72 C20 - Wiring through firewall question

[jvictorv33]

Hey All -

Putting in new stereo/amp for my son... drilled through firewall for amp power cable (crutchfield kit). Snaked 8 gauge wire through supplied grommet and inserted. Current test shows everything is fine from battery to and through in-line fuse box and in the cable immediately pre-grommet pass through. Current test immediately past-grommet inside the cab shows no power.

Fire wall current force field? What am I doing wrong?

Thank you for the help.

[Myoldtruck]

Your using a digital volt ohm meter? at battery posts you have 13.4 volts about. from neg post of battery to outlet side of the fuse do you have 13.4 volts about. If you do then the fuse is good. You can also check continuity thru the fuse with the meter showing fuse is good. The wire in the truck, that your not reading the 12volts at, where do you have your meter grounded to while checking inside the truck. You may have a bad cab ground.

I grounded battery to the engine block, block to the frame. Cab to the frame and bed to the frame using #6 stranded wire. you also have the factory ground straps from firewall to the valve covers.

[dfwright]

You would probably be better off measuring voltage to ground at the ends of the cable, rather than current. I assume you are using an ammeter with an inductive pickup loop to measure current? It's possible you are picking up stray inductive readings from nearby cables outside the firewall -- the firewall itself would tend to block the stray inductance.

[jvictorv33]

Thank you... no, not that sophisticated yet. Just using the "bulb in the handle probe." Will be picking up a meter tomorrow - see where that leads.

[Myoldtruck]

Harbor freight for the volt meter, small red one is free with a coupon or like 8.00. I keep one in all my old cars and bikes.

[RustyPile]

Originally Posted by jvictorv33

Thank you... no, not that sophisticated yet. Just using the "bulb in the handle probe." Will be picking up a meter tomorrow - see where that leads.

My guess is you probably don't have your test light attached to a proper ground.. A little lesson to help you with proper nomenclature.. In your case, a test light isn't used to test for current flow, only the presence of voltage.. If you need to know how much actual voltage you have, a multimeter set to read voltage must be used.. In order to read current flow (amperage), a multimeter set to the amperage setting is used...

[rpmerf]

Originally Posted by RustyPile

My guess is you probably don't have your test light attached to a proper ground..

I always test my ground connection test light by touching the test light to something I know is hot - battery or something in the fuse block to verify I have a good ground before testing. Got to be sure your equipment is reading accurately before using it to test.

[jvictorv33]

Thanks guys - very helpful. Meter secured, so will be testing per your advise later today. One other bit of info, just for fun:

With my continuity tester clipped to the ground I secured for the amp inside the cab, I got the bulb to light up by hitting the constant power wire from the fuse block (attached to the stereo head). Assume this at least somewhat validates my ground.

From there, I shifted to the end of the remote wire (at the amp) that runs from the connection at the stereo. Bulb lights up there too. Shift to testing the power cable running from the battery (at the end where it connects to the amp) (using same grounding point as the other positive tests) and no reaction from the tester. That's when I jumped to inside the engine compartment and started testing there.

With alligator clip on the negative terminal of the battery, bulb lights up on the post, on the ring connector of the power cable, the "in" side of the fuse connector, the "out" side of the fuse connector and when poked through the sheath of the wire just before it passing through the grommet in firewall.

It's when I get inside past the firewall - I get nothing. Wire on tester doesn't reach all the way from the battery, so it's not a perfect test given I had to switch grounding points, but literally nothing from the power cable past the firewall... It goes from lighting my tester to nothing in the span of 1" in the cable (pre fire wall and post fire wall).