Voltage issues

[cabz]

I have been having an issue with my backup lights on my 82 j10.
They are super dim!.
I checked all the connections all the way to the transmission plug.
What I found is that circut had only 9.8 volts, so I dug some more and found that this circut also has my choke on it.i traced the wires all the way into the cab thru the bulkhead connector. Still only 9.8v.

So for some reason I had started the truck and walk!
14.00 volts at the rev lights and choke s circut.

How can I have 9.8 when the truck is off and 14 when running?
BTW the batt voltage was 12.60 and all the other things in the truck are fine

[nograin]

Automobiles have two power sources: Battery & Alternator.
Power flows from whichever one can supply at the higher voltage.
> Battery can supply power at approximately 12.5 Volts.
> Alternator can supply power at approximately 14 Volts when its running.

So when the engine is running, the entire system should be at the alternator's output voltage. The electrons will move through the wires between the item that is switched on, in your case the reverse lights, and the alternator. The amount of electrons moving at a given time is measured in amps. Kindof like we measure molecules of water moving in gallons per minute. Voltage is like pressure in the water line.

Lets take the analogy to the battery and the alternator.
There's 14+ volts pressure in the line with the engine running. That pressure is coming from the alternator. When the reverse light is turned on, the electricity will flow all the way through the wires, switch, lamps and then back to the alternator through the chassis.

As long as the demand for power does not exceeed the alternator's capacity, it will supply the power at the 14ish Volts.

The battery is different. Its more like a big storage tank. It has a lot of capacity. It can supply a couple of hundred amps - at least for a short time. But it will lose pressure (voltage) as it supplies power.
If the battery voltage drops below 9.5 Volts with just a few lights on, that's a weak battery. Put it on a slow charge (5 amps or better 2 amps).

A good battery should not drop below 9.5 Volts during starting.

On the pre-86 SJs, and many other older vehicles, the alternator does not directly feed the battery. Rather the battery and the alternator output wires are joined at a welded splice with the main electrical feeds.

The ammeter only shows battery charging or discharging. It does not show alternator output. So if you turn on the lights with the engine off, the meter will show discharging. After starting, the meter should show charging for a few minutes.

Tip. Recharge a low battery on a battery charger, not with the alternator. Extended periods of high current (30 amps plus) through the wires will damage connections, may burn out the alternator stator, and will cause the battery to get hot - which at best shortens its life.

[cabz]

Thanks for the reply.
I understand basicaly how that alt and battery work.
And I understand that the circuits will read battery voltage with the engine off and alt output voltage while running.
What I don't get is 9.8 volts on a circut when my battery has 12.5.

The battery is new and never drops to 9.8 it stays 12 or so while off.
Every other thing in the jeep works correctly and shows 12 v.
When I put my meter on the rev lights and start the jeep I get 14v alt voltage .

[nograin]

Ah. OK.


A couple possibilities:
From what you've posted you may already have covered the first one but I'll mention it anyway. Even though the battery is new, it could be low on charge. A fully charged new battery should be very close to 12.8 V with no load. A recently charged battery will have a surface charge that can be quickly disapated by flipping on a light for a 10 seconds.


Related to this is verify the alternator's output voltage at idle with an extra lamp on. Dome light or parking lights would do if you don't want to put it in reverse with your head under the hood. Measure at the alternator output stud. Also note if the battery is recharging (ammeter) as that adds to the load.



The second possibility is a poor connection somewhere in the circuit. Generally speaking there's some loss of voltage (resistance) anyway just due to the length of the reverse light circuits. A technique to narrow down locations of excessive resistance is to measure voltage drop along the path current is flowing. This provides a more sensitive means of identifying resistance and isolates the measurements in the positive wiring from the ground wiring.


Hypothetical example.
Chock the wheels/e-brake on, etc.
Voltage across the battery is 12.7
Key on:
Voltage across the battery is 12.6
Select reverse, voltage across battery is 12.6
Voltage to ground at reverse lamp is 9.6 Volts with one lamp on (probe in the other lamp socket).


It would seem that there is a loss of 3 Volt between the battery and the reverse lamp. But where?

Place the voltmeter's positive probe on the battery positive terminal and (with an wire extension) place the second probe on the reverse lamp terminal. Meter shows 2 Volts, not three.

Now we know that 1 volt is lost in the grounding, and 2 volts lost in the positive wiring.



Lets narrow the resistance in positive wiring.
With the key on, power is flowing to the ignition, the alternator field, as well as the backup lights. The ammeter shows around 5 amps discharge which is about what we would expect.

I don't know the details of how your '82 is wired. On an '84 and '85, the backup wiring and ignition is fed through the main splice and then the key switch.

Meter positive probe on the battery positive, place the second probe on the solenoid junction. 0.0 Volts. No measurable resistance there.
Then place the second probe into the backside of the bulkhead connector. 0.1 Volt. Small loss
Next, place the second probe on the alternator output terminal. 0.9 Volts
Whoa! That's a lot for only 5 amps.
We know there is no current flowing between the alternator output and the main splice. Therefore that wire is simply acting as a probe extension to the main splice. We now know that there is a significant resistance between the connecting terminal on the firewall and the main splice.


We also know that there still is another 1 Volt loss somewhere between the main splice and the reverse light.

[cabz]

Issue solved.

I had wired the choke into the original choke circuit,
What I didn't realize is that someone had fed the choke from another circuit so it blew the fuse for the backup light and was somehow back feeding.
I pulled the jumper for the choke and replaced the blown fuse. All is good now.

Thanks for the help.