Trouble with voltage drop

[leftybass209]

Having a voltage drop problem with my 72 c10 350.

Engine wiring harness is an m/h harness for HEI. the rest is stock. The wiring for the 100 amp si style alternator has been upgraded to 10 ga. The alternator wiring is 8 ga to a bus bar. Sensing wire to the bus bar. Other wire ties in with the 10ga power wire before it hits the bulkhead. external regulator, and amp gauge wiring has all been unpinned from the harness completely. There is no resistance wire to the distributor, it's a normal stranded copper wire.

I have 14.6 volts running on the engine side, battery. Every check, at every point, is alternator output voltage with no drop. Same voltage running into the ignition, with around a .2-.3 drop running out of the brown 12ga wire to the accessories.

Any accessory i use creates a voltage drop, and that voltage drop includes on the HEI power wire.

My voltage gauge (conversion pulling power from cluster) ticks widely with the signals, and drops to around 12.2 if i have headlights, wipers, blower motor going.

Things that ive addressed
New grounds for core support, battery cable to block, cable from block to frame.
Two grounds off each head to the cab.
two grounds off the frame to the underside of cab.
Two grounds off the frame to the bed.
Each taillight, reverse light, and signal light has a separate ground to fame.
New cluster circuit board
used two different alternators 63a and 100a
used two different ignition switches, 1 original 1 new

I'm running out of things to check without tearing apart the fuse block and checking for corrosion on the connections behind the glass fuses. Could the brown/white wire from the ignition to the fuse block, which ultimately ties to the alternator wiring cause the issue? Alternator charges fine, has a diode in the wiring to prevent backfeed/run-on.

Any help would be appreciated. Sorry for the long-winded post.

[dmjlambert]

Could you explain this more? "My voltage gauge (conversion pulling power from cluster)"

I don't follow what you're saying. From what wire or point is voltage gauge measuring from?

When you're getting "around 12.2 if I have headlights, wipers, blower motor going" do you continue to have about 14.6 at your bus bar and at battery?

[leftybass209]

Originally Posted by dmjlambert

Could you explain this more? "My voltage gauge (conversion pulling power from cluster)"

I don't follow what you're saying. From what wire or point is voltage gauge measuring from?

When you're getting "around 12.2 if I have headlights, wipers, blower motor going" do you continue to have about 14.6 at your bus bar and at battery?

Sorry, what I mean to say is I've done a volt gauge conversion using an 80's c10 gauge, positive connected to the temp gauge positive. Using multimeter hooked directly to the battery, there is no significant voltage drop when any accessories are used. Using a multimeter on the fuse block, and acc ignition wire, it is verifying the voltage gauge needle movements; as more accessories are turned on, the voltage drop gets worse.

I ruled out the gauge itself by connecting directly to the battery, and it reads correctly.

[RustyPile]

I'm assuming you have the buss bar installed near the battery?? If so, you have the voltage sense wire terminated in the wrong place.. It should terminate farther "up" the main power distribution wire.. Somewhere near the firewall bulkhead connector.

[dmjlambert]

Check how much voltage drop you get with accessories running at both sides of the firewall bulkhead connector. You probably have a 10 or 12 gauge red wire going into a position on the bulkhead connector. Measure it there by inserting your test meter probe into the back of the connector to make contact with the terminal. This is done while the bulkhead connector is connected to the firewall. Inside the cab repeat measuring at the connector. There's not anything to tearing apart the fuse panel, no biggie. It just fastens to the firewall with 2 screws and you move it out of the way to gain access to the firewall connector. While you have it unfastened, you can make measurements of voltage at the terminal connections on the back side of the fuse panel. I think you'll be able to figure out the main cause of the voltage drop. It's probably one of the connectors if I had to guess. It might need a cleanup of the connector.

I've been rolling it around in my head about how best to get the main power wire voltage into the cab without relying on just one position of the bulkhead connector, to improve the wiring in my truck. I haven't settled on any plan yet.

With all that said, the voltage drop is probably not a serious concern unless it's actually causing you problems. If it's just causing you worry, that may not be enough. These trucks were not engineered to have stellar electrical systems. So, you could power the gauge from the original black/white wire previously going from the alternator to the ammeter you replaced, or power the gauge from the original black/brown wire previously going from the battery to the ammeter you replaced. One of those wires will probably give you a better and more steady gauge reading.

[dmjlambert]

And what RustyPile says about placement of the sense wire is solid info. If you place the sense wire at a point in your harness where you want the voltage to be steadiest, the alternator will produce output that results in 14.6 volts at that point. If you power your volt gauge from that point also, you would be measuring the ability of your alternator to adapt to different current demands. There's always voltage drop along wiring, especially wiring with a lot of current flowing in it and with resistance of connectors along the wire.

[bhap]

I'd suggest upgrading the wire size from your buss bar to the fuse box and maybe adding relays for your headlights. The headlights are easy to wire from your buss bar to a relay using the original low and high beam power wires to trigger the relays. The other point mentioned above is the bulkhead connectors. Resistance causes voltage drops and heat. I've seen connector plugs that have melted from poor connections. Any other accessory you can power from a relay will take stress off the rest of the harness. I'm a big fan of madelectrical.com - lots of good information.

[leftybass209]

Originally Posted by RustyPile

I'm assuming you have the buss bar installed near the battery?? If so, you have the voltage sense wire terminated in the wrong place.. It should terminate farther "up" the main power distribution wire.. Somewhere near the firewall bulkhead connector.

Good call. I do believe it's ran all the way over to the busbar, it's just been a while since I wired it up. I'll see about terminating it closer to the bulkhead.

Originally Posted by dmjlambert

Check how much voltage drop you get with accessories running at both sides of the firewall bulkhead connector. You probably have a 10 or 12 gauge red wire going into a position on the bulkhead connector. Measure it there by inserting your test meter probe into the back of the connector to make contact with the terminal. This is done while the bulkhead connector is connected to the firewall. Inside the cab repeat measuring at the connector. There's not anything to tearing apart the fuse panel, no biggie. It just fastens to the firewall with 2 screws and you move it out of the way to gain access to the firewall connector. While you have it unfastened, you can make measurements of voltage at the terminal connections on the back side of the fuse panel. I think you'll be able to figure out the main cause of the voltage drop. It's probably one of the connectors if I had to guess. It might need a cleanup of the connector.

I've been rolling it around in my head about how best to get the main power wire voltage into the cab without relying on just one position of the bulkhead connector, to improve the wiring in my truck. I haven't settled on any plan yet.

With all that said, the voltage drop is probably not a serious concern unless it's actually causing you problems. If it's just causing you worry, that may not be enough. These trucks were not engineered to have stellar electrical systems. So, you could power the gauge from the original black/white wire previously going from the alternator to the ammeter you replaced, or power the gauge from the original black/brown wire previously going from the battery to the ammeter you replaced. One of those wires will probably give you a better and more steady gauge reading.

I'll have to do some more probing this weekend, we got hit with rain for the last two days. I agree it's not a serious concern, except that when voltage drops on the distributor i assume it's affecting spark voltage and possibly causing the engine to load up more at idle than it usually would otherwise, when accessories are running. It's not a problem i'm chasing daily, but I always default to making sure current wiring is tip-top before adding stereo or additional accessories.

Originally Posted by bhap

I'd suggest upgrading the wire size from your buss bar to the fuse box and maybe adding relays for your headlights. The headlights are easy to wire from your buss bar to a relay using the original low and high beam power wires to trigger the relays. The other point mentioned above is the bulkhead connectors. Resistance causes voltage drops and heat. I've seen connector plugs that have melted from poor connections. Any other accessory you can power from a relay will take stress off the rest of the harness. I'm a big fan of madelectrical.com - lots of good information.

I do have headlight relays, and the main harness wire from busbar to bulkhead has been upgraded to a 10ga. It's impossible to get the physical connection into the bulkhead with 8ga or larger with the original style packard terminals. I don't plan on running ANYTHING additional not equipped in 72 through the original harness, with the exception on the Alt and HEI upgrades. I'll run anything additional from the battery to a new fuse panel and or relays.

a little more info- Any time i disconnect a harness or connector, I unpin each connection and clean it with a wire brush before reconnecting with a touch of dielectric grease.

Thanks for all the help so far. Fantastic community as always

[leftybass209]

Update: tore apart the fuse panel, pulled and cleaned all the connections, and reinstalled. Moved the sensing wire closer to the main harness power feed.

None of this has helped. Even though the headlights are on relays, even pulling the headlight switch out has roughly a 1.5 volt drop from 14.6 to 13.08, which doesn't make much sense considering the relays.

I do have a diode in-line from the alt. To the brown/white stripe wire at the ignition switch. I do not have a resistor and I've read that I may need a 47 ohm resistor in line to protect the diodes in the alternator, depending on if the wire itself is a resistance wire. I couldn't get a reading on a wire checking for ohms.

If the diodes were damaged though, wouldn't that cause a low/no charge symptom?

[Dead Parrot]

Check the ground/negative battery cable and the engine to cab ground cable.

[leftybass209]

Update: the issue is fixed.

It wound up being the fuse panel power wire being too long. After taking the suggestion above of moving the sensing wire, i still didn't move the harness feed. The excessive length was enough to cause a significant voltage drop on all accessories that got exponentially worse as accessories were added. I bought larger packard terminals to run a larger gauge wire into the factory harness, but it physically didnt fit between the plastic and lock in, so my wire was limited in gauge.

I still have some small stretches of cca wire needing to be replaced bit overall this has made an amazing difference on voltages seen at the accessories.

I discovered this after reading the M.A.D electrical page regarding chevy wiring harnesses.

Thanks for the suggestions

[RustyPile]

Originally Posted by leftybass209

Update: the issue is fixed.

It wound up being the fuse panel power wire being too long. After taking the suggestion above of moving the sensing wire, i still didn't move the harness feed. The excessive length was enough to cause a significant voltage drop on all accessories that got exponentially worse as accessories were added. I bought larger packard terminals to run a larger gauge wire into the factory harness, but it physically didnt fit between the plastic and lock in, so my wire was limited in gauge.

I still have some small stretches of cca wire needing to be replaced bit overall this has made an amazing difference on voltages seen at the accessories.

I discovered this after reading the M.A.D electrical page regarding chevy wiring harnesses.

Thanks for the suggestions

As for the issue of getting a large gauge wire to attach to the terminals at the firewall connectors.. Here's what I did.. I upgraded the main power distribution wire from the buss bar near the battery to the firewall connector.. I ran a 10 gauge wire to a point about a foot just shy of the firewall. At that point, I spliced in two 12 gauge wires long enough to reach the firewall connector along with the alternator sense wire. I connected one of these wires to the "socket" where the 12 gauge wire originally connected.. I connected the other wire to a blank, unused "socket"..

Inside the cab, I separated the main power feed at the junction where it splits and feeds the headlight switch, fuse box, ignition switch and horn relay. I connected the horn relay feed and headlight feed together and put a terminal on it and plugged it into (doesn't matter which one) one of those two main power "sockets".. I connected the feed for the fuse box and the feed to the ignition switch together and put a terminal on it and plugged the wire into the other main power "socket".. This modification divides the amperage flow into two separate terminals at the firewall connector.. If nothing else, this eliminates the need for headlight relays.