Low voltage on run / ignition key position

[Rubican]

Jeep TJ Rubicon, 2003, manual Trans.

When the key is in "Run" or "Start" position all of my ignition voltage is at 7.8V . Symptoms: Dash cluster instruments are 'dead', on signal LED are working, accessories like lockers, not working, ...
Battery is loaded with 12.8V, same voltage at power distribution fuse box.
Measuring all fuses that are working with IGN power have 7.8V on both sides of the pins, which is unusual as well. So something is really wrong there.
Didn't take the steering wheel column apart yet in order to check key switch.
So from your experience what is the most likely location of troubleshooting.
Bad fuse box, bad relay, or key switch??
Anybody has a wiring diagram what wires are connected from key switch in "Run" position.
Thanks.

 

[Jerry Bransford]

If the battery measures 12.7 when the ignition is off but 7.8 when the ignition is on or in the Start position, you either have a nearly dead battery or a bad/corroded/dirty battery connection.

Physically remove both battery connections then wire brush both battery posts and the inside of the battery connectors. A $4 battery brush from your local auto parts store makes that job easy as it includes both a male and female wire brush for the posts and connectors.

If that doesn't help take the battery to a competent battery shop and have it load tested. A load test is the only way to know the battery's true condition and state of health.

 

[Goatman]

Load test the battery. You can have excellent voltage showing, but have few amps backing up those volts. A load test can rule that out easily. Then go from there. A poor connection (+ or -) can allow voltage but restrict amperage.

 

[qslim]

Your factory service manual is in the "TJ Resources" section of this forum, it has all the wiring diagrams for your TJ.

 

[Rubican]

Thanks for the replies. I guess it needs a bit more of an explanation about my application.
I have currently finished a LS swap and got the LS engine running already.
There are 2 new batteries with new battery wiring/terminal clamps. The Jeep 'old' battery wiring with new terminal clamps connects to the Jeep under the hood fuse box . Two separate alternators supply each battery independently.
Batteries share only Ground. The Jeep alternator/battery is supplying all Jeep accessories, devices and is hooked up as from the factory. There are no changes to the 'old' Jeep harness, except it will miss the Jeep engine.
Because it was a manual I disabled the clutch security switch in the cabin/glove fuse box.

So something in the Jeep under the hood fuse box must cause the issue I think. All direct fuse battery connection like relay 30 pins have the same voltage as the battery. Only ignition based accessories are getting 7.8V only.
And pulling accessory fuse both sides of the plug-in connection have 7.8V, where only one plug-in connector should have about 12V. This is strange. Currently I have no idea.

There is only one other indication of a problem, the Jeep does not remember the keys. the Sentry Key Indicator Light comes up. But I don't think it causes such an issue as I have. ???

 

[Rubican]

It appears the SKIM module does have a major effect indeed. After reading tons of posts, I'm pretty sure now.
1. Gauges dead
2. Jeep alternator not charging
3. IGN starter cable no power when key on 'Start'
4. Low IGN voltage in under the hood fuse box

How is it possible the Skim module does not accept my two original factory Jeep keys? WTF
[The PCM was without power about 4 years ????]

 

[tgr22]

Did you solve this issue? I’m having the same issue with an LS swap but the low voltage only happens when the engine is hot. I’m only getting 7.8 volts off of the ignition and I get one click from the starter. I can jump the starter wires to get it started or wait for it to cool.