[Peter Serio]

Post #20 brings up a good point, if the tach tries to read 30 degrees of dwell off of Pertronix what you end up with is a skew error. As pointed out in several posts above; at idle or just higher your readings may seem perfectly fine but as you increase RPM the error grows. At 3,000 RPMs you may be indicating more or less +-350 "off" and at 5,500 the error is typically around 800 RPMs or greater.

I apologize if I was not clear in my previous post.

When the cars were new AC Delco offered optional transistorized ignition. This was only available from 64-66 on the GTO series, (As I recall that there was a version of this for full size Pontiacs only starting in 1963). For the GTO in 1967 they completely redesigned the option and called that Capacitive discharge. From 1968 to 1970 there was no optional electronic ignition set-up offered for Pontiacs. Hence, GM only used one circuit board which is the one you see most often in the hood tachs. Starting in 1971 with the introduction of the 455HO available in GP, Lemans-GTO and came as stock in all Trans Ams. They introduced a brand new distributor with was small; like a normal points distributor size; but had electronic parts inside (no points). That 1971 design later morphed into what everybody is famular with today: "big cap HEI". Those systems all employ variable dwell timing.


During the early years (64-67) if you ordered a new GTO with the high-performance ignition system AND rally gauges the tach had a different circuit board to accommodate the variable dwell. And there are 2 wires running to the back of the tach instead of just one.

GM was a huge company with almost unlimited resources. As such they attracted some very talented people as engineers and designers. GM would never have (the circuit board) different design part if there would be no reason to do so.


When you have electronic ignition (either vintage or modern; Pertronix, MSD-6A or Mallory you can't read the RPM signature signal same as you can with points @ 30 degrees. In a nutshell you have to "read" off of the edge of the square wave, not the duration.

I know of no adapter, that works in all cases. basically the circuit board needs to be custom made and/or replaced with the right one to read off of the ignition system you are running. And you have to run an additional wire that has 12 volts+ on it from your fuse block to the back of the tach.

I know MSD sells an adapter but from people I have heard back from this only works about 1/2 the time. As a general rule I don't like adapters because you invariably wind up with redundant circuitry; which can slow down response time.

[Old Goat 67]

Well said. Thank you Peter.

Charles

[1beautifuldaughter]

Yes, thanks very much for the great will sorted conclusion

[Judge273]

I was not aware the 455 HO was available in the GP for 71?

[HO LEMANS]

This thread is an eye opener indeed but I now have another question (maybe it's dumb but I'm gonna ask anyway) So if the original tach's cannot correctly read the pertronix signals what about the new reproduction tach's will this affect them the same way?

[1beautifuldaughter]

no question is a bad one, But I'm glad you asked it, lol I just had a talk with my buddy on the phone about exactly that, It would stand to reason that the aftermarket world would see the trend and make it an option at the very least. I guess a simple call could clear that one up, but you will probably end up with someone at the other end that dosn't know. I dont know if its true but I have heard that aftermarket tachs can be picked out from 30ft, that they dont look the same, maybe thats just one version?

[1beautifuldaughter]

From my past experiances with aftermarket parts, they have usually turned out to be like that can opener that buy at the flea market for a dollar that has the little gold made in china sticker that looks like a can opener and it feels like a can opener, but IT WONT OPEN A CAN !

[Old Goat 67]

Originally Posted by 1beautifuldaughter

From my past experiances with aftermarket parts, they have usually turned out to be like that can opener that buy at the flea market for a dollar that has the little gold made in china sticker that looks like a can opener and it feels like a can opener, but IT WONT OPEN A CAN !

But, their reasoning is "Gotcha".

[Peter Serio]

From what I have seen all new reproduction hood tachometers use a modern circuit that will read off of original points or anything upgraded, such as electronic or transistorized. Pertronix MSD-6A or GM-HEI. The key here is to study the wiring.

For the circuit it takes to de-code anything with variable dwell you need at least one transistor in the tachometer circuit. This transistor has to have a 12 volt "+" supply voltage in addition to what is being read (i.e. the RPM signal).

Basically, what you are looking for is a minimum of 4 wires. Black= Ground; Grey = light illumination (nitetime). Red, yellow or orange which is your 12 volt power intput (12 volts + with key on, zero volts with key off). Lastly your signature RPM signal wire which is going to connect from the bottom of the tach to the pulsed or "counter" terminal which is typically the negative side of the coil.

Since there are/ or have been at least 5 or 6 sources of people and/or companies making reproduction hood tachometers.

(Some have been producing tachs for 15- to 20 years or longer) there is a wide variation of case designs, evolution; not to mention possible wire colors.


I did see at least one "early" reproduction which piggybacked the 12 volt "+" supply wire and the nitetime illumination into one wire; for ease of installation I am guessing.
This resulted in the light(s) being on all of the time if the engine was running. This really was not a problem back then as it was one of the first reproductions available. People were eager to have something new that would work.

You could not dim them, the lights are not all that bright to begin with.


These are just the very basics, if someone would be kind enough (our host, maybe?) to post photo(s) of available tachs.

Originally Posted by HO LEMANS

This thread is an eye opener indeed but I now have another question (maybe it's dumb but I'm gonna ask anyway) So if the original tach's cannot correctly read the pertronix signals what about the new reproduction tach's will this affect them the same way?

[HO LEMANS]

thanx for the detailed reply that is really helpful I am including a couple of pics because I am not seeing the red or yellow wires here so if you could "decode" the wiring on this as I received no diagram with this hoodtach and now I am confused --- your help is VERY much appreciated
thank you
H.O.

[Peter Serio]

The wiring appears to "change colors" as it runs thru the plug. How many wires total, in the plug, it looks like just 3 to me.

I see 3 gray wires, 3 black and one brown at the tach baseplate. Then in the plug-in I am seeing just one black, one brown and what looks like a black with a pink tracer? Are there wires that change colors and one or two that stay the same? Which wires start out as more than one and then as they join into one connector what color comes out?

[HO LEMANS]

pics of the inside of the tach
Also to answer the question of the wire that changes color at the plug -- that would be the dark brown wire as it is the black with pink tracer when it comes out of the plug

[Peter Serio]

Black= Ground, Gray = 12 volts "+" power whenever the key is on or run position. (Take off of the fuse block, one of the acc power ports.) I would run it thru an extra 2&1/2 or 3 amp in-line glass fuse for safety. MAKE sure it's 12 volts + key on; ZERO volts=key off!!!

Black w/pink tracer is your RPM signal input (negative side of the coil) in most cases.

[1beautifuldaughter]

I inquired about a repo hood tach that is sold at The Parts Place,Supposedly looks exactly the same as original with upgraded lighting and the heater tube connection, and asked if it would work with petronix ign. they answered,,, This is a reproduction of the original and is disigned to work with original point type system.
I wonder if it says anything in the paperwork to that affect when you buy one????

[HO LEMANS]

1beautifuldaughter the tach in the above pics is the one Peter and I have been discussing is indeed from The Parts Place and from what Peter has concluded (and I absolutely trust his word) it should work fine with the Pertronix installed
H.O.

[1beautifuldaughter]

thanks I appreciate it, good to know.

[1beautifuldaughter]

Well I ordered a hood tach from the parts place that says it accepts the factory heater vacuum hose and that it is 100% like the original in fit and finish. It is very nice but not the same as original, The original tach on my car has a chimney vent hook up for the hose from the heater, the replacement comes with some little plastic vent that you screw in somewhere different thats half the size. I guess I'm supposed to drill an additional hole in the hood. Pisses me off! and the vacuum hose is very small compared to the much larger original. I just don't get why things can't be described as they are, Is it me? I am pretty certain that my tach is original. does anyone know about this? I sent them a mail to see if I can return it. I think I would rather have the original tach not working properly than something that is clearly a repo. Am I wrong? If something is supposed to be exactly like the original shouldn't a blind guy not be able to tell the difference ?? If they take this back I guess I'm going to have to see if I can get mine repaired and/or re born in some way. Opinions, If you are perfecting a car ultimately to make it best it can be for sale one day, should I consider this a worth while issue? I would think a purist would rather have the original part on a are car with an issue than a improper repo. ???????

[1beautifuldaughter]

my original tach is on the right in the photo, The word Pontiac is not the same size and the RPM part is different, not that this is a problem, but still . Its not the same. And I'm sure the factory made the hose this larger size for a reason. I think its time to retire from this hobby and start collecting die cast.

[HO LEMANS]

I wonder if they (whoever supplies the Parts Place)the style of the hood tachs or if it's just because the one I ordered was for the R/A II - IV because as can be seen in the pic mine does have the drain where it belongs unlike your new one but I also see that my light sockets are black not white ?!?!?

[1beautifuldaughter]

Yes I see now in your photo yours did come with the proper vacuum port, Damn, So close yet so far ! I can't see where a ram air car of any sort would make a difference. I think its possible its just a different version from a different diet like you said. It also has a sticker on it that says it was tested in early 2010. What the frigg, I also see that yours has the light plug closer together like original, this one has a larger bulge underside for the tach mechanicals and the plugs are further away, My hood has a 3 3/4 in hole and the template they sent has a 4 1/4 hole, I don't even know it will fit well or be able to remove the lights without removing the tach. I couldn't try it because I didn't want to take it out of the vacuum packed wrapping its in so they will consider taking it back. And it was packaged like a monkey at the zoo did it, You ever see a square box look round? and if you shake it ,it sounds like cracker jacks. Well enough complaining. I have a bad feeling about this, They charged me 17.00 shipping and now if they take it back they will probably ask me to pay another 17.00 even though its not what they said it was. I can hear it now " Well Sir this is the best part available on the market today" " you should have asked before you bought it" I don't know we'll see. If I have ti spend almost 40.00 on shipping for nothing I will probably try to put the correct vent on it myself and chalk it up as another aftermarket experience. God help the poor fool who has to buy floor and trunk panels these days and make them fit ! OH I think I said I was going to stop complaining, thanks for pointing out that yours is correct,thats going to be helpful in my argument ,,Appreciate it