I have a voltage drop in the cab of my disco as measured at the OBD port, the voltage at the alternator and battery are good 14.2 to 13.8 but the cab drops to 13.1 to 13.5 after about 20 minutes. I need to figure out where the issue is,the fuse box voltage matches the OBD port.
Is there a primary positive lead into the cab?

The truck runs fine, this more proactive.

 

The Brown/Pink wire fed by the 100amp fuseable link 4 feeds the passenger compartment fuse block.

When you measure the low voltage, what are you measuring across? If you measure good at the batt/alt but bad directly at the passenger fuse block, your next step is to measure from that positive point on the pass fb to the batt neg post or the alt case. If that is OK, then reverse and measure from that pass compartment gnd point to the batt pos post.

My WAG would be a bad ground, or fuseable link 4 getting soft.

 

@H20nSnow I checked via the OBD port and a fuse in the fuse box, indicated voltage starts correctly perhaps 1. or .2 down from a direct measurement at the battery (Via basic voltage gauge sitting on my dash -temporary), and confirmed with a multimeter.
I will clamp to link 4 and the engine ground and see what I get, while driving.

Thanks very much for the help, now I know where to start.

 

Also what are you using as a ground when inside? If that ground has some resistance then that will affect your reading. Try to read inside with positive and the ground out on the battery ground and compare the readings. This will isolate which side has the issue.

 

I have not had chance to run further tests getting the house ready for sale, I will update this week. Thanks for all the suggestions, and I have lots of wire on the voltage meter so running to battery ground while testing the interior is easy.

 

Ok got some testing done

Fusible link 4 positive
Battery Neg 13.9 to 14.1
Negative at Alternator bracket 13.9 to 14.1
Negative to engine compartment fuse block 13.4-13.6 - any ideas on why that would be so low?

 

Could be a rusty or loose connection between battery positive and the fuse box.

On the other hand, it is normal for voltage to drop whenever current is flowing. For example, 0.5V drop is normal for flow through a diode. Voltage also drops a little due to resistance of copper wires whenever current flows. The voltage will change depending on the electrical load, so headlights ON vs OFF will give different readings. What are the voltages at the same points when the engine is off (no current flow)?

 

@JohnZo The voltage is the same with engine off. I kept the positive location the same through all the tests, it seems to be ground related. I expected some drop just not .7 to .9. Running all the lights hi beam, plus driving and rear fog lights drop the 1st test to 13.8 engine running

 

Richard, why is this voltage disparity important to understand? Isn't anything 13v+ all that's needed in the cab? Please help me understand. Thanks.

 

@mln01 I am OCD about these things, the cab voltage should be stable but it drops constantly after startup until it hits around 13.0. If it just stuck at 13.4 I would be happy but it keeps dipping down.