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Old 02-12-2006, 11:50 AM
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Idiot light to Gauge question.

With a lot of help from this board I've just about completed a change over from a very basic idiot light panel to a complete factory guage panel in a '68 Chev C20.

My remaining three questions are; Tach, Batt, and Temp.
1. Tach. I hooked up the gray wire to the dist. side of the coil, pink to a 12V source, and grounded the tach. (This is a factory GM tach and harness) Started the engine, but the tach didn't respond . Where'd I go wrong here?

2. The Batt gauge has two ribbons in the printed circuit that go to the harness plug. I jumped the correct wire from the harness plug to the gauge terminal then went from the other term to ground. Started the engine and the gauge stayed on the "-" side. (With the ign. off, the "hot" wire to this gauge reads 0 volts. With the ign on, the voltage is about 4.) The charging system on this truck works.

3. Temp. I have to change the sending unit in the engine which is a no brainer, but the gauge in the cluster has four posts. Two of them are incorporated in the printed circuit, (these are horizontal) and there are two vertical studs that come through a ceramic (resister?) piece. I am going to use the same wire that serviced the idiot light for the sending unit, but I'm not sure where it wants to go.

I've studied a wiring diagram, but I suspect I'm comparing apples to oranges because I'm modifying the order of the wires in the harness block, and have dropped the oil press wire. (ie The oil press is wet now, and the tach is isolated from the board.)

Thanks for the help!

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Old 02-12-2006, 03:10 PM
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Doc here,

Assuming this is NOT an HEI, on the Tach, did you wire the Grey To the - side of the coil? (The Dizzy side) And when you first turn the key on does the needle "Jump" a bit?

If not, check your 12 volt source & Fuse and your ground again. If those are good, and your on the - side of the coil, your tach calibration board is bad.

On the Volt Gauge, Is this a TRUE voltage gauge? on an AMP gauge? If it's a Volt gauge, you only need switched power and ground to it, you need not interweave it into the laminate card at all.

If it's an AMP gauge, you need to wire it across the vehicle load , usually at the fuse link and the buss wire powering up the rest of the car.Either way, you really need not interface it to the laminate card.

On the temp gauge, Are we talking a direct replacement for the stock to laminate card gauge?..If not, it won't (probably not) Work without some serious mods to the laminate card..

Which would mean, If you just wanted to use the laminate card as a mount (so it looks stock) you'd need to isolate the circuit trace foot pads (where the laminate bolts to the gauge) and with a scalpel, cut those traces off the pads and leave about 1/8 gap between them (so they have no contact) and hardwire the gauge to the cluster plug for power and ground. Where you cut the laminate card , on the trace side, get some nail polish from the Wife, and paint those traces to insulate them.

The Drawback here is some of those circuits are series and parallel wired traces, So you have to be careful so as not to separate one of those traces, I.E..The ground from the Temp gauge has a trace to it, then from there another trace to the fuel gauge..ETC..meaning if you accidentally separated those in addition to the foot pad, you'd lose the function of the fuel gauge ..follow what I mean here?

As far as finding the sender wire, Easy..just put a DVOM set for R X 1 on the ohms scale on the sender wire and and probe the cluster connector until you find that wire, from there you can either hardwire it or probe it to the laminate card and hardwire it from there to the gauge..I prefer the connector, those laminate card traces are wimpy at best, and a snagged wire can rip several inches of trace off the board , affecting who knows what..

Also on SOME of those Early GM tachs, you needed a Tach filter to buffer the high pulse shock down a bit (usually with HEI)..and if it ran without one for any length of time, the gauge will be damaged..also some required a separate "Box" in addition to the head unit..but usually are a signal and case ground to the head unit, not 3 wire..I assume you bought this used.

Maybe post the Schematic back and the Gauge diagram and type's and I can help more...

Doc
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Old 02-12-2006, 05:17 PM
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Hi Doc, Thanks for the come-back.
I haven't gone out to trouble shoot the system yet......we have some snow to deal with tonight. Tell me again why I live up here??

The dizzy isn't HEI so I should be able to figure that out. The hot wire (pink) has a terminal that plugs into the power source, but off of this is a short wire - two or three inches - with another terminal plug. (Think of a "Y" with one long leg to the tach & a short leg to ??) Would this be to a filter although it's not in series, it just branches off the main line.

The Batt gauge...I don't know if it's volts or amps. The face just has D on left and C on right. (A typical generic factory gauge) Momentary full vehicle voltage pegs the needle pretty quick. I think the gauge is volts because it's wired through the circuit card to the connector plug with some pretty light ribbon through the card. I would think full vehicle amps could smoke the ribbons.

The Temp gauge is a factory unit ( The whole gauge cluster is a rebuilt GM unit with the bezel and a new circuit card out of a C40-C60 truck with air brakes. Since I had air bags I thought I'd go masochistic and change the whole dash for a gauge. I guess that's why they call it hot-rodding) Any how, I have isolated the wire that comes from the original sending unit, can it be so simple as changing the sending unit, hooking up the wire to one side of the gauge and grounding the other side? In this case the ground side would run as you described through the card and incorporating several other instruments.

Thanks for your help....I've gotta go deal with the snow.
Later,
Tim
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Old 02-12-2006, 06:43 PM
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Quote:
Originally Posted by slomotion
Hi Doc, Thanks for the come-back.
I haven't gone out to trouble shoot the system yet......we have some snow to deal with tonight. Tell me again why I live up here??

The dizzy isn't HEI so I should be able to figure that out. The hot wire (pink) has a terminal that plugs into the power source, but off of this is a short wire - two or three inches - with another terminal plug. (Think of a "Y" with one long leg to the tach & a short leg to ??) Would this be to a filter although it's not in series, it just branches off the main line.
The Stock Tach filter plugs into the Tach wire (white on an HEI) the filter is not used I don't think on a standard points type ignition or lower voltage coil.

Quote:
The Batt gauge...I don't know if it's volts or amps. The face just has D on left and C on right. (A typical generic factory gauge) Momentary full vehicle voltage pegs the needle pretty quick. I think the gauge is volts because it's wired through the circuit card to the connector plug with some pretty light ribbon through the card. I would think full vehicle amps could smoke the ribbons.
Does it look like this? (0 to 60- and 0 to 60+)


If so it's an AMP meter..Those can be wired as a head unit on the laminate card, the shunt , remotely located so no high current passes into the cluster..so beware.

Quote:
The Temp gauge is a factory unit , can it be so simple as changing the sending unit, hooking up the wire to one side of the gauge and grounding the other side? In this case the ground side would run as you described through the card and incorporating several other instruments.
Most factory units have three terminals (not counting lighting) S, G and I, The I is ignition switched, the "S" is sender, and the "G" is ground, you need to verify that these match up when (before) you put the gauge into the card..If they do..then your good to go..If not, you need to modify the card or consider another location.

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Old 02-13-2006, 09:07 AM
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I was checking an old shop manual for this dash panel wiring. It shows an ammeter.
There are (2) #20 wires running to the ammeter. (105blk &106blk/wht)
The other end of these wires are connected across the fuse link. Both wires are fused at 4 amps with inline fuse holders.
This is a factory setup. I have seen these wires and fuses in the harness even when no (optional) ammeter is installed.

What this means is they are using the fuse link as a meter SHUNT. The ammeter (in the dash) is actually reading voltage dropped (IxR=E) across the SHUNT.
If the fuse link blows the 4 amp fuses will also blow, thus protecting the ammeter.

The ammeter will deflect on the + side when current flows to the battery (Charging).
The ammeter will deflect on the - side when current flows from the battery (discharging).

vicrod
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Old 02-14-2006, 12:19 PM
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Thanks Guys,
The closer I get to the end of this tunnel the more I'm beginning to think this gauge cluster thing is sorta like an orchestra - all of the instruments have to be playing at the same time! I have "re-programed" the truck harness connector to match the inst. cluster printed circuit card, and with a volt/ohm meter could cipher out what was what (with your help) but in a couple of cases the gauge wouldn't respond in a stand-alone scenario which is what was bugging me. Once all of the connections were made it seems they do much better!

Thanks again,
Tim
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