Help please-Inst gauge needles jumping up, then down

[Coronet-24]

I just bought a 1971-era powerboat with twin Volvo 305s and one of the first things I did was to replace some corroded and single-strand wiring. Before this, all the instruments worked correctly. Now, when the engines are running, all the needles on all the instruments jump up high for several seconds and then jump low for several seconds, and this cycle continues for as long as they're running. I haven't yet checked the actual battery voltage with a DMM, but will be doing that this evening. Could this be wrong alternator wiring? Any other ideas?
Thanks for any help.

 

[Bondo]

Re: Help please-Inst gauge needles jumping up, then down

I just bought a 1971-era powerboat with twin Volvo 305s and one of the first things I did was to replace some corroded and single-strand wiring. Before this, all the instruments worked correctly. Now, when the engines are running, all the needles on all the instruments jump up high for several seconds and then jump low for several seconds, and this cycle continues for as long as they're running. I haven't yet checked the actual battery voltage with a DMM, but will be doing that this evening. Could this be wrong alternator wiring? Any other ideas?
Thanks for any help.

Ayuh,.... Welcome Aboard,... I gotta believe ya screwed something up, if they all worked, Before ya fixed 'em,...

Retrace yer steps, maybe you'll find the problem,...

 

[NYBo]

Re: Help please-Inst gauge needles jumping up, then down

Welcome to iboats!:welcome:

In case you need a wiring diagram for the instruments, courtesy of Silvertip:

 

[glenn property of pam]

Re: Help please-Inst gauge needles jumping up, then down

where did you replace the wiring, at the dash or at the engines? which gauges are going up and down and on which engine (sorry I know its a pain but gauges like the volt meters or fuel give a idea of whats going on)

 

[Coronet-24]

Re: Help please-Inst gauge needles jumping up, then down

where did you replace the wiring, at the dash or at the engines? which gauges are going up and down and on which engine (sorry I know its a pain but gauges like the volt meters or fuel give a idea of whats going on)

Thanks for all responses. To answer glenn property of pam, I changed nothing in the instrument panels themselves; all changes were at the engines:
I replaced all wirenuts with marine butt connectors, or the entire length of the wire.
I replaced the old battery cutout switches with a single ignition-protected OFF-1-2-BOTH switch.
I removed dangling wires.
I added an AC marine type dual battery charger and wired it exactly according to the instructions.
There are 2 engines, and dual stations, so there are 2 port engine temp gauges, 2 starbd engine temp gauges, etc.. All the meters/gauges including OP, temp, are jumping up and down in unison. The tachs seem normal.

The boat was transported 600 miles by tractor-trailer from Ontario to Long Island, so something could have been jarred on the trip, or, yes indeed, I could have screwed something up.

 

[Coronet-24]

Re: Help please-Inst gauge needles jumping up, then down

PS: Fuel gauges work normally.

 

[Bondo]

Re: Help please-Inst gauge needles jumping up, then down

Thanks for all responses. To answer glenn property of pam, I changed nothing in the instrument panels themselves; all changes were at the engines:
I replaced all wirenuts with marine butt connectors, or the entire length of the wire.
I replaced the old battery cutout switches with a single ignition-protected OFF-1-2-BOTH switch.
I removed dangling wires.
I added an AC marine type dual battery charger and wired it exactly according to the instructions.
There are 2 engines, and dual stations, so there are 2 port engine temp gauges, 2 starbd engine temp gauges, etc.. All the meters/gauges including OP, temp, are jumping up and down in unison. The tachs seem normal.

The boat was transported 600 miles by tractor-trailer from Ontario to Long Island, so something could have been jarred on the trip, or, yes indeed, I could have screwed something up.

Ayuh,... Sounds like the Ignition circuit is what's makin', 'n breakin',...

As All the gauges are powered by the Ignition circuit,...

 

[glenn property of pam]

Re: Help please-Inst gauge needles jumping up, then down

sometimes the wire is pushed to far into a crimp connection so the insulation gets crimped instead of the wire so a wiggle test with someone watching the gauges might work.