to the passenger side back in February and has been parked since. The only reason I even mentioned it was because I thought it interesting that the symptom was present in the '99 and then when I bought the '00, it has the exact same symptom. Seems to me there is a poor connection somewhere, but haven't found it yet. Ran it for an hour and a half yesterday afternoon with the lights on and A/C on full blast with volt meter on at varying rpms and it never acted up once. This car will do this sitting still at an idle or on the road @ 65 mph or points inbetween. Thanks.

techman1963 · 04-14-2012, 22:04

There appears to be a voltage drop in the keyed power circuit (green wire) to the 3 pin connector to the alternator. Circuit is good from the fuse, but somewhere between the fuse and the alternator is a bad connection, ie, pinched , damaged, etc. The wiring diagrams that I have are sketchy at best, but I assume that the circuit goes from the fuse to the instrument panel for the charging light and then to the alternator connector? I am tempted to run a wire from the fuse to the alternator, but I would rather fix it right than fix it with a bandaid. Thanks.

techman1963 · 04-20-2012, 19:49

Found a service bulletin dealing with '99 and '00 Cougars with the 2.5 engine that are having the pulsating light symptom. The fix is to reloacate the voltage sensing wire (red) from the engine compartment fuse block and loop it to the B+ terminal on the alternator. This took care of the pulsating lights, but I still have a charging issue. The keyed power circuit (green w/black stripe) to the alternator will not carry a load. If I install a jumper wire from pin 4 to pin 15 of the instrument panel connector, the keyed power circuit will definately carry enough current to light a 12v bulb. The problem is that the charging system is still inoperable. I've had the new alternator off twice and checked it on 2 different test stands. Both tests indicate that the alternator is working properly. All ground points have been loosened, cleaned and retightened. Seems to be a problem with the circuit with in the instrument panel. Thank you.

techman1963 · 07-22-2012, 00:08

After determining that the alternator is functioning properly and the battery is more than suffecient, I determined that the issue is in the charging indicator light circuit within the instrument cluster. As I mentioned in my previous post, the head light flickering issue was resolved by moving the voltage sensing wire to the B+ terminal on the alternator per the Ford service bulletin. The charging issue was resolved by completely disabling the alternator exciter circuit from the instrument cluster and created my own circuit by running a wire from an unused keyed power terminal in the engine compartment relay/fuse panel to the exciter terminal on the alternator. I installed an inline fuse to protect the circuit. The only problem with this modification is that the alternator light in the instrument cluster is non-functional, but I can live with that since replacing the instrument cluster was not a financially feasible option.

**Bert** · 07-22-2012, 07:32

Thanks for the update.

**CatSkinner** · 07-22-2012, 08:01

I missed most of your posts, but it looks to me like you might have bypassed your voltage regulator, but I'm not sure how you could've done that. I'f you've got a steady, regulated potential coming from the alternator, pay ne heed to the BS I just said. Here's a diagram for a 2000 charging system if you want to jump back and regain your instrumentation.

techman1963 · 07-22-2012, 09:36

I guess I should've been more specific in how I ran that new circuit. The regulator is not bypassed and I now have good steady charging voltage (13.5V).
The instrumentation works just fine, just the circuit for the alternator light has been disabled. I have the schematic for the charging system, but thanks anyway.