78 620 charge light issues

[perattzi]

Hello all, need a little direction with this one. Sorry but this is a long post lol.

 

Back story: about 1 month ago my charge light came on while driving home, I thought crap my alternator is going out. The next day I go out to try and investigate and the truck starts right up and the charge light goes out like normal after starting. Checking the voltage at the battery it was around 14.5 revved with no lights, 13.x with lights, at idle it was around 12.8 or so. Seemed normal, if not a little high after 2000 rpm. So figured all is well, maybe just a fluke, or perhaps with the lights off the regulator is letting it overcharge at revs.

Then about a week ago the charge light starting coming on again, usually at idle and would dim and go out when driving with normal revs. Sometimes it would pop back on and other times not at all, whether at idle or driving revs. Seemed totally random. Then one evening I left my headlights on for several hours by accident and killed the battery. I took battery out and tried to recharge..only getting to about 12.4volts resting. The battery sticker said it was 9 years old! So I figured time for a new battery. So I bought a Les Schwab 500cca 60 month battery, new battery cables, (ground straps are also newer from when I had issues with my throttle cable seizing!) Installed battery and the charge light stayed on for a short time then goes off. Then was normal for a few days, no light at all. Then when driving home on Monday I stopped at a store  and was idling for maybe 10 minutes then drove off up a hill and the truck sputtered and died. At this point the charge light was not on, so I had no warning. I tried to restart the motor and nothing, battery was discharged so much that starter would not turn over. I waited a few minutes and tried again and got the truck to start. I managed to drive it home without using headlights or turn signals. It died once on the way and I had to get a jump which worked getting me home. The battery was below 6 volts when I checked it at home. Seemingly I had been driving on just the battery for awhile. But why no charging light if the battery was not being charged? I recharged the battery, it is right around 12.9. Which I believe is good for a 6 cell. 

 

So I looked further into it today and noticed that my oil pressure light was also not coming on when you turn the key to on(not start) I pulled the bulbs and swapped them to the parking brake socket and they both worked there. Then I pulled the oil pressure wire off the sending unit and grounded it, still no light. So I did what was probably the dumb thing and removed the gauge cluster to look at the 12 pin connector and printed circuit board, everything looks fine, nothing shorted etc. All fuses are good, 12v to both sides of fuse holders. So I just started tugging on the loom going to alternator/oil pressure switch...the oil light then popped on. So I unplugged the round 6 pin connector that connects the wires to alternator, oil pressure switch, b/y to starter, and a smaller loom that heads down towards the transmission. The large black wire terminals were badly corrorded/oxidized and the large yellow's were so corroded that the male pin had broken away from wires. So obviously this is a problem. I cleaned up the black connectors and pulled the two ends of the yellow out of the connector halves and cut away to clean wire and jumped them with a spare wire. Still no charge light when I turn the key on. So my question is where should I test from here? My friends are like 'just take it to Autozone and have the alternator tested". But I understand that the charge light is integral to the alternator starting the charging process. So it seems silly to test the alternator until I figure out why the light does not come on. Am I way off base here? Thanks for any tips! P.s. I have the factory manual but it doesn't provide much insight at all into the charge light... and the black/white wiring diagram makes my head want to explode haha.

[datzenmike]

Charge, oil and brake lamps are all powered from the same fuse. When the ignition is turned to the ON position all three should light. As long as one lights then they are getting power.... do they?

 

I believe this is the 10 amp fuse on the left side closest to the bottom of the dash. If any doubt, replace it, and make sure the fuse is firmly in the fuse holder. If the fuse box is dirty or corroded, unplug, remove, take the fuses out and soak overnight in a solution of vinegar, lemon juice and salt. Rinse and dry in the morning and install new fuses.

 

 

Now if the brake and the oil pressure lights are working and the alternator isn't then check the plastic plug on the back of the alternator. It must be clean and snug and making good contact. The charge wire on it is the White/Red stripe.

[perattzi]

Thanks for the reply! Yes the brake and oil lights come on when you turn key switch, but not the charge light. So red/white stripe powers the charge light? As a side note, the truck is in really good shape overall, most wires/connectors look new inside cab. Under hood the usual wear and degradation, but barely any non-factory connections hacks etc. I will just redo/replace all the connections at the 6 pin connector and alternator and report back. Thanks!

[datzenmike]

One side of the lamp is powered from the ignition switch. The other end (the White/Red stripe) goes to the alternator and two things happen....

 

If charging normally there is 12 volts on both sides of the lamp. As the potential is the same on both sides no current flows and the light is out.

 

If the alternator fails to charge or is weakly charging the 12 volts from the ignition flows through the lamp into the alternator and through it to ground.... and it lights.

[banzai510(hainz)]

I dont think you need to replace all the connections at the 6 pin connector.

 

Try reseat all conncetion first and clean.AS MENTIONED!!!!!!!!!

Bet that fixes it

 

reseat the ignition switch also

when key is to ON there is 12volt going to the alt also

 

 

just saw a connector with a resunk pin on another thread recently

 

 

have alt cked again, I dont know if they ck the volt reg on those things as Napa. I assume so and load test it

 

 

these work good before you get stranded

http://mx.innova.com/en-US/Product/DetailProduct/4a35a1b5-c4f3-41fa-a632-f7ad1f12a77b

[perattzi]

Good tip on the battery monitor, that is pretty neat! 

So I replaced only the corroded/damaged connectors on the 6 pin, replaced the eyelet connectors (r/w, black) to the alternator and cleaned up the 'T' shaped connector (yellow, b/w) on the alt. I'm am using quality connectors with a ratcheting crimper...so no autozone bs. Still no charge light with key switch to on. Then I unplugged and replugged the ignition switch. Bam the charge light came on. So I started the truck and the light went out... then I put a voltmeter on the battery...12.5....revved the motor and no change whatsoever, so obviously not charging. But why did the light come on and go out?! I turned the truck off for a minute and then turned it back to 'on' and no charge light again. No amount of unplugging and reconnecting or tugging, wiggling of the wires to the switch ever made the charge light come back on, yet the brake and oil lights are on. Fuses are still good. Is it possible that when I started the truck something shorted? It must be something so silly. It seems like if the other warning lights are getting power from the switch then the charge light should be too correct? Thanks again for the help guys! Just hoping the rain stays at bay since I'm commuting on my motorcycle til I get the truck straightened out haha.

[datzenmike]

Yes the oil brake and charge are all powered from the same source. That's not the problem as the other two do light up. The other side of the lamp must go to ground in order to turn it on. Normally the non spinning, non charging alternator is the path to ground. This wire does not go through or have anything to do with the ignition switch... it goes directly to the bottom of the 'T' shaped connector on the alternator

 

Do you have air conditioner? If so this wire passes through that round engine harness connector. Non air con vehicles don't

[perattzi]

The truck came with A/C yes, but the hoses, blower, compressor etc. were removed and sitting in the bed of the truck when I bought it. However, I doubt the wiring was removed. So does the r/w wire just ground through the body of the alternator to the engine? Or is there a path through the alt? If I directly ground the r/w before the alt the light should come on no? And that would rule out the connector/shorts in wiring harness(though if it were a short to ground on the side going to alt shouldn't the light always be on?).

[datzenmike]

Power flows from battery to ground. In this case when the alternator is not charging  it grounds through one of the diodes

 

Like this...

battery.... ignition switch...fuse box.. charge lamp.... alternator.... ground

 

There are lots of connectors on the way but we know power does get to the dash lamps.

 

On air con trucks the White/Red stripe wire from the CHARGE lamp in the dash is one of the wires on that round harness plug behind the alternator. If that plug is corroded or loose and the wires not making contact then it can't go to ground through the alternator. Have someone wiggle it while the key is on and see if the lamp flickers.

[perattzi]

Ok so I just touched b/w wire (which is on the bottom of the 'T' connector) to ground and the charge light comes right on. Also, I hear a relay near the wiper fluid click. Sooo what is this telling me? haha. Sorry I sound so clueless. I usually don't have this much trouble with electrical items. 

[datzenmike]

Well if grounding that wire fixes the problem then the alternator isn't doing it's job. Is the alternator well grounded? There should be a Black wire to the alternator case.

 

 

The relay click is also normal. That's the choke heater relay. It needs power from a charging alternator to work properly

[perattzi]

I believe the alt must be grounded fine because I grounded the B/W wire to the large grounding strap bolt on the engine block and it illuminated the Charge light and powered the choke relay, then I touched it to the alternator case and it had the same effect. So it seems something in the path to ground through the alt has failed, a diode perhaps like you said. And I'm back where I started, thinking I need a new alternator ;) Was hoping it was just something cheap and silly, like the corroded connectors. Seeing as just how bad some of them were.  I wouldn't mind repairing the alt I have if practical, but probably a false economy. Thanks again.

[datzenmike]

It's a bit of work to narrow the problem down only to find out that it seems to be the alternator. All I can say is what if it wasn't. This would have turned up several easy fixes and prevented buying an unneeded part. Well now you know even more about your truck.

[perattzi]

Indeed! Thanks for the help, much appreciated! 

[banzai510(hainz)]

I unplugged and replugged the ignition switch.????????

 

WTF I said !!!!!!!!!!Gangsta!!!!!!!!

 

The Nips made battleships and BIG ONES Im sure they know how to wire a little car

[perattzi]

I unplugged and replugged the ignition switch.????????

 

WTF I said !!!!!!!!!!Gangsta!!!!!!!!

 

The Nips made battleships and BIG ONES Im sure they know how to wire a little car

Lol yes but it was NOT the fix. I cannot explain why it made any difference bc the other gauge lights were getting power. Explain why it then turned off but did not work again? 

'The Nips'? C'mon man...

[datzenmike]

Short for Nipponese. I hope the word short doesn't offend anyone...

[banzai510(hainz)]

it then turned off but did not work again?

 

 

My 521 did this and blow a fuse. reseat the instrument panel and worked fine after that.I think they call it a hight resisitance short

But I clean all connections.

 

521s have a hard time in engine bay with moisture

[perattzi]

Nips is definitely derogatory, though I don't think banzai meant it to be.

[datzenmike]

Naw he would have prefaced it with 'fucking' :lol:

[perattzi]

So, I finally got a replacement alternator. Apparently it was sitting on my neighbors porch for awhile thanks to UPS. It is a reman. Mitsubishi part, looks a little different than the original but it fits/aligns up just fine. My question is where do I hook up the ground wire? It is not marked like on the old part. There is the BAT + terminal and T block connector points that are marked but no 'E' marked for the ground. There are two threaded spots on the alternator body. One to the left of 'B' post at around 8 o'clock and one to the right around 4 o'clock. I'm thinking it doesn't matter which one and it just grounds through the alternator body but I'm not sure. Thoughts? Disappointingly it came with no instructions and the threads need cleaned up. Thanks for any help!

 

[datzenmike]

Either will work. The length of the 620's Black ground wire may decide for you.

[perattzi]

Thank you!  :thumbup:

[Eccles]

my truck has the same alt in it

[perattzi]

How has been working for you?

I am gearing up to really fix this truck and make it nice with a fresh coat of red paint but I need it to be at least running to get it to and fro. It has such good bones. Off topic-were these trucks undercoated from the factory? Mine has what appears to be an original or older thick black rubber like coating on most of the undercarriage. The front end area does not and shows a bit of surface rust here and there but the rest is coated and thus rust free. The only real rust damage on the entire truck is from leaky batteries up front on the wheel well. The floorboards are immaculate.