Voltage to helm gauges, what should it be?

no they should essentially be the same at the meter and the battery.
Most overlooked is ground wires.
They need to be just as clean and tight as any positives it is a full circle in any electrical circuit.

 

There can be a difference from what you read at the batteries with a Volt meter and what you see at the helm. Could be accuracy of the meter, another factor is voltage drop. What size wire that is ran up to the helm and how far that distance is could affect it….

 

There will be a slight drop ~.2-.6v or so depending on how long the wire run is and the gauge wire used. But there should not be more then a volt at all The older boats usually have heavier wiring and are pretty good on the 12v side. As mentioned it sounds like a possible dirty ground some where, those are the toughest to find.

As long as your testing with the Fluke meter and not relying on the needle gauges you should be fine to t-shoot. Try measuring the voltage along the path of the wiring at every junction point including the 12v panel. Also keep in mind hat sometimes the connection will look fine but corrosion will be between the connection itself.

 

You may see a VERY slight drop in voltage between the two places - but as noted above, very small and relatively insignificant. The amount you're seeing is certainly not insignificant... also not surprising given that the electrical system is about 35 years old. And, of course, the biggest factor here is that it's an electrical system on a boat and it's subjected to harsh conditions.

Is everything else behind the dash seeing the same low voltage? Or is just the volt meters?

Run a temporary ground from one of the meters to a known good ground... or even a battery. See if that changes things. If it does - concentrate on the ground system. If it doesn't change things, look at the positive side.

In the end, you may very well have corrosion/broken wires (sometimes some of the wires break, but not all) on both sides.

Start with things in the bilge and behind the dash. Those areas are more likely to have the issue than somewhere in between.

 

Check for corrosion at the cannon plug

 

Good find. Start there - especially if your switch feeds the gauges you tested (likely). It could be the wires/connections on the switch... or could be the switch itself. Check the supply wire to the switch vs feed wire to accessories.

Edit: This was already mentioned, but since it's both switches, a common ground is still high up on the list of culprits.

 

Your boat doesn't have a Mercruiser cannon plug. But you should have a terminal block either by the helm or in the engine room with breaker(s) on it.

Your gauges are fed directly from the engine wiring harness and I am not sure if there is a plug for that, but I don't think so, not in 89'.

 

Analog gauges and mechanical engines. For the Temp and pressure gauges the ground is the bias from the senders on the engines. Grounds to the gauges are required for lighting (obviously) and to operate the voltage, tachometers / hour meters. The gauge power comes from the key switches and engine ignition/start switches. Port powers port and starboard powers starboard. The question is are you getting the same voltage drop on both sides? And, where are you picking up the ground to measure the voltage?
It makes me wonder what has been added to the ignition circuit wiring over the years - quite possibly a voltage drop due to loads on that long 18 gauge wiring.....

 

Do you have a lower helm also?

 

In a perfect world, voltage will be the same as it is at the battery. However, we know there will be a little voltage drop by time its reaches the helm. Any more then .5V would be a concern for me.

 

Brige across the Ign switches and see if it's the switches.

 

OK, so it seems you are narrowing down where the issue is.

-- You have the proper V at the bus bar
-- You DO NOT have proper V at the gauges

-- Follow each wire that leaves the bus bar and temporarily remove it from where it next attaches at. Measure V at the connector you just removed.

 

Now see if you have the same drop across the ground buss.

Check for any multiconductor plugs that join the main harness at some point. 1 loose or dirty pin can screw everything up.