Lights pulsing at idle

[m880cummins]

Both my truck and my dad's do this. When idling any light will pulse (bright/dim). Both are 1989 engines/electrical systems and have no issues and charge fine. Is this normal with other trucks or is there something wrong? I have tried several different voltage regulators and made sure of good grounds, connections etc.

Charlie

[Tacoclaw]

Here's what Ive found. My truck was doing the volt gauge "wiggle wiggle" thing. All the lights would flicker, etc...I finally went out and did some real diagnosis to figure out why, since my previous truck did the same thing. After several tests of wiring, and consulting the factory service manual, I found a voltage drop of 0.8 volts to the blue wiring to the regulator. Tracing the wiring in the manual, I decided to check the fuses that are common with this circuit. Fuse 11, 12, and 13 all had the same drop, their source is one of the "key on" circuits in the ignition switch. I suspect the switch is the culprit, but I haven't checked that far yet. I have however used a jumper wire from the battery to the regulator, to try the charging system. It functions perfectly with the jumper wire attached, and pulses with it removed.
There is still the possibility that the source that FEEDS the ignition switch is weak. The feed originates from the heavy cable from the alternator/battery connection, thru a fuse link, thru the bulkhead connector, than to the ignition switch.
I will post my final findings when I confirm the repair. Hope this info helps.

can't tell you what's wrong, but I can tell you how to fix it.
What happens is, somewhere in the wiring to the voltage regulator there is resistance. This causes the regulator to "see" lower voltage than the rest of the system, causing the regulator to turn the field windings "on" when it really doesn't need to. The extra voltage overcomes the resistance and suddenly the regulator "sees" way to high of a voltage reading. This causes the pulsing you're seeing.
The way I overcame this, was to add a wire from the battery, to a relay, feeding the blue sense wire on the regulator. After doing this, the voltage gauge is rock-steady and I get a dvm reading of steady 14.4 volts.
I did have a problem when I did this at first. I used a hella relay without an internal suppression diode. Relays that don't have a suppression diode release a 300 to 500 volt short duration spike from the coil at very low amperage when turned off. This fried my regulator after the third time I shut the truck off. I put a relay in it with a suppression diode and it has been fine ever since.
Now, the best way would be to find the actual source of the voltage drop, but this is very simple way of overcoming the problem.

[Mark Nixon]

My theory on the voltage drop is that it's partly the fault of the fusible links.
Any time you heat a wire almost to the point of burnout, it creates more resistance in the wire once it cools.
A fusible link will be more susceptible, because it's DESIGNED to burn out under overload.

NOWHERE else in the wiring harness is there any other point that the wire gauge is corrupted as much as in the fusible link area.

Mark.

[m880cummins]

Thanks for the tips guys.

As a perfectionist/OCD person I HAVE to get to the bottom of this. The fat 10ga wire running from the alternator to the battery is about 5ft too long and has a fusible link. I am thinking of shortening it to the proper length, cutting it where the inline fusible link is and putting a real fuse it. The question is what size fuse should I need for this? 100amp?

Charlie

[BobS]

Mine has always had the slight high rate flicker. It charges just fine. Just for giggles I put my digital voltmeter in the AC position on the battery terminal connection at the alternator and found an AC voltage present. I been running it like this since 2004 and I just ignore it. I figure there's a shorted out diode in the alternator. I'll find out eventually when it quits charging and I rip it apart to rebuild it.

[freedomisntfree]

I had that same issue and was in the market for a PA performance 200amp single wire alternator... slapped it on and that flickering went away....

[m880cummins]

Definitely the alternator. I took the fusible like out and it is hardwired to the battery, voltage regulator has a good ground, new regulator, all wires checked and checked again. New alternator someday...

[TRENDZ]

You will put a direct replacement alternator on and it will do the same thing. These alternators don't have fade out like alot of others. This is why you get the pulse. when the regulator says charge it charges. when the regulator says dont charge, it drops to zero output, not a fade to zero as with others. If the voltage the regulator sees isn't what the alternator output is, you will get the pulsing. As a perfectionest, you need to find the voltage drop that is present at the blue reference wire on the voltage regulator. If there is a voltage difference from that wire to what the running battery voltage is, you will get the pulse. An easy test is to connect a jumper wire from the positive battery post to the terminal on the regulator that has the blue wire.

[cougar]

I've had all 3,
bad wiring
bad regulator
bad alternator
Best place to start is to bypass the wiring.

[ahale2772]

Interesting story... Bout three weks ago, my truck kept dieing. Charged ok but had a serious draw, chaced the draw to a recently installed tach, I had powered the tack to battery, not key on like i should have... doh... dumb move.. anyways, i fixed that, but i think i killed the alternator after killing the battery a few times. Battery kept dieing and I traced a draw back to the alternator. SOOO, I replaced the alternator thinking one of the diodes shorted out. New alternator goes in...no charge.....

Turns out... the old man who owned my truck before me, removed the regulator... Ive got now clue how my truck has been charging. there were 2 wires spliced into where the reg should have been, leading into the cab, both broken off??? so im at a loss there...

But besides my little story here, I installed a regulator and fixed the wiring, now i have some serious high voltage, almost 15 at idle with nothing on, and about 14.4 with the lights on and heater on. serious light pulsation at all rpm with no load

Long story short, Im going to jump out the blue regulator wire. Thanks TRENDZ

PS where can I find a relay with a supression diode?

[Tacoclaw]

Since you're apparently around school buses all the time I'd check out the relays on them, Alex. I've been using the big 4-blade 70amp relays from semis on all my stuff since I got them from an old Mack cab on our hill.

[ahale2772]

busses have some nice ones, but how do I know they are supressed?

[ellis93]

O'Reillys has a Borg-warner brand relay plug and pig tail that has that diode made in. Most newer Ford trucks relays have them,could rob one from one. Of course your truck could reject that

You'd have to open up the relays to tell.

I'll see if I can find one and I'll get a pic

[Tacoclaw]

Just look at the diagram on them that tells what blades do what.


^Stolen from some electronic site, ignore the transistor stuff.

It'll show it there if it has one.

[ellis93]

Crap pic
http://mob1011.photobucket.com/albums/a ... QuanBn.jpg

The pink/blue blur