grounding gauges

[stevep]

Hi all -- I am replacing my 1972 oill pressure and water temp gauges with new SW gauges from MMI. The 1972 gauges have only 2 posts: 1 for 12 volts and one for the sender wire. The new SW gauges have 3 posts: 1 for the sender, 1 for 12 volts, and 1 designated G for ground.

Now I have two questions: do the new SW gauges really require grounding on the A4?
Do you know if any of the wires in the original wiring harness that lead to the instrument panel are ground wires?

Thanks very much for the assistance.
Steve

[sunnnnseeeker]

I don't know about the wiring harness in your boat but I am positive that the ground is required and at least one of the wires is a ground, somewhere. If the ground is not in the wiring harness it must be somewhere else. I would suspect that the 1972 gauges are grounded as well. Since you say there is no ground wired to them are they grounded through their installation relying on the case of the gage to be the ground? In any case this is easy to figure out. Trace and identify the wires. You'll find the ground wire. The SW gauges will not work without being grounded.

The battery negative post wire is most likely connected to the engine block. I suspect you have one or two smaller wires connected to the engine for ground also. There must be a ground to the ignition switch and it is likely that this is the ground for the gauges as well. On my boat all the original ground wires are black in color. This is standard practice to use black as ground in DC applications. So if you have some blake wires in your boat look at them first for being the ground wires.

Let me know what you find.

Thanks.
Nathan

[SEMIJim]

Now I have two questions: do the new SW gauges really require grounding on the A4?

My guess would be if the gauges have it, they need it. Or, if they don't need it, at least the manufacturer thought it would be a Good Idea to hook it up.

Do you know if any of the wires in the original wiring harness that lead to the instrument panel are ground wires?

This is as much dependent on what your boat manufacturer did as on the fact you have an A4. The wiring harness' on our 1976 Pearson P30 aren't going to be the same as those on whatever kind of boat you have, unless it's also a 1976 Pearson P30 .

I would just find a ground wire the hard way: By finding out what else had/needed/used ground and tracing it. Lacking that: I'd just run one. But if you don't know electrical systems, I imagine that might be problematic.

There is a book I've often seen recommended: Nigel Calder's "Boatowners Mechanical and Electrical Manual". Perhaps that would be a good book for you?

Either way: You'll be needing a multi-meter for that boat. I saw recommended on another forum a DVM for about $40 at Harbor Freight. It seemed like a decently-built piece of equipment, so I bought it. It even has built-in dwell angle and, IIRC, tachometer functions . (Recommend you store it on the boat in a large freezer storage bag to keep the moisture out. Maybe throw a desiccant pack in with it.)

Jim

[rigspelt]

Hi all -- I am replacing my 1972 oill pressure and water temp gauges with new SW gauges from MMI. The 1972 gauges have only 2 posts: 1 for 12 volts and one for the sender wire. The new SW gauges have 3 posts: 1 for the sender, 1 for 12 volts, and 1 designated G for ground. Now I have two questions: do the new SW gauges really require grounding on the A4? Do you know if any of the wires in the original wiring harness that lead to the instrument panel are ground wires? Thanks very much for the assistance.
Steve

My amateur diy vote would be yes to both. As suggested, trace black wires that connect to the engine block.