How Much Voltage Drop Is Too Much?

[Kola16]

Howdy all! I am trying to determine whether my wiring harness and instrument cluster has too much resistance and draw and needs updating.

With my 3-wire ignition switch in the off position, the hot wire reads 12.45 volts, which is what the battery reads. With the switch in the run position, which sends power to all the gauges, I am only getting 11.30 volts on the hot wire. I am only getting 10 volts on the wiper wire with the wiper going. Is this normal? Should I be seeing this much voltage drop? This is without the engine running so no alternor charging. Thanks for the advice!

 

[tpenfield]

The voltages sound low. A battery, in good condition and 'at rest' should read about 12.6 volts. With the engine running (i.e. alternator charging) the voltages should be in the 13.5 - 14.5 range.

You may get some voltage loss at the helm, but 1+ volts seems excessive. It may be time to check/clean all the main connections, and see if you are getting voltage loss at the ignition switch or any of the circuit breakers or terminal strips.

Also, if some electronics have been added over the years, the main power wiring to the helm may not be adequate.

 

[Scott Danforth]

Howdy all! ....Sould I be seeing this much voltage drop?

No

Start cleaning connections

 

[Lou C]

I had that same problem with my ‘88 Four Winns. Cleaned all the connections etc. Thought it might be inadequate grounding & tried running an extra ground from the fuse box back to the engine ground but that didn’t affect it. What did was adding a supplemental power lead from the battery switch common post (fused) with 8 gauge wire. Had that problem for years, thought it was a flaky volt meter till I started checking things….

 

[Silvertip]

Measuring voltage of a battery with no load is a rather useless test. Measure it with stuff turned on. Now if you show 12 volts or more you proved you have a reasonably good battery. If voltage drops below 12 volts you have a sick or discharged battery. But by all means clean and tighten all connections at the battery and helm.

 

[sam am I]

If V(2,4) = 11.30V and if *V(1,3) = 12.45V with S1 on and a normal load R3.

V(1,3) = V(1,2)+V(2,4)+V(3,4) eq. 1 (summing voltage in a closed loop)

Then from eq 1 V(1,3) - V(2,4) = V(1,2)+V(3,4) = 12.45 - 11.30 = 1.15V

V(1,2)+V(3,4) = 1.15V

The problem is voltage drops caused by higher then normal resistance R1 and/or R2 (usually cause by bad connections due to corrosion in battery posts clamps, wire crimps, spade lugs, switch contacts, fuse clips, etc, etc)

Use your meter to measure V(1,2) and V(3,4) while S1 in on, you might need 10/20 feet of spare test wire, any size will do, could even use one leg of an extension cord.

e.g., if V(1,2) = 0.1V, then V(3,4) = 1.05V (should be able to verify this w a reading), you then have a ground issue R2.......lol.

* if V(1,3) = 11.30V when V(2,4) = 11.30V, your battery is obviously weak, you really don't need to go much further trouble shooting until you get a descent/charged battery.

 

[Kola16]

Thanks for the replies y'all. Almost all my wiring and gauges are the original from 1979 so you confirmed my suspicions. I also left the key on and drained my cranking battery twice. I did not realize it was weak since it charged right back up and kept 12.45 volts.

Rewiring my boat it is, along with a new battery.

 

[Scott Danforth]

Should just need to clean connections vs rewire