75 Tach wiring @ the coil
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,173
Likes: 1
From: Liberty Missouri
75 Tach wiring @ the coil
I am about to install a new distributor in my car that’s not an HEI. Where does the tach lead go on the coil + or -. I’ve been through the search function and am tired of learning how to wire the back of the tach
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I keep this handy, you'd never guess how many people ask this question. (I deleted the dia., it's not needed in this).
I just re-read this too! The factory tach will not work with the regular distributor. Are you running an aftermarket one? If you are trying to hook up the factgory 75 tach to a non hei (points) distributor, it will not work.
Willcox
I just re-read this too! The factory tach will not work with the regular distributor. Are you running an aftermarket one? If you are trying to hook up the factgory 75 tach to a non hei (points) distributor, it will not work.
Willcox
Last edited by Willcox Corvette; Jul 20, 2009 at 07:03 PM.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,173
Likes: 1
From: Liberty Missouri
At the moment I have it hooked to a factory HEI that is powered by a MSD 6A box. I am trying to change over to a Mallory Unilite distributor powered by the 6A box. I have an MSD tach adapter I’m just not sure if I need to run it or not. I’ve seen guys running similar setups and their tack is working. I just don’t know if these are current or voltage triggered tachs.
Instructor
Joined: Jun 2009
Posts: 129
Likes: 2
From: Nashville, TN
so I have a computer science degree and have read alot of electrical wiring diagrams in my life.... I cannot figure out which leads goto the tach in the diagram? little help....
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Have you gone to the MSD web site. They have PDF instructions on how to wire the boxes up. If you can't find this or don't have it email me, I have a PDF copy on my office computer I can send you or post back and I'll email it here to me and take a look at it. Maybe some else that has more MSD exp. will chime in. . . . If I remember correctly do you not plug a wire in to the 6AL box?
Oh, and just a note: MSD will not tell you and I can not find it anywhere in the instructions but you should never charge a car with the box hooked up to power. And a super no no is to try to start it with a charger hooked up to it. I never knew this until I called the tech line after blowing another box!
Willcox
Oh, and just a note: MSD will not tell you and I can not find it anywhere in the instructions but you should never charge a car with the box hooked up to power. And a super no no is to try to start it with a charger hooked up to it. I never knew this until I called the tech line after blowing another box!
Willcox
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
so I have a computer science degree and have read alot of electrical wiring diagrams in my life.... I cannot figure out which leads goto the tach in the diagram? little help....
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
You need a square signal to pulse the tach on this car. I'll go head and email the schematic to my home later tonight and look at it. I think there is a tach connector on the side of the box. God knows I should remember but I don't. I have the 6AL in two of my cars. http://www.msdignition.com/instructi...pdf?terms=6200
Willcox
Last edited by Willcox Corvette; Jul 20, 2009 at 07:10 PM.
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Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,173
Likes: 1
From: Liberty Missouri
Have you gone to the MSD web site. They have PDF instructions on how to wire the boxes up. If you can't find this or don't have it email me, I have a PDF copy on my office computer I can send you or post back and I'll email it here to me and take a look at it. Maybe some else that has more MSD exp. will chime in. . . . If I remember correctly do you not plug a wire in to the 6AL box?
Oh, and just a note: MSD will not tell you and I can not find it anywhere in the instructions but you should never charge a car with the box hooked up to power. And a super no no is to try to start it with a charger hooked up to it. I never knew this until I called the tech line after blowing another box!
Willcox
Oh, and just a note: MSD will not tell you and I can not find it anywhere in the instructions but you should never charge a car with the box hooked up to power. And a super no no is to try to start it with a charger hooked up to it. I never knew this until I called the tech line after blowing another box!
Willcox
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The tach on your car picks up square signal from the distributor. This wire must be routed to the msd box. It should be preset to pulse for a 8 cylinder car from the factory.
The tach already receives power and ground thru the stock wiring harness at the back of the tachometer. If you turn on the ignition right now the tach should go to what we call a zero set! This is power and ground applied and no pulse signal coming in from the distributor. Basically the tach is waiting for signal.
So if I’m correct, the wire that went to the distributor on the “tach” connector should now pass on to the side terminal on the MSD box and you should be fine.
The tach already receives power and ground thru the stock wiring harness at the back of the tachometer. If you turn on the ignition right now the tach should go to what we call a zero set! This is power and ground applied and no pulse signal coming in from the distributor. Basically the tach is waiting for signal.
So if I’m correct, the wire that went to the distributor on the “tach” connector should now pass on to the side terminal on the MSD box and you should be fine.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Also, there is a tach filter between the wires for the tach and the distributor. I would highly advise using this filter in your new system.
I should have answered you a little simpler above. No, if the tach is stock, it is waiting for signal and that should be all you need to supply from the MSD box. No voltage should be applied to the signal wire.
Make sense? (oh I deleted the pic, I didn't know you were working with MSD).
Willcox
I should have answered you a little simpler above. No, if the tach is stock, it is waiting for signal and that should be all you need to supply from the MSD box. No voltage should be applied to the signal wire.
Make sense? (oh I deleted the pic, I didn't know you were working with MSD).
Willcox
Last edited by Willcox Corvette; Jul 20, 2009 at 07:28 PM.
Instructor
Joined: Jun 2009
Posts: 129
Likes: 2
From: Nashville, TN
Wilcox, here's what I'm taking from what you're written:
a stock tach will have 3 inputs
1: + (12V)
2: - (GND)
3:the square wave from the distributor (stock) or MSD box (if installed)
correct?
a stock tach will have 3 inputs
1: + (12V)
2: - (GND)
3:the square wave from the distributor (stock) or MSD box (if installed)
correct?
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Yes, here is the board plugs.
Here is where the 6AL generates signal.
Willcox
Hold on, I'm not done. . . Check back.
Here is where the 6AL generates signal.
Willcox
Hold on, I'm not done. . . Check back.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Ok, this may do it! I hope and god knows if I'm wrong someone chime in. I'm trying to type a letter for a guy on the C4 fourm. . . He really got drilled by another shop... so bad he was ready to sell the car. . . To the shop. . . .
Oh, the colors of the wires are only to show the wires. I did not look at the schematic to pull the color for you. I think the tach wire is white for the filter.
Oh, the colors of the wires are only to show the wires. I did not look at the schematic to pull the color for you. I think the tach wire is white for the filter.
Last edited by Willcox Corvette; Jul 20, 2009 at 07:55 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
so I have a computer science degree and have read alot of electrical wiring diagrams in my life.... I cannot figure out which leads goto the tach in the diagram? little help....
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
Is there a reliable and easy way to test (with a multimeter) if the output from the distrubtor is within spec? Maybe my tach really is bad?
My hunch is the tach is fine but its wired wrong. With the car off its shows ~2200, once I turn the key on it goes to ~3600 and stays there.
Last post. . . You can't test the output with a multi meter. You'll need an oscilloscope to ready the signal.
Instructor
Joined: Jun 2009
Posts: 129
Likes: 2
From: Nashville, TN
Thank you so much willcox!
Pro
Joined: Jul 2004
Posts: 605
Likes: 0
From: Alta Loma CA
I'm wondering how you got it to work. I have my same tach wired directly to the MSD output. It has been fine for several years but lately has only worked intermitently.
So I'm wondering how did you resolve the issues?
So I'm wondering how did you resolve the issues?
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I just noticed, you have a 73. Did someone install an electronic tach in the car?
If so, if you are going thru the filter, you may have a loose wire where it goes in (or out of) the actual filter.
They tend to break over time. Check that and then if you don't see a problem there, post back.
Willcox
If so, if you are going thru the filter, you may have a loose wire where it goes in (or out of) the actual filter.
They tend to break over time. Check that and then if you don't see a problem there, post back.
Willcox
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,173
Likes: 1
From: Liberty Missouri
I if you have a 73 and it does have an electronic tach I would wander if the needle was removed when it was installed. If you pull the needle off for installation you need to power it up before you put the needle back on. FYI ….. My tach reads 0 when the car is off when I turn on the switch it goes to 1500 and doesn’t move until I get above that rpm. That drove me nutz for the first few months now I just accept that is how it is….yep I gave up:o