Iggy

We decided to take the boat out to see the fireworks on the fourth. This was only the second time we had her out at night and I decided to check the running lights to be sure they all worked before we left the house. They did.
Well, when I turned on the instrument lights, without having the engine or ignition on, the fuel gauge jumped from empty to half. I switched them off and the gauge went back to empty.
That night while running I turned them on to see what would happen. The gauge maintained a proper reading but the voltmeter dropped from 13V to 10V.
Switched them off and everything went back to normal.

Any Ideas??????
I'm thinking I've got a bad ground someplace.

Bulldog

Iggy- it does sound like there is a common ground connection between the lights and instrument cluster.

Bad grounds act funny because they cause voltage which changes on the ground side of gauges, etc. which are often wired to a common point. The voltage changes depending on the current- more current, more voltage change.

1- Quick check-
Run a test jumper wire from the battery - terminal to the ground terminal of a gauge, and try switching the light. If the gauges are now steady, you have a bad ground connection.

2- Isolate-
You can isolate the bad ground using the same test jumper from step 1 and a voltmeter. Remove the jumper from the instrument ground. Connect the VM - lead to the jumper (still connected to the negative battery post) and turn on the lights, etc. Now measure for voltage at the grounding leads to the instruments. Ideally you should read 0 volts, but if there is a bad ground you may read several volts.

When this has happened, it is a good idea to do maintenance on your connections at the battery, starter, and helm. All of them. Disconnect the battery first, and do one connection at a time. Clean them and put a light coat of conductive grease on afterwards (white lubriplate works good).

let us know what you find
and good luck
Bulldog aka Ronnie

Iggy

Bulldog, thanks for the procedure.
I had cleaned the battery teminals prior to this discovery so they won't have to be done. I'll have to check for the stray voltage and clean everything up.
Besides coating all the connections with a grease, is there a spray coating that could be applied after the connections have been cleaned? Maybe a Mil Spec product?
I know grease works, used it for years on auto battery terminals, but it tends to make a mess of things and is a dirt collector.
I'm a believer in "do it right-do it once". Then all it needs after that is periodic inspection.

I'll let you all know what I find.
Thanks again.

Bulldog

Iggy-
You can wipe the excess grease off after the connection is done- it is primarily needed between the connection surfaces. I never tried it but Mercruiser makes a spray can 'Corrosion Guard" that may be for this purpose. I need to find out and will let you know.

Audiofn

Corrosion guard will work for this purpose. However be aware that it is sticky. We used it on the outboards all the time to keep the electronics dry.

Jon