Voltmeter Power Source Question

[memsiej]

Looking at Tim's widely-circulated diagram for the ammeter bypass, I'm trying to draw one up for an install of a voltmeter. I read through Cody's old thread on his voltmeter install but there was no mention of wiring. I know one post needs to be a ground, and I read that I can grab power from anywhere under the dash that powers on with ignition, but as an electrical rookie I'm hoping for more concrete suggestions/instructions on a power source. Am I overthinking it, and I can just grab from that new red/yellow splice?

Diagram so far:

[tgreese]

You want somewhere that turns off when the key is off. A voltmeter is a tiny drain, but it is a drain and will run down your battery eventually.

Are you installing a voltmeter below the dash, or replacing the ammeter with a voltmeter?

I can show you what I did. I replaced the ammeter with a voltmeter in the cluster.

ClusterBack.jpg

Can't scan it because it's a bound book - best I can do is a macro shot with my camera.

This is from the '82 TSM and shows the back of the instrument cluster. I believe an '84 is the same. You see pin 11 and the trace that's highlighted in pink. That is the pin from the ignition switch that feeds power from the ignition switch to the gauges and the indicator lights.

You also see pin 4 on the flat connector. That is connected to the screws marked GR that ground the top edge of the cluster.

I'm good at soldering. I polished a small spot on the pink trace and soldered a wire to it. The new voltmeter replaces the ammeter which is the two studs coming through the circuit board at M1 and M2. One of these now goes to ground (with a lead under one of the screws? Don't recall exactly) and the other connects to the wire soldered to the pink trace.

The voltmeter then turns on with the other gauges, when you turn the ignition switch.

[memsiej]

Noted. So if I'm not good at soldering (I'm currently learning), can I use one of these to take from the wire feeding into 11 on the round connector, and then run that wire to the voltmeter? https://www.amazon.com/120-PCS-Wirefy-E ... 123S&psc=1

And yes, hoping to follow the Cody write up and replace the ammeter with a voltmeter.

[tgreese]

You could use one of those, but they are considered nasty. Should be sorta ok under the dash, where the wires are protected from the weather.

Suggest you test your tap on a separate wire of the same gauge, before you tap into the important under-dash wire. The TSM will tell you what gauge the wire is, or it's sometimes printed on the wire in tiny illegible script.

Soldering a wire to the trace is not that hard. Maybe you can find someone to help you with it.

[memsiej]

It's something I'm excited to learn, that's for sure. I had breaks in my PCB that were preventing a blinker, a gauge, and some other lights from working. Successfully soldered across the breaks and fixed it.

Someone else drew up this diagram to show how to power the voltmeter without using a splice. I'm assuming the green is a 14g. This may be the route I go since I'm not yet confident in soldering.

[tgreese]

That will work but the voltmeter will be on all the time. Maybe not significant, depending how often you drive. To me, it's satisfying to see the voltmeter jump to life like the other gauges when you turn the key.

Note the fusible link is only about 6" in length. The rest of the run to the battery should be 10 ga wire. Too much resistance to make the whole run 14 ga.

https://www.amazon.com/gp/product/B001B ... UTF8&psc=1

You need two of them. One is in the wiring harness already, and one for the new connection from the alternator direct to the battery. You are protecting the dash connection and the alternator from the battery.

[memsiej]

Well, maybe I'll start with the above diagram for now and if that voltmeter draws too much between drives (I don't drive it very often) I'll switch is up to power from a different source. Maybe by then I'll feel confident soldering to the PCB.

Thanks for your help!

[tgreese]

NP! Take care.

[memsiej]

You'll love this. I accidentally drilled my holes for the voltmeter too low, and they contacted the trace below. To fix, I cut that trace and soldered (sloppily) a new trace up and around, coated with liquid tape to be safe. THEN remembered "didn't Tim mention something about how I should just solder a wire off this exact trace to power the voltmeter?"

Luckily, in all this learning, I have a new PCB on it's way shortly. This thing is turning into quite the frankenstein.

[tgreese]

Shall I critique? Looks pretty much ok to me. I think you could use this circuit board, if the meter is not too low in the window. The voltmeter does come close to that trace.

[memsiej]

Never straight forward. Went to do my re-wire (your way, with the jumper to the pink trace) and under the hood I did not find factory wiring, or at least not factory colors.



I'm guessing that the black coming out of that loom is the "yellow", and somewhere it's changed. But because this doesn't match what I was expecting to find, now I'm second guessing my rewire plan.
My best guess is disconnect red from the alternator post and run THAT to the starter solenoid (where black/yellow currently is), disconnect and disregard black/yellow, and then run an additional wire (with fusible link) from alternator post to starter solenoid. This will allow red to still power the other dash components, I can tape off both red and yellow under the dash since the voltmeter will have it's new power from the pink trace and the other post will be a ground wire.

Am I on the right track?

[tgreese]

Suggest you pull it all out of the plastic loom (split plastic tubing) and see where it goes. You need the wiring diagram for '84, if you don't have that.

S is solenoid? Likely that the black wire is a fusible link and connects to the yellow 10 ga wire to the dash. Possible the colors have changed, but I doubt it. Follow it further up the loom and you'll likely find a splice that connects to yellow 10 ga wire.

The solenoid side has the only fusible link in your Jeep. The red wire goes to the dash, feeds the dash and lights, goes through the ammeter, and back down to the battery via a fusible link.

What you show is backwards from my scheme, if that's what you want to do. Are you reading my post on IFSJA? http://www.ifsja.org/forums/vb/showthread.php?t=184885 Some would take the yellow wire and use that for the alternator, and use the red wire to feed the dash. That seems like extra steps to me - the red wire is already connected to the alternator, and the yellow wire goes to the dash via the ammeter.

The '84 diagram on the Tom Collins site (TC) is not very legible, and I don't want to figure it out tonight. If you don't have the actual '84 TSM, you can photocopy what's on TC and trace through the circuit in my diagram. I'm sure it's the same aside from details, if you have an ammeter from the factory. It's too much for me to trace this out for you tonight. Maybe tomorrow.

[tgreese]

You might also look at the wiring diagram for a 1976 CJ-5. https://oljeep.com/gw/76_tsm/Section22.pdf

This is exactly what you want to make. In 1975, Jeep switched to a voltmeter for the CJs, and the six cylinder uses the Delco alternator that you have. Jeep puts both the Motorcraft V8 alternator and the Delco six cylinder alternator at the top and bottom of the same diagram, so don't be puzzled by that.

Here, ther is no yellow wire - both the red and yellow in my diagram are red. 12D and 12C red are the same as the yellow in my diagram. Note also that 12E - the voltage sense wire - just bends back to the terminal on the alternator for 12B. You can do this too.

[memsiej]

I have all the wiring diagrams printed from Tom Collins and the manual.

Why keep the yellow, if it's no longer being used at the dash? The ammeter is gone, the voltmeter gets power from the PCB via a soldered wire. Red still has a purpose- to feed other dash components- so it might as well stay.

[tgreese]

You can have a single loop - red to ammeter to yellow to battery - or you can have two wires: red from alternator to battery, and yellow to dash.

You need to connect the alternator to the battery somehow. I don't understand your reasoning.

[memsiej]

I think I get this. Left is what I did, Right is what you were saying I should do. I think both should work (well, I have no doubt yours does. I'm hoping mine does).

For whatever reason, when I was in the trenches this way made more sense to me, but would be easy enough to revert back should it not work. In both, the alt is wired to the starter, as are the dash components.

[tgreese]

Yes, that's right. Choose the way that you like. My way appeals to me for obvious reasons (it's mine) plus I think it uses the existing wires better. My way needs a splice where the ammeter was to reach the dash feed, yours does not. Your way abandons the yellow wire and adds a new wire, mine does not.

[memsiej]

High five.
The only thing that I don't love about all this is that both methods have a large-ish wire (from alternator post) and the wire from the electrical connection on the alternator meeting up with that fusible link. Maybe my link isn't large enough, but it felt like I was doing from two medium-large wires to one medium. But then again, there's much I don't know about all this.

Thanks for your help through all this. I'll fire it up tomorrow and see what happens.

[tgreese]

Great. Looking at your diagram, I presume both the gray connections are each fusible links.

The main difference between the 14 ga fusible link and the 10 ga wire is its resistance. Heating of wires is the product of its resistance - for the same current, the more resistance, the more heat produced. 10 ga wire has a resistance of about one ohm per 1000 feet of wire, and 14 ga is about three times that. The fusible link is just plain wire with a special insulation, meant to contain the fire if the link ever actually fuses. Both the length and the diameter is important. Those are chosen so that the link is not a significant additional load on your 10 ga wire, and it does not get hot enough to matter in daily service.

If you short the battery to ground, that's say 700 amps. That 9" fusible link has a resistance of 3/12000 * 9 = 0.00225 ohms. P = I^2 R, so that's 1102 watts at that link - apparently enough to fuse it. Your alternator makes maybe 60 or 80 amps peak into a completely dead battery. Say 70 amps, that's 11 watts in the link. Apparently that's safe, esp. since the link is now hanging in free air under the hood.

If the skinny wire bothers you, you can upgrade the alternator link with say 8 ga wire and a 12 ga link. JMO - not really an issue, esp. with the wire hanging out in the air. Quality of your connections is important though; if you crimp, make your crimps really solid.

I'd also point out - having the sacrificial component (link or fuse or circuit breaker) as close as possible to the power source is correct. You could put the link next to the alternator and that would be the same electrically. However, the whole length of the 10 ga wire is now unprotected from shorting to ground and that is less safe.

[memsiej]

The fusible links come off the starter solenoid, so they are as close to the power source as can be. I'm worried they're too short though at only 5-6". We'll see.

Now I'm just curious, and enjoying learning- what is the benefit of fusible link over an inline fuse? Seems to me the latter would be preferable because if it blows it's easier to replace on the road. But then, I guess the question would still be "why did it blow". Is there a reason to go link over fuse?