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Old 02-10-2020, 06:42 PM
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armyfsj armyfsj is offline
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Voltage drop

Ok so I have been gone for a while but I’m back with a 79 Cherokee Chief with an LS swapped in it. It still has the TH400 and QT transfer case. We had it running and driving fine until a few days ago when the voltage started dropping on the body harness. The LS harness will stay a steady 11.8 key on engine off but the body side will only sit at 9-10 and drop to 8-9 when the fuel pump cycles. We have cleaned/fixed all the grounds and connectors we could find. Any place in particular we should look on the vehicle side?
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Old 02-10-2020, 07:33 PM
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nograin nograin is offline
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Sounds like the alternator isn't working.

When you check voltage of the battery and everything is off, that's only a general indication the charge state.

When the electric fuel pumps or other devices start drawing power, the battery voltage will drop. How much it drops depends on the power draw. In other words the other part of power, current, is just as important as the voltage.

An analogy is water pressure in a house. Lets say say the pressure is 50 psi. If you're taking a shower maybe it drops to 48 psi, and if someone flushes the toilets and someone else starts running the washing machine - you know what happens.

Battery being a chemical cell is a little different but it too loses pressure (Voltage) as the outflow (amperes) increases. If a battery is in pretty good condition and reasonably well charged, it should go from roughly 12.8 to somethng above 9.5 Volts when turning a typical old school starter and starter solenoid. IIRC that's one of the tests in the MR 253.
Every time the fuel pump cycles, its drawing more current. Right? So that's one of the things you are seeing.
-----------------
Another thing you may be seeing is voltage drop in the circuit due to resistance.
The way to track this down is direct measurement of the voltage drop. Put one probe on the highest voltage point and the other test lead on the next connection downstream. Write down the voltage and move the probe to the next connection point. Keep doing this until you reach the device that power is flowing through. Then move the high voltage probe to the ground side of the device and use the second probe on the bettery negative terminal, and any connections in between.

When you're all done, put the battery on a slow charge. Actually depending on the charger, you could have it charging during the test.

The larger the current flowing in the circuit, the larger and more obvious the voltage drops will be.
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Last edited by nograin : 02-10-2020 at 07:44 PM.
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Old 02-10-2020, 08:07 PM
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Maybe these illustrations of voltage differences will help.
https://www.forabodiesonly.com/mopar...ost-1972559825
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carries wood inside
no "wood" outside
My other car is a fish
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Old 02-10-2020, 10:03 PM
Dave Jeeper Dave Jeeper is offline
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There are 2 sides to the circuit which will affect voltage, the plus 12v side and the ground side. To test the resistance of the ground side of the circuit: Disconnect the positive cable from the positive side of the battery. Leave the negative cable connected to the battery. Then take an extension cord, jumper cable or other large gauge wire and connect it to the ground side of the battery or the negative cable lug. Make sure the battery terminals have no corrosion as well as the inside of the cable lug. Clean the corrosion off the outside of the lug where you are attaching the new wire. Run the wire through an open door or window to the circuit where you are experiencing the voltage drop. Attach one lead of your portable voltmeter to the new wire and the other lead to the ground side of your circuit which has the voltage drop. Switch the meter to resistance and you should have a very low reading (maybe 5 ohms or less). You can now take the meter and new ground wire to another spot, perhaps the circuit under the hood and test the resistance of the ground side of the circuit there. If the resistance is more than an ohm less than from inside the car, then the ground path may be causing the voltage drop.


Now, to test the plus 12v side of the circuit. Remove the negative cable from the battery. Connect the positive cable after cleaning the battery terminal and the inside of the positive cable lug. Attach the new wire to a clean spot on the positive cable lug. Now take resistance measurements on the positive side of the circuits which you previously tested. If the resistance is much higher than the ground side, the positive circuit path is at fault for your voltage drop.


If the above test is too much trouble and you have already cleaned all of your grounds (battery terminals and cable lugs, cable lug to cable wire connection, body ground, frame ground, engine ground, firewall ground in engine compartment and dash circuit grounds to firewall or floor), then you could look at the fuses, relays and switches that carry the current for your circuit. Replacing all fuses or cleaning the fuse blades, replacing relays with new relays, replacing or cleaning the electrical terminals inside the ignition switches on the steering column and replacing the starter switch may decrease resistance on the positive side of your circuits and restore full voltage to the inside of the vehicle. Corroded connectors could also be reducing voltage.



It might be easier to run a new fused positive wire from the battery through a grommet in the firewall to the inside of the vehicle. Then use your circuit with low voltage to switch a relay that will deliver the full voltage from the fused line that you just ran to the circuit requiring a full 12v. Using a relay will make sure that the 12v from the battery is only applied when the wire with less than 12v is energized.



Howell recommends this relay circuit for their fuel injection control module inside the cab when voltage at the fuse box is one volt or more less than the voltage at the battery.


Even though we normally refer to battery voltage as 12v, a healthy electrical system should have a voltage at the battery between 12.5v and 13.7v (approximately) with the engine off.



I like to use dielectric grease or ordinary high temperature grease on all ground connections and the battery terminals to prevent corrosion.



I hope that this is helpful, that I have explained what I was trying to say clearly and that I didn't ramble on too much. I'm an engineer and have been working on auto electrical circuits for more than 40 years.
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Old 02-11-2020, 11:38 AM
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Most mullti-meters can't measure resistance accurately enough to identify poor connections in an automotive circuit.


10 amps through a 1/2 ohm worth of resistance will see a 5 Volt drop.
We'ld like to see a lot than that that in either of the main feeds.

V = IR
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carries wood inside
no "wood" outside
My other car is a fish
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Old 02-11-2020, 11:49 AM
Dave Jeeper Dave Jeeper is offline
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Most inexpensive voltmeters will show a couple of ohms with the leads shorted. It is good to measure this baseline and then subtract it from your circuit readings if you are trying to get an actual resistance reading. If you are comparing one circuit to another for relative resistance, it is of less concern and not necessary.
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Old 02-11-2020, 12:29 PM
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Your LS harness should be completely isolated/standalone from original wiring.
With it's own fuses/relays etc. if it isn't? that would be the first thing to address.
Having it using any of old/existing 79 wiring is asking for reliability issues.
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Old 02-11-2020, 12:38 PM
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armyfsj armyfsj is offline
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The LS is isolated, we have narrowed it down to the interior factory fuse panel only receiving 8 volts from its feed wire. The LS pcm and harness all have full voltage.
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Old 02-11-2020, 12:53 PM
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Probably not *it* but I had a 79 that both main feeds had been rubbing on cowl for decades clean through tape & insulation to bare copper.
Way up under the dash next to fuse block.

Either start @ battery and trace towards fuse panel, or from fuse panel towards battery. You will find it.

I've seen others with damaged fusible links, they weren't blown but HUGE voltage drop right after them.
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Old 02-11-2020, 12:55 PM
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I was thinking about just replacing that feed wire altogether, for the reason you just listed.
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