78 - distributor/tach combo makes error?
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Joined: Dec 2005
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From: Brownsboro Texas
78 - distributor/tach combo makes error?
The 78 I have has a 71 pumped up engine. It has the original 78 distributor and tach. However, the tach reads constantly almost double what the actual speed is. The tack works, but is not accurate.
Any of you have an idea as to what I could check to make it right?
Buddy
Any of you have an idea as to what I could check to make it right?
Buddy
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
Have you double checked the engine RPM with a timing light or auxiliary tach?
I suspect if it's really that far off that the tach board is shot. I suspect a new one can be ordered fairly easily, and the instruments are much easier to get to for that year.
A member named Aceco is very keen when it comes to tach boards...maybe send him a PM.
I suspect if it's really that far off that the tach board is shot. I suspect a new one can be ordered fairly easily, and the instruments are much easier to get to for that year.
A member named Aceco is very keen when it comes to tach boards...maybe send him a PM.
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
Not really but Aceco might know that for sure.
The tach board is where all the calculations come from since it just interprets the rotations of the rotor via an electric signal. It converts that into what you see you needle do through the mechanism of the tach.
I can't really imagine there is a mechanical problem unless the results are constantly changing. If it's got the same results while being wrong at all times I would suspect the tach board first.
Again, Aceco is a whiz with the electronic tachs and will have some valuable input.
Former Vendor
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Buddy,
If the tach is reading double, then it has to be the board. The tach reads a square single from the distributor input and there is nothing that would double this reading besides the board!
If the tach is reading double there is nothing else it could be. If you want to remove the tach and send it to us, we will test it for you for free.
If you need the board, we have them in the forum special price section of our site too, here is the link: http://willcoxcorvette.com/product_i...ducts_id=25779
Willcox
If the tach is reading double, then it has to be the board. The tach reads a square single from the distributor input and there is nothing that would double this reading besides the board!
If the tach is reading double there is nothing else it could be. If you want to remove the tach and send it to us, we will test it for you for free.
If you need the board, we have them in the forum special price section of our site too, here is the link: http://willcoxcorvette.com/product_i...ducts_id=25779
Willcox
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Le Mans Master
Joined: Sep 2002
Posts: 5,233
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From: Myrtle Beach SC
Remove tach/speedo cluster cover. You also have to remove small screws that attach the lens cover from the tach/speedo.
than you'll remove the hex screws that have the tach/speedo bezel from the actual tach speedo devices. Three screws and you've got the tach out of the vehicle. You'll see the board on the back side.
Note the location of the screws and nuts and the hole location on the board before removal so you get them back correctly.
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From: Brownsboro Texas
Thanks hunt4. I did this once on my 82, but in that case, it didn't fix a thing. The 82 tach still does not work and I put a new board and a new filter and checked all the connections at the distributor. I have temporarily given up on the 82 tach.
But, once the new board gets here, I will see if it fixes the 78.
But, once the new board gets here, I will see if it fixes the 78.
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Joined: Dec 2005
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From: Brownsboro Texas
It don't work - at all
Well, the new tach board came in thanks to Wilcox. I got around to putting it in today, and guess what - It don't work at all with the new board. I took it out and put it back three times to be sure I got everything tight. 
So, I put the old board back and it continues to read in error.
The tach board package says for 78-79 Corvette, so I assume it is the correct one. However, the circuits ARE different than my original.
Any suggestions as to what should be my next step? All the parts are still lying in the front seat.
Thanks,
Buddy
So, I put the old board back and it continues to read in error.
The tach board package says for 78-79 Corvette, so I assume it is the correct one. However, the circuits ARE different than my original.
Any suggestions as to what should be my next step? All the parts are still lying in the front seat.
Thanks,
Buddy
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Well, the new tach board came in thanks to Wilcox. I got around to putting it in today, and guess what - It don't work at all with the new board. I took it out and put it back three times to be sure I got everything tight.
So, I put the old board back and it continues to read in error.
The tach board package says for 78-79 Corvette, so I assume it is the correct one. However, the circuits ARE different than my original.
Any suggestions as to what should be my next step? All the parts are still lying in the front seat.
Thanks,
Buddy
So, I put the old board back and it continues to read in error.
The tach board package says for 78-79 Corvette, so I assume it is the correct one. However, the circuits ARE different than my original.
Any suggestions as to what should be my next step? All the parts are still lying in the front seat.
Thanks,
Buddy
Contact Willcox, and arrange to send the tach board and your distributor to them. He offered to test it and I suggest you take him up on that. Also maybe look for the tach board part number on the one that you pulled out because it may NOT even be '78 correct and the one from Willcox IS.
Just another possibility.
Thread Starter
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Joined: Dec 2005
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From: Brownsboro Texas
My tach on the 78 is now working properly 
Thanks for a fair amount of help from Wilcox
I ordered the new circuit board from Wilcox and it arrived quicker than I could get to putting it in. At first, I put it in and the tach did not operate at all.
After emails with Wilcox I learned that inside that little plastic pouch the board came in are 3 small insulating washers that I needed to use. Once I did that, everything just went into place.
I now have a good tach again. NOW, I have to do another thread on the speedo!!!!
thanks to all of you that contributed and thanks a whole lot to Wilcox for the amount of time he spent answering my emails, even over the holidays.
Buddy
Thanks for a fair amount of help from Wilcox
I ordered the new circuit board from Wilcox and it arrived quicker than I could get to putting it in. At first, I put it in and the tach did not operate at all.
After emails with Wilcox I learned that inside that little plastic pouch the board came in are 3 small insulating washers that I needed to use. Once I did that, everything just went into place.
I now have a good tach again. NOW, I have to do another thread on the speedo!!!!
thanks to all of you that contributed and thanks a whole lot to Wilcox for the amount of time he spent answering my emails, even over the holidays.
Buddy
Le Mans Master
Joined: Sep 2002
Posts: 5,233
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From: Myrtle Beach SC
Wilcox does a great service for forum members and is always readily available...a nice service for forum members. I've used them/him as well. It would be instructive if the boards came with instructions in the first place which would avoid the guesswork. But than that's the nature of the aftermarket and these tach circuit boards.
Le Mans Master
Joined: Oct 2005
Posts: 6,245
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From: Glen Allen, VA
On you speedo, if it's not working, disconnect the cable from the trans, then chuck the cable in drill and run it backwards. If the speedo works, then you know the problem is in the trans, and my guess would be you just need new speedo gears.
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From: Brownsboro Texas
C3 4ME,
You beat me to the punch. Yes, I did the drill motor trick and the speedo works. My real question regards the drive gear. the driven gear is fine.
I bought new seals for the speedo cable and put them in as it was leaking. While doing this, I made sure the driven gear and cable were properly seated and they fit correctly.
Since I have not done much on auto trans before, it will be new territory for me.
Can the drive gear be removed from the trans without pulling the tail shaft? Is there a simple way of changing out the drive gear?
I have read a couple of other threads on the forum regarding the calculations for getting the indicated speed close to actual speed with having the correct drive and driven gear combination. So, I am interested in how to get the gear out quickly and easily.
thanks for your help and thoughts!
You beat me to the punch. Yes, I did the drill motor trick and the speedo works. My real question regards the drive gear. the driven gear is fine.
I bought new seals for the speedo cable and put them in as it was leaking. While doing this, I made sure the driven gear and cable were properly seated and they fit correctly.
Since I have not done much on auto trans before, it will be new territory for me.
Can the drive gear be removed from the trans without pulling the tail shaft? Is there a simple way of changing out the drive gear?
I have read a couple of other threads on the forum regarding the calculations for getting the indicated speed close to actual speed with having the correct drive and driven gear combination. So, I am interested in how to get the gear out quickly and easily.
thanks for your help and thoughts!