Lights Flashing While Idling -
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Unread 09-25-2011, 07:22 PM   #1
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1974 CJ5 
Join Date: Apr 2011
Location: Lawrence, KS
Posts: 16
Lights Flashing While Idling

I have a problem I'm trying to figure out, I'm not 100% sure of the details here, but it's what I've observed so far.

To start with...I have a 1974 CJ5 with the 232 I-6. It's pretty stock under the hood, specifically I'm using the Motorola 37 amp alternator, Motorola Voltage Regulator, and a new battery (a few months old).

I just got the Jeep back on the road a few months ago and it's been doing well, but it started cranking slow, then one day it wouldn't start, would just click. Popped the clutch to start it and drove it home.

I tested the battery voltage and was getting just around 12 Volts, but when I'd try and start it all it would do is the clicking. If I hook up jumper cables to my car, I could start it. I charged the battery a week ago and it tested at about 13.5 Volts. I've been driving it for about a week now, and haven't tested the voltage again. I'll be sure to do that.

However, here's the problem I'm wanting to get some help with. After charging the battery I drove the Jeep over to Autozone to have them test the alternator. When he tested it, he said that everything tested ok, but that the alternator was only putting out about 30 amps. After telling him it was only a 37 amp alternator, he seemed to think that would be fine. I drove home, but when I got home and pulled the Jeep in the garage I noticed my headlights were flashing on and off, as if I were turning them on and off, as opposed to dimming and coming back up. This wasn't just the headlights, but also the dash lights, and I'm assuming side markers and tail lights, but I haven't confirmed this. As it was daytime, I didn't notice it until I pulled in the garage.

I took the Jeep out again a few nights ago and didn't have any problems with the lights flashing while I was driving (moving), but when I would come to a stoplight and sit, they would start flashing, once I started driving again, they seemed to stay on, but once I would come to a stop they'd start flashing again.

I'm not sure what to make of all of this, especially since they're flashing on and off, as opposed to just dimming.

Any ideas what's going on?

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Unread 09-25-2011, 11:51 PM   #2
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1978 CJ5 
Join Date: Jan 2009
Location: Massillon, Ohio
Posts: 5,075
Is it more of a pulsing? Mine were doing that, where the lights would rhythmically be dim/bright/dim, but they never went out.. My volt meter would pulsate with them. Mine would do it at a frequency that increased with RPM, and then once it hit a certain RPM it would cut to half the frequency..

I took the alternator apart, planning to take the diodes out and test them, and then also swap from a spare alternator any parts needed, (Spare was the OE 36A, I have a 78A in it now) and I found that there was a bluing on the commutator rings of the rotor. I used some crocus cloth and polished the bluing off, and reassembled (diode trio was ok). Fired it up, and it is fine now. I've caught it doing it every now and then lately, so I figured I'll have to do it again some day. I'm guessing that leaving the key 'on' and not running has heated a spot on the rings where the brushes were sitting, causing the blue spots.

1978 CJ5 258/T177/D300, '86 D30/D44 WT axles, 'glass tub windshield frame, and fenders, 31x10.5 BFG A/T, TDK galv'd frame - DD and weekend toy
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Unread 05-23-2014, 07:35 AM   #3
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1974 CJ5 
Join Date: Apr 2011
Location: Lawrence, KS
Posts: 16
I know this is a really old thread, but wanted to update it to let everyone know what the problem was. Turns out the headlamp switch was the problem. It was old and I'm assuming had weak connections which would cause it to get really hot and that would cause it to fail. Once it cooled it would light up again until it overheated. Long story short a new switch has me up and running at night and my lights are so much brighter now too!
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Unread 05-23-2014, 07:44 AM   #4
Mike Romain
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1986 CJ7 
Join Date: Nov 2007
Location: Off the Grid in Chester Grant, Nova Scotia, Canada
Posts: 12,684
Thanks for the update and a tip on the cause for you. The stock switches only are rated for stock headlight bulbs. If you put halogen headlights in, the breaker in the switch will eventually overheat and blow out again. The more high beam use, the faster it will get cooked. You need to wire in relays so the switch only triggers that and the headlights get direct fused battery power.

When halogens first came out they came with relays for this. Most modern vehicles come with relays.
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