Fixing the wiring of my tach - I'm dumber than a blonde!

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7milesout

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Hey guys - So I've got this 72 Plymouth Scamp. It has a Sun Tach (model ST-602) tach already on it. The tach works. But it is wired up hot to the battery + post. Luckily, there is a disconnect about a foot off the battery. When it is connected, the tach is ON, and the light is on. It will drain the battery. It doesn't matter the key switch or anything else. So, last night I tried to figure it out.

In summary ... I SUCK at anything electrical. I'm a mechanical engineer for YEARS. But when it comes to electrons, I have no idea what they're gonna do.

Here's the deal. On the back of the tach it has the following connections:
  • Volts 12 (nothing terminated to this tach post)

  • IGN. SW (terminated via a black wire, routed to battery + post)
    • The bulb conductor is looped around and crimped into this IGN. SW eyelet.

  • Trigger (terminated via a blue wire, routed correctly to ignitor)

  • GRD / NEG (terminated via a yellow wire, routed up under the dash to a ground, although I don't prefer it's location, it is on paint)
When the connection is made to 12V under the hood, the tach light is on continuously, and when the car is driven, the tach works correctly.

Last night while messing with it, I disconnected it from 12V under the hood. Then I changed the black wire from the IGN. SW post to the Volts 12 post. I then attempted to connect it back to the battery under the hood. It didn't like that, and demonstrated it's objection with a loud pop and sparks all up in my face. At that point I felt as stupid as I always do when dealing with electrons.

I know what the "Volts 12," "IGN. SW," "Trigger," "GRD / NEG," and all that mean. I'll just be damned if I can figure out how the electrons are going to feel about me messing with them.

Seems to me that, I should disconnect the black wire from the IGN. SW and put it on the 12 Volts post. And then, add a new wire to the IGN. SW and terminate that wire to a 12V switched source (like the ignition of course). At that point, I'm assuming the switched source will keep the circuit to the 12V side of the tach "open" when the car is off. And only close the 12V circuit when the ignition is switched ON. This would turn the tach and it's backlight off when the car is off ... no?

2 questions:
  1. Is my previous paragraph correct?
  2. Without tearing into the steering column, where could I find access to switched power?

7milesout
 
you should have a ground wire, a switched 12v power wire, a ignition signal wire and one going to the instrument lights so when you turn on the headlights you also light the tach.
 
Here you go.
Probably the toughest part is to find 12v switch power.
Go to the fuse panel, and unscrew the screw to free it up.
Get a test light. Hook one end up to ground, and touch each fuse turning the key on and off.
That is how you find a 12v switch power. Usually there is a piggy back terminal where you can connect the 12v to switch wire to. The same test is used to find the headlight fuse. This is the fuse that the white wire hooks up to.
Hope this helps.

autogage2.jpg
 
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I did a search, did not find squat. Can you post a photo of the rear of the meter as taken out of the case?

This might be an old type needing the old black sender box. "The internet" was little help
 
Hey guys - I've been working on the Scamp when I have time. And the time I've had available went toward replacing the harmonic balancer and setting timing. Well that's done now. So, I'm back on the tachometer. Since starting this thread, I procured a circuit tester as you guys suggested. It came in handy with some LED's I mounted in the ceiling of my pick-up's camper shell (blue LED's ... look nice).

And today I went out and messed with the Scamp and used the tester on the fuse box. Just a while ago (today), I purchased fuse taps for SFE (glass) fuses. I'm going to tap the "ACC" fuse for 12V switched power, and I'm going to tap the "Lamps" fuse for power to the tach bulb. There's no built in ports to tap power.

Somebody asked for a picture, see below. Currently, the 12V wire (black wire below) attaches straight to the battery, with a disconnect near the battery. But I'm going to change the black wire to terminate to the same location as the battery "-" post. And then terminate it to where the yellow wire is in the picture (NEG post). I will move the yellow wire to where the black wire is in the picture. Currently the yellow wire is attached to a (painted) ground under the dash. I'll remove it from the painted ground and attach it to the "ACC" fuse for switched power. The trigger is really the only thing wired correctly.

Seems odd that the left post in the picture is not terminated to (like the other 3). But that's how I found it. The bulb wire has been crimped in with the 12V source wire, and that's why it is on all the time whenever I push the connector together by the battery under the hood. That I knew. I'm going to snip that, probably do a butt-splice on that blue wire and run that blue wire to the fuse box "Lamps" fuse. That would provide power to the tach back light only when I turn on the headlights.

Let me know if you read any problems with what I plan to do.
upload_2017-5-21_23-12-15.png
 
upload_2017-5-23_20-47-44.png

CLOSED! After finishing the Harmonic Balancer, I got back on this. I thought about what was right and what was wrong, based on the schematic attached a few replies above. I also bought the circuit tester and some glass fuse taps. The picture in the previous post is BEFORE, the picture just above is AFTER.

Before:
  • Switched Power (pole) was terminated straight to the battery (black wire, with a disconnect near the battery).
  • Backlight (dark blue wire) was crimped in with the Switched Power, which was hardwired to the battery.
  • Trigger was correct (light blue wire).
  • Negative was grounded on paint under the dash (yellow wire).
After:
  • Switched Power, I pulled the yellow wire over to that pole, because it was grounded near the fuse box.
    • I removed it from the painted ground and terminated it to the ACC fuse, using a glass fuse tap.
  • Backlight, I snipped it off the black wire crimp, and extended it to under the dash, then terminated it to the LAMPS fuse, using a glass fuse tap.
  • Trigger, I did not change.
  • Negative, I pulled the black wire off the switched power post and moved it to the NEG post.
    • I then disconnected the termination from the battery + pole, and attached it to the battery - post, as there was an attachment readily available.
Bada-Bing, Bada-Boom. DONE! Worked like a charm.

That little black thing at the bottom is some kind of calibration for the tach. I noticed it was sitting at like 600 rpm (when off). Then I remembered I had fiddled with that thing not knowing what it was. So I fiddled with it again, and set the tach at 0 rpm.

Overall, I don't like this tach. I may update this later. But it is good for now.


7milesout
 
REOPENED!

Well, it ain't over 'til it's over. I don't know if it was working completely correctly after re-wiring or not. But I noticed over the weekend when driving it that the tach will not go over 2,000 rpm. Right on 2,000 rpm it just refuses to go further. It will bounce around from 2,000 some also, when I'm steady cruising above 2,000 rpm.

So, as I was out, I decided to mess with the wiring. The illumination seemed to be working fine, so I thought it was a grounding issue. I placed the ground at another location, no change.

And subsequently, I got gas. After filling I noticed the gauge would not read full. It would stay just over half. Later I pulled the connector off the ACC fuse tap to check the fuse tap to make sure it was still located properly (and it was). But while under there it appeared that the wire was chaffed from where it ran through some brackets. I rerouted it. But it still won't go above 2,000 rpm.

I'm thinking that wire is compromised now, so I'm going to replace it. After rerouting the wire, the fuel gauge works fine. Very strange. It could be the fuel gauge is not related ... but it is very coincidental.

QUESTION: Does a chaffed ACC (switched power) wire sound like the likely cause?


7milesout
 
This may not be a wiring problem at all. It may be problems right in the meter for the tach
 
Yeah, that is possible. But since I just played around with the wiring, and since I'm dumber than a blonde, expect that the dumb person who just played with the wiring had something to do with it. Next chance I get I'm going to replace that wire.
 
Was driving the Scamp around locally and checking the tachometer, it has decided to work again. On its own. Which leads me to believe that the tach is no good. So, I will tolerate it for a while, and in the meantime, I'm moving on to the exhaust project. After the exhaust, I will just replace the tach.


7milesout
 
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