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Discussion Starter · #1 ·
My 1978 Ramcharger has the factory tach in the dash. It has three wires coming out the back and one terminal without a wire. The one terminal on the back with no wire I am guessing goes to the distributor as it has the letters 'dst' under it. Does anyone know what all these wires are for and where it hooks in to the distributor? Does 'dst' indeed need to be hooked to the distributor. With this RC, it is a great truck but some of the wiring has been unhooked. I am cleaning all this up and everything is working great so far. Thanks for any help.
 

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factory tach? I don't think so. Factory would never lable a wire coming off a component.
-stew-
 

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i have seen factory tachs out of trucks, ive seen the marks on some wires. just ask if someone has one out, to see if the wires are marked. i know ramdozer just got a rc with factory tach. he could maybe see where the wires go. or steveo might get a wiring diagram to help ya out.
 

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Discussion Starter · #5 ·
This is a factory tach, no doubt to that. It is chrysler, in the factory tach position and has factory wiring. The wires are not marked. What is maked DST is the back of the tach at one of the connections. I was wondering if this one needs to be hooked up. Thanks for the info and I will check with the other members mentioned. Thanks again. It could be this thing is hooked up and does not work. I just need to get in to all the wiring and trace it. Course if one is missing, can't do much tracing there.

KB
 

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For what it's worth....

I have an aftermarket tach in my RC and here's how it's wired.

Red wire -- 12vdc
White wire -- illumination
Black wire -- ground
Green wire -- to coil...I believe the negative post but I'll have to check to be sure. (maybe someone else knows for certain.)
 

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Wire colors may be off as I am unsure for '78, but my guess for the three wires that are there: (let us know if these are correct or not)

Red or Blue: Ignition switched power
Black: Ground
Orange: Dash lighting
Not sure what color teh factory used for the tach signal wire, but unless your dist has a terminal on it labeled "tach output" or "tach signal" the 4th wire from the gauge should go to the coil negative terminal, as stated above. (I have only seen the tach output terminal on chebby dist's w/ HEI ignition, and aftermarket dist's like MSD)

-SM
 

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Discussion Starter · #9 ·
Thanks everyone, this is helping. I have traced the wires a bit and here is what I have.

The orange does go to the dash for illumination
The grey is marked ign at the tach and is going to the fuse box to the ignition accessory fuse
The white is marked grd for ground and is plugged in to the wiring harness and I assume being grounded with the rest of the components.
I found a black wire that is factory bundled with the grey ign wire and has a plug that is correct to plug into the connection marked dis on the back of the tach. (it is marked dis not dst, my mistake earlier). I plugged this into the dis connection but it was real loose so I replaced the connection.

The problems I have is;
#1 - no illumination. I cannot figure out how the orange wire comes out of the back of the tach. It has a spring in it at the tack but does not come out upon turning or twisting a bit.
#2 - I am not 100% sure the black is the correct wire to go to dis. It had the correct connector, it was hanging right where the tach plug is and it is bundled with the other tach wire. When I hooked it up and tested it, it only moves slightly up when the key is turned to on and it does not register any rpms upon starting the truck, so simply, not working yet. I traced the black to behind the instrument cluster and lost it back there, then my daughter was wanting me to come in and get her out of the tub. (shes only 2 guys) So, priority takes place and those women get bossy reeeaaal early in life. But, progress is made but nothing works. I will plug away some more and keep my eye on this post for help. Thanks all.
 

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I have a factory tach right in front of me. The wiring has the numbers #4085379
87940

Has white, grey and black wire. The white wire has male spade connector and is for a ground. Black wire has a female spade connector and is the tach signal wire. The grey wire has a connector that makes a 90 degree angle and is just pushed on.

There are two holes in the back of the tach one should be covered in tape the other hole closer to the ground terminal is for the light. It should have an orange wire with a bulb on the end of it that is pushed into the hole.

I hope it works but chances are your tach may need work. I found a place to rebuild them for a $ 150.

Anybody have one rebuilt before? Hope that helps Mike
 

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Discussion Starter · #13 ·
Okay, I think everything is hooked up correctly at the tach.

All wires are accounted for and hooked up but the tach only surges a bit when the ignition is cycled on. It registers no RPM's while running.

Now, 4 questions.

1) What is a good way to test the output of the black wire coming from the ditributor?

2) What exactly does the black wire supply to the tach so the tach can then register this as RPM's?

3) Where does this black wire hook in to the distributor?

4) What is a good way to test the tach? Can an auto parts (NAPA) test one?

Kevin
 

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1- If your engine runs, the black wire is working.

2- The black wire is connected to and controlled by the ECU. When its time for the coil to fire, the ECU breaks ground (resulting in a broken ground at the coil, via the black wire), its this decelleration of current (which is the result of breaking ground) at the coil that creates the spark within the coil to send to the distributor.
So, everytime the black wire breaks ground, it represents when a cylinder is fired. The tach uses this and converts it to RPMs, although I don't know how the tach does this.

3- It doesn't....see above.

4- The only way I know, is to hook it up properly and if it doesn't work, its bad. ???
 

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Discussion Starter · #15 ·
I need to clarify.

1) I was questioning testing the black wire at the tach. I can not trace the black wire out from behind the dash unless I do major disassembly. I was wondering if I can just use a volt/amp meter to test it and then I would know it was the tach. If I wasn't getting any reading at the tach, I know that something was up with the wiring or connection.

2, 3 and 4) So, make sure I understand. The black wire coming from the tach goes to the ECU not the distributor. The truck could still run jsut fine if the black wire was fudged or not connected somewhere between the ECU and Tach, that is what I was trying to rule out.

I have and aftermarket tach in the basement and I can hook it up and see if it gets anything from the factory wiring.

Thanks

KB
 

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Discussion Starter · #16 ·
Well, it's the tach. I hooked up a tach I had in the basement to the factory wiring and it worked fine. Thanks to all for helping me figure out the wires and such. I will look at getting this one repaired or finding another or using the aftermarket one I have.
 

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There are a number of places online that repair tachs,etc. Here's a few...
www.redlinegaugeworks.com
www.dashusa.com
www.tachman.com
www.gaugeguys.com
www.bobsspeedometer.com
www.autoinstruments.com
www.williamsons.com
www.american-classic.com
www.speedometershop.com
www.ccspeedometer.com
www.dandmrestoration.home.mindspring.com
www.paspeedo.com
www.clocksandgauges.com

 

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kybishop said:
I need to clarify.

1) I was questioning testing the black wire at the tach. I can not trace the black wire out from behind the dash unless I do major disassembly. I was wondering if I can just use a volt/amp meter to test it and then I would know it was the tach. If I wasn't getting any reading at the tach, I know that something was up with the wiring or connection.

2, 3 and 4) So, make sure I understand. The black wire coming from the tach goes to the ECU not the distributor. The truck could still run jsut fine if the black wire was fudged or not connected somewhere between the ECU and Tach, that is what I was trying to rule out.

I have and aftermarket tach in the basement and I can hook it up and see if it gets anything from the factory wiring.

Thanks

KB
Oops, my bad, I thought you were talking about the black wire at the coil negative, the one you would hook the tach trigger wire to. Sorry. :-[
 
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