Help with electrical issue in my '73

[mrk5]

I drove the 73 at night for the first time and I noticed the lights are cycling in brightness. It's the headlights and dash lights. The previous owner wired the light for the aftermarket tach straight to power and it cycles also so I suspect the entire electrical system is doing it.

I also noticed the dome light cycling during the day with the headlights off. The needle of the tach seems to move with the cycling lights and I think I can hear it in the idle a little to.

I had noticed the ground strap between firewall and engine was disconnected. So today I installed a new one I had on-hand. I also cleaned up all the grounds I could see in the engine bay; battery to alternator bracket, battery to frame, firewall to block, battery to core support, and both headlight grounds. In addition to that I also installed relays in the headlight wiring.

I also tightened the alternator belt. I even put some belt dressing on the belts.

After all that, the cycling is still occurring. If anything it seems like it might be cycling faster. The lights don't go out, they just cycle in brightness.

Before I started the work above, I checked the battery voltage with the truck idling. It was cycling between 14+ to 15+ volts (the + means it was some decimal value). I didn't really think to watch the voltage long enough to see if it was cycling the same as I noticed the lights were. My focus was making sure the battery was charging, and I really thought re-installing the firewall to engine ground strap was going to fix it. This truck has an amp meter, and it doesn't move.

It doesn't seem to cycle when stopped in gear idling about 600RPM. I start to notice it when idling in Park, which is about 750RPM. It's gets more pronounced as the RPM increases.

I'm suspecting there is something wrong with the alternator. I can't figure out if this truck has an external regulator. It's definitely not where the regulator is on the 66, and I can't find one anywhere else in the engine bay. If it's there, it must be tucked in under or behind something.

My guess would be it has an internally regulated alternator. Based on the little searching I did, internal regulating started about 1973. I"m thinking if it could have been either way, this truck probably is internal because it has AC. On the other side of the argument, the truck has the amp meter not a voltmeter. I always thought that change came with the use of internal regulators.

I don't want to just replace the alternator because if it is externally regulated, that might be what's failing. Not to mention the whole problem of putting the wrong alternator in.

Any thoughts would be appreciated.

 

[Capt Ron]

If the small 2 wire plug goes into the back there is an external regulator. if it goes in the side it has an internal regulator.

 

[diesel4me]

Sometimes the diode trio in the alternator loses one or two of the diodes and it will still charge "some" but not to full capacity and it'll make the lights flicker or pulse--if your truck had an idiot light instead of an amp meter you'd see it glow dimly at night when the diode trio is failing...usually a new voltage regulator comes with a diode trio,it's not a very difficult job to put a new regulator and diode trip in an alternator but if your not up to the task its just as easy to install a used or parts store one..

But first I'd look for any bad wires or corroded connections,and the fusible links to make sure they aren't half cooked--to prevent replacing parts for nothing..

 

[6872xtc]

I am betting that you have a bad regulator in the alternator. I haven't ever seen one of the 10SI units put out anything close to 15 volts with a good regulator. And the pulsing is not right either.
I believe that you have a 10SI and using the identification @Capt Ron posted will tell you internal vs external regulator.

While you are checking power connections, check to see if the power is close to the same voltage at the alternator, then battery and fuse panel. It will tell you if more cleaning is needed.

Post up a pic of the alternator and we can help identify it too.

 

[bp71k5]

I’ve also heard those “1-wire” alternators can act weird like that. Might be something to check.

 

[mrk5]

Unfortunately that's probably the best picture I have now.

I feel like it's the SI type and it has the plug on the side going by memory. It's definitely not 1 wire.

I left the truck at the shop so I probably won't be able to do more investigation until Monday. I did order one of these to act as an easy voltmeter in addition to being a phone charger.

 

[mrk5]

Here we go, a picture before the carb swap. It shows the plug which I think confirms 10SI.

 

[diesel4me]

1972 was the last year for the external voltage regulators on all GM's..

 

[6872xtc]

Yes, 10SI.
@500$k5 said that you may have a problem with the "sense" wire. One more thing to check. It's possible to permanently jump it to the charge stud, but I don't know how that affects the ammeter.

 

[500$k5]

If you have a bad connection in the sense wire, which is the red wire in the plug-in connector, it can cause this problem, or it could just be a bad regulator. The sense wire goes down to the batt stud on the starter. It has a fuse link, at the end. If you jumper this wire to the charge stud, on the back of the alt, it will sense the voltage there, and possibly fix your problem.

 

[mrk5]

Cool, I will try jumping the sense wire and see what happens.

Yes, 10SI.
@500$k5 said that you may have a problem with the "sense" wire. One more thing to check. It's possible to permanently jump it to the charge stud, but I don't know how that affects the ammeter.

I'm questioning if the gauge even works, to be honest. It never moves but the one in the 66 does. Maybe it just means my battery is fully charged.

 

[6872xtc]

Cool, I will try jumping the sense wire and see what happens.

I'm questioning if the gauge even works, to be honest. It never moves but the one in the 66 does. Maybe it just means my battery is fully charged.

The ammeter is using a shunt (?) so that it dosen't actually have the charge circuit in the cab. My '72 has little fuses in the corners of the radiator support which protect the circuit. One blows, and the gauge quits. Not sure what the '73 has.

 

[500$k5]

Sense wire shouldn't affect the amp gauge.

 

[mrk5]

So I'm thinking it's the internal regulator. At around 750RPM or less, it holds at about 14.8V. If I run the RPMs up to 1200-1500, the voltage will vary between the 14.8 all the way up to a little over 16V and it's constantly moving around. At that lower idle, it was holding the 14.8V steady. And that number is consistent between the multimeter and the little volt gauge on that car charger I linked above.

I've noticed if the truck sits for several days, the battery doesn't seem to crank very strong. I'm wondering if it has been a little messed up from being over charged. It read 12.3V on the battery this morning before I started messing with it.

 

[6872xtc]

So can you get the battery load tested when you go get an alternator?

 

[mrk5]

I'm a little on the fence about the battery. I REALLY prefer AGM batteries and the top-post, side-terminal combo, so I'd kinda just like to get a battery regardless. But since I'm going to be spending money on an alternator, I may have this battery tested at the place I'd buy the new battery.

There was a ton of corrosion on the positive cable when I cleaned up the connections.

Side note, I just realized I don't have any kind of battery retention in this truck. It also looks like someone fabricated a new battery tray at some point but didn't add a hold-down clamp.

 

[6872xtc]

Sounds like a solid plan, but you think that I may be TOO old for much...

 

[mrk5]

.....I may be TOO old for much...

That's why I was counting on your help with a truck older than me......

 

[diesel4me]

It's not good to let a battery bounce around,it can shake the material off the plates..it might be it has a loose internal connection between the cells ,or one cell isn't good,and that could make the voltage vary and account for the flickering..(though it's probably the alternator's regulator or diode trio)..or both are failing..

 

[mrk5]

It's not good to let a battery bounce around,it can shake the material off the plates..it might be it has a loose internal connection between the cells ,or one cell isn't good,and that could make the voltage vary and account for the flickering..(though it's probably the alternator's regulator or diode trio)..or both are failing..

That's part of the reason I like AGM batteries too. I'm keeping your post in my pocket so I can spit it back out when I give the wife a receipt for a new battery.... Except I'm taking out words like "might".