How do the voltmeters work in these trucks?

[kipps]

I'm holding two instrument clusters.

An '87 cluster has three pins on the back of the voltmeter.

A '91 cluster has an ignition hot and a ground going into the voltmeter, and a separate wire going to the battery light. It looks like the battery light needs to be grounded through that wire in order to work. I may have misread the printed circuit, though.

How does that three-pin voltmeter work? How does the battery light work?

I'm working on a LS swap, and am trying to literally merge the old and new harnesses, including the new underhood fusebox.

[ray_mcavoy]

The voltmeters have a current limiting resistor ... it's the rectangular white ceramic piece on the back with a patch of paint in the center.

One terminal serves as the +V input. It connects directly to one side of the resistor and has no internal connection to the meter movement. The 2nd terminal serves as a connection point between the other side of the resistor and the meter movement coil. This terminal has no external connection to the cluster. And the 3rd terminal is the ground for the meter coil.

I don't think I've ever seen one of these clusters with BOTH a voltmeter and a battery light ... they usually have just one or the other. Although wiring diagrams will sometimes lump both together (often using dotted or dashed lines to indicate optional wiring) so you have to look carefully when following those circuits. The battery light (when used) typically has one side of the bulb connected to power via the ignition switch and the other side connected to the alternator's voltage regulator.

[kipps]

On the battery light, I realized that I don't know if that light was used or not. Most of the bulbs were missing when I got the cluster. The harness was cut six inches from the cluster. That scrap of a harness had a brown wire that ultimately went to the battery light socket.

The second pic shows the backside of the voltmeter when installed. Note that the right pin does not contact the circuit board. For anyone who's putting one together from scratch -- resist the urge to add that connection. It looks possible, but should not be there.

For that matter, it looks like the oil and temp gauges have a similar two/three pin arrangement with a resistor between the two terminals.

The third pic is simply a backside comparison of the '87 and '91 clusters for interest's sake. Note the speedometer connection on the left of that big plastic box. Top to bottom, it's signal, ground, and hot.

One other comment for future reference; these clusters had completely different pinouts. The wire colors were completely consistent, but the position of the wires in the connector were different.

ray_mcavoy, thanks a lot for the help. As I look through some more of these 87 connectors, I'll probably have more questions.

[ray_mcavoy]

Originally Posted by kipps

The second pic shows the backside of the voltmeter when installed. Note that the right pin does not contact the circuit board. For anyone who's putting one together from scratch -- resist the urge to add that connection. It looks possible, but should not be there.

Yes, that's the terminal I was describing that only serves to connect the resistor to the meter's internal coil and does not have any connection to the cluster's printed circuit. This arrangement puts the current limiting resistor in series with the coil inside the voltmeter.

Originally Posted by kipps

For that matter, it looks like the oil and temp gauges have a similar two/three pin arrangement with a resistor between the two terminals.

Yes, the oil pressure & temperature gauges (as well as the fuel gauge) do have a similar looking terminal / resistor arrangement. However, these gauges each have 3 connections to the cluster's printed circuit ... power, ground, and sending unit.

[sweetk30]

funny i just had 2 1987 clusters apart 1 basic with gauge and other silverado with clock .

both had 2 pin volt meters in them but housing and circuit board had 3 clips in them .

[andyh1956]

Do the resisters on the gauge backs have specific Ohm Values that can aid in trouble shooting?
Thanks

[ray_mcavoy]

Originally Posted by sweetk30

funny i just had 2 1987 clusters apart 1 basic with gauge and other silverado with clock .

both had 2 pin volt meters in them but housing and circuit board had 3 clips in them .

Interesting ... sounds like someone had those clusters out before, saw the empty hole, and unknowingly stuck a clip in there.

As shown in kipps photo, there is a trace on the printed circuit that appears as though it would create a short circuit between 2 pins if clips were installed in both locations. But looking closer at the photo, it appears as though the portion of the trace surrounding the unused hole is covered by the plastic lamination on the printed circuit. So a clip snapped into that hole would be electrically isolated and not cause any harm. Although I still wouldn't recommend installing a clip there because there is the potential for it to rub through the plastic and short out the voltmeter.

And in case anyone is wondering why that unused hole is there in the cluster housing & printed circuit, I believe that is simply a left-over from the older (73-75) clusters that used ammeters instead of voltmeters. The ammeters had their 2 terminals side by side and did have a clip in that outermost location.

Originally Posted by andyh1956

Do the resisters on the gauge backs have specific Ohm Values that can aid in trouble shooting?
Thanks

Yes, and the color of the paint patch in the middle is used to identify them. Most of the 76-87 GM truck voltmeters I've seen have a light blue resistor that measures around 127 Ohms. I can't remember exactly where I saw it, but I think one of the Corvette restoration parts websites has a picture or chart showing lots of the other colors & values.