More dumb Tach and Gauge questions.

[Super Birthday Snake 11]

Sorry for the continued questions on this but got some more dumb gauge questions.

 

I had a thread before about moving to aftermarket but I'd much rather get my stock ones working. My main issue I think will be my tach but I've been looking around and starting to think I might have wired it wrong. At the moment it gets slight reading, about 500rpm tops.

 

I have a painless wiring harness along with an RJM engine harness. The painless harness says I need to put it's wire into the Signal (red?) and the RJM harness also says to put it's wire into the signal. At the moment I've got the painless side (marked Tach to Coil) going into the Black wire and the RJM going into the signal.

 

As I've got an EFI engine, the RJM harness has a plug to the coil so I presume that the RJM wire is correct in going to the signal but that leaves me wondering what should be going into the black wire?

 

With the other gauges, is the only ground I need in the printed circuit plug? I don't need grounding wires on the fuel or temp gauge to the chassis or anything?

 

 

Thanks!

[TheRktmn 15]

The factory tach wires in series. RED tach is switched 12V power IN. The BLACK wire is OUT. On a stock harness the tach goes in the wire going from the ignition switch to the + side of the coil.

 

Note the 'pink' resistor wire between the tach and coil - it is necessary for the tach to function correctly and component longevity (a fancy way of saying full voltage through the coil can screw it up).

 

The Fuel, Temp and Oil gauges all ground through their sensors (sending units) on the motor.

 

The only things that need a ground on the cluster are the bulbs and instrument voltage regulator (that little silver box). The grounds on the plug and chassis mounting points should be sufficient, afaik there is no separate ground wire from cluster to chassis.

Edited May 21, 2014 by TheRktmn
added pink wire note

[Raven R code 281]

talk to the tachman!

[Super Birthday Snake 11]

The factory tach wires in series. RED tach is switched 12V power IN. The BLACK wire is OUT. On a stock harness the tach goes in the wire going from the ignition switch to the + side of the coil.

 

Note the 'pink' resistor wire between the tach and coil - it is necessary for the tach to function correctly and component longevity (a fancy way of saying full voltage through the coil can screw it up).

 

The Fuel, Temp and Oil gauges all ground through their sensors (sending units) on the motor.

 

The only things that need a ground on the cluster are the bulbs and instrument voltage regulator (that little silver box). The grounds on the plug and chassis mounting points should be sufficient, afaik there is no separate ground wire from cluster to chassis.

 

Hmm not sure how the RJM harness is designed if it includes some sort of resistance, would imagine so but I guess it depends if other classic mustangs had a need for this. I think that's definitely the correct wire to have going into the input (RJM tach) but not sure what to do with the output then, from that diagram looks like it's going to the coil negative?

 

talk to the tachman!

 

I'll drop him an e-mail!

[TheRktmn 15]

Look at the diagram. The circuit goes from the key to the tach RED, out the tach BLACK through the resistor wire to the POSITIVE (power, + ) side of the coil. The distributor connects to the negative (-) side of the coil. All FoMoCo factory 12V ignitions prior to electronic ignitions used the pink resistor wire.

 

You said the RJM harness was the engine harness. The resistor wire is in the main harness under the dash.

[Super Birthday Snake 11]

Look at the diagram. The circuit goes from the key to the tach RED, out the tach BLACK through the resistor wire to the POSITIVE (power, + ) side of the coil. The distributor connects to the negative (-) side of the coil. All FoMoCo factory 12V ignitions prior to electronic ignitions used the pink resistor wire.

 

You said the RJM harness was the engine harness. The resistor wire is in the main harness under the dash.

 

Yes sorry, I don't have the standard main harness, I have a painless harness so no resistor wire. I figure that the RJM tach wire will get the power from the ignition and into the red wire on the tach so then if I use my Coil B+ wire on the painless harness to the black that might work?

 

Thanks.

[TheRktmn 15]

The RJM wire is a signal wire and your factory tach does not use a signal so ignore that one.

 

The painless harness should have a wire going to the coil + side. You will need to cut this wire under the dash. It has to be cut, not tapped into. Connect the side from the ignition switch to the tach RED (input). Connect the other end that goes to the coil + side to the BLACK (output) tach wire.

 

As stated earlier, the factory is wired in series. This means that it senses CURRENT, not a wave signal like a modern or three wire tach. The tach has to 'read' the current passing through it to the coil.

 

If you are running an aftermarket ignition like an MSD -6 box, RTR distributor or Pertronix 3 you will need a signal adapter to convert the wave. What ignition system do you have?

Edited May 21, 2014 by TheRktmn
Added aftermarket ignition info

[Super Birthday Snake 11]

The RJM wire is a signal wire and your factory tach does not use a signal so ignore that one.

 

The painless harness should have a wire going to the coil + side. You will need to cut this wire under the dash. It has to be cut, not tapped into. Connect the side from the ignition switch to the tach RED (input). Connect the other end that goes to the coil + side to the BLACK (output) tach wire.

 

As stated earlier, the factory is wired in series. This means that it senses CURRENT, not a wave signal like a modern or three wire tach. The tach has to 'read' the current passing through it to the coil.

 

If you are running an aftermarket ignition like an MSD -6 box, RTR distributor or Pertronix 3 you will need a signal adapter to convert the wave. What ignition system do you have?

 

Makes perfect sense thanks for that. My coil b+ wire goes from fuse box to fuse box but if memory serves I had to use this as part of the ignition to power the EFI, I'll need to double check that. I've just got standard ignition for a 89 Fox.

 

Thanks again for your help.

[danno 128]

If those harnesses you have came with schematics, it would be a real huge help if you could post them. Maybe take a picture of them and post it?

 

The factory tach in a 69/70 Mustang was not at all a tachometer by the standards they started using later. It was an ampmeter. It measured the electrical current going to the ignition coil. It just happens that the current to the coil is exactly proportional to the engine speed.

 

The result is that using a newer wire harness or typical tach wires will not work for a 69/70. It will also not work if you have solid state ignition, like Pertronix. Then you need a tach adapter.

 

The factory tach with factory wires is quite simple. But when an aftermarket harness and other factory options are added, it gets real complicated real quick. Another word of advice, be real careful, or you can burn up the factory tack if not done correctly. If you try something and it does not work, disconnect things immediately.

 

Start with the schematics, and we can see if we can help. Also, do you have electronic ignition?

[danno 128]

I missed reading a part if your discussions. It now seems like you need the tach adapter. Like the others said, contact tachman.

[Super Birthday Snake 11]

Ignition is just standard Fox 89, presume it's electronic.

 

Chassis harness manual:

 

http://www.painlesswiring.com/Manuals/10101.pdf

 

Engine harness manual:

 

http://www.ronfrancis.com/images/TDZ75-INST.pdf

 

With regards to the adapter, what would that accomplish exactly? I e-mailed tachman earlier so will see what he says.

[Super Birthday Snake 11]

Ok, managed to get a bit more info. The RJM harness tach wire comes from the Negative side of the coil so is no good, so I presume the adapter would then turn this into a positive?

 

I think if I can run my Coil B+ to both sides of the tach that will work although that's highly dependent on whether I had to use that for ignition.

 

If I had to use that for ignition then I guess I'd need to get the adapter to work on the RJM tach wire, although I'm not sure how that could be modified to run in series...

[danno 128]

I am confused a bit. I do not see any connection in the RJM harness for a coil. But I guess that does not make any difference.

 

Maybe you will not need a tach adapter? Your question about running the coil b+ side to both sides of the tach gets you close, but that will not work. You need to connect the coil b+ side to the tach. Typically the coil +B side goes to the switched ON side of the ignition switch. This provides +12 volts to the coil when the key is on. Instead of running this wire from the switch to the coil, you need to run it to the tach.

 

Look in figure 8.2 of the painless instructions. The 2 wires for the tach go exactly where the 2 wires to the ballast resistor are. Assume you are using the tach as the ballast resistor, and put it in the exact same place in the wiring. Also be careful about the polarity. The tach MUST be wired with the correct polarity, or you will damage it. I will need to check my schematics to see what is the correct polarity.

[Super Birthday Snake 11]

I am confused a bit. I do not see any connection in the RJM harness for a coil. But I guess that does not make any difference.

 

Maybe you will not need a tach adapter? Your question about running the coil b+ side to both sides of the tach gets you close, but that will not work. You need to connect the coil b+ side to the tach. Typically the coil +B side goes to the switched ON side of the ignition switch. This provides +12 volts to the coil when the key is on. Instead of running this wire from the switch to the coil, you need to run it to the tach.

 

Look in figure 8.2 of the painless instructions. The 2 wires for the tach go exactly where the 2 wires to the ballast resistor are. Assume you are using the tach as the ballast resistor, and put it in the exact same place in the wiring. Also be careful about the polarity. The tach MUST be wired with the correct polarity, or you will damage it. I will need to check my schematics to see what is the correct polarity.

 

You're confused? :)

 

Ok got you, I'm pretty sure my wire 920 (where it has the ballast resistor in the middle) goes from my ignition to power my engine harness. No idea what the polarity is.The RJM harness has a plug for the coil, the coil is stock from an 89 so needs a plug connector and this then picks up power from the harness in general which is from the pink wire 920 I'm sure.

 

I can get an msd tach adapter http://msdignition.com/product.aspx?id=4709&terms=tach%20adapter here in the UK but not sure if it's what I need, I've asked on the MSD forum but not had an answer. I presume this would take the negative signal from the tach wire and make it readable by the tach. That's what Painless said anyway.

[danno 128]

If you replace the ballast resistor in figure 8-2 with teh tachometer, the wire between the coil and the ballast resistor shoudl connect to the pink wire on the tach. The red/green wire on the tach should connect to the ignition switch, where it is shown as "916 RED" in the schematic.

 

Do you have a voltmeter that will also check for resistance ( or ohms)? That might help in the trouble shooting.

[danno 128]

WRONG! I said it should be connected to ethe "916 RED", and I was wrong. It should be connected to the "920 PNK".

 

One more item. You have a wire from your electronic ignition that connects to the + side of the ignition switch to provide power to it. I do not see how this is connected in the schematics you mentioned. But this wire should not be connected to the wire between your coil and tach. It should be connected to the same place the red/green wire on the tach is connected. You can see this in figure 8-3. In this figure, it goes from the ignitin module ot the "920 PNK" wire on the ballast resistor.

 

One final word. If you try this and it does not work immediately, shut everything off. If something is connected wrong, you can damage things if you leave the power on. If you shut it off immediately, you can greatly reduce the chance of this happening.

 

Are you confused enough yet?

[Super Birthday Snake 11]

If you replace the ballast resistor in figure 8-2 with teh tachometer, the wire between the coil and the ballast resistor shoudl connect to the pink wire on the tach. The red/green wire on the tach should connect to the ignition switch, where it is shown as "916 RED" in the schematic.

 

Do you have a voltmeter that will also check for resistance ( or ohms)? That might help in the trouble shooting.

 

Yip got a voltmeter so can check for resistance, what should I be looking for it to be?

 

WRONG! I said it should be connected to ethe "916 RED", and I was wrong. It should be connected to the "920 PNK".

 

One more item. You have a wire from your electronic ignition that connects to the + side of the ignition switch to provide power to it. I do not see how this is connected in the schematics you mentioned. But this wire should not be connected to the wire between your coil and tach. It should be connected to the same place the red/green wire on the tach is connected. You can see this in figure 8-3. In this figure, it goes from the ignitin module ot the "920 PNK" wire on the ballast resistor.

 

One final word. If you try this and it does not work immediately, shut everything off. If something is connected wrong, you can damage things if you leave the power on. If you shut it off immediately, you can greatly reduce the chance of this happening.

 

Are you confused enough yet?

 

 

I'm pretty sure I've got my coil ignition wire (PNK 931) in with the Coil B+(PNK 920) on the ignition switch so what you've said makes perfect sense. I've got a Tach Adapter on the way (MSD 8920) that I managed to pick up cheap here in the UK so I'll give that a go as well. Should be able to give both a go at the weekend and will post my results.

 

Thanks again for all this help, massively appreciated :)

[Super Birthday Snake 11]

Ok so got the Tach working.

 

Had to use the tach adaptor wired with the 920 pink spliced into the red on the tach, the red on the adaptor to the black on the tach and the white on the adapter into the tach RJM wire. Sorted!

 

I tried putting the tach in between the pink 920 wire as per 8.2 on the painless harness but got nothing.

 

Got a new circuit for the dash as well and this one is worse than the last so still no fuel and temp gauge. No idea where to go from now on this.

[danno 128]

Whoo hoo! Got the tach working, good job.

 

I was trying to figure out how you would connect the painless wiring harness in the dash to the factory instrument panel. Is that your next challenge?

[Super Birthday Snake 11]

Whoo hoo! Got the tach working, good job.

 

I was trying to figure out how you would connect the painless wiring harness in the dash to the factory instrument panel. Is that your next challenge?

 

Thanks again for your help :)

 

Well, that's the next thing, the painless wires (fuel sender, instrument lights, turn signals, full beam) and the RJM wires (Engine temp, oil, engine light) I've just hooked into the plug but I get nothing from it other than a few bulbs, am I missing something?

 

I've tried 2 brand new circuits and in the newest I get 2 bulbs on and the older one I get 2 bulbs and the turn signals. Nothing from either gauge.

[Super Birthday Snake 11]

Well, dumb really is the correct use in this thread.

 

My fuel gauge is now working fine, not started the car yet as it's on stands with the clutch off (another thing altogether) but presume my temp gauge will too.

 

The solution was simple, I'd mis-read the instructions/diagram about 30 times and thought that the painless wire referring to voltmeter meant ammeter... :blushing:

 

So I got my voltmeter + wire spliced into the plug on the 4 or 5 connectors and off it goes, works perfectly! The joy of seeing that fuel gauge move was something else.

 

Anyway with that and my tach now working, just need to get the bulbs on the go as only got 2 working and my turn signals don't work either.

[danno 128]

I have been there and know that dumb thing quite well. The bulbs and turn signals should be easier. It is summer, gotta get it done...

[Super Birthday Snake 11]

I have been there and know that dumb thing quite well. The bulbs and turn signals should be easier. It is summer, gotta get it done...

 

Yeah I'll get onto them as soon as I've finished changing my clutch and sorted out my driver door glass allignment. I'll get good use out of this car yet!

 

Thanks for all your help by the way.