77 Tach probs
Thread Starter
Pro
Joined: Apr 2005
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From: Illinois, USA
77 Tach probs
Okay, I'm looking for a little assistance on this one. 
I've replaced the circuit board on the tach and it has gone from floating about 500 rpm to drop to zero on key on. I tried a bypass of the tach filter with no success.
When I first fired up the whole deal, revving the engine generated a tiny amount of movement, but that's all I've seen so far. The new circuit board didn't fit as well as the original (large), so I am not sure if the board was shorted out during the connection of the plug to the back of the tach.
Of course, its no small task taking that dash apart (used it as a reason to clean it up good), but I want to make the next one count. Did I miss something? Any way I can throw a signal to the tach to see it climb (if so, what is the voltage range?)
thnking about pulling that dash again.
Thanks for your wisdom.
Brian.
I've replaced the circuit board on the tach and it has gone from floating about 500 rpm to drop to zero on key on. I tried a bypass of the tach filter with no success.
When I first fired up the whole deal, revving the engine generated a tiny amount of movement, but that's all I've seen so far. The new circuit board didn't fit as well as the original (large), so I am not sure if the board was shorted out during the connection of the plug to the back of the tach.
Of course, its no small task taking that dash apart (used it as a reason to clean it up good), but I want to make the next one count. Did I miss something? Any way I can throw a signal to the tach to see it climb (if so, what is the voltage range?)
thnking about pulling that dash again.Thanks for your wisdom.
Brian.
Heel & Toe
Joined: Aug 2004
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I posted the same problem with my 76 yesterday and have not gotten a reply. My *guess* is that my plug is not re-seated firmly as I can see a little bit of the lower post, and I do not recall if that was the case with the original circuit board. I do not want to remove my dash again because it was an all day project and it might not get me anywhere anyway
Thread Starter
Pro
Joined: Apr 2005
Posts: 603
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From: Illinois, USA
Originally Posted by wmrvette1976
I posted the same problem with my 76 yesterday and have not gotten a reply. My *guess* is that my plug is not re-seated firmly as I can see a little bit of the lower post, and I do not recall if that was the case with the original circuit board. I do not want to remove my dash again because it was an all day project and it might not get me anywhere anyway
When you try to push the connection on, does it seem like the circuit board has a lot more "flex" than you'd expect? I'm afraid to crank on it for fear of just wrecking it, but then again if it already shorted out -- it could be shot.
Based on the connector, it seems like it should make electrical contact pretty quick upon insertion, so I am hoping it's all the same. There has to be some kind of test for this. I assume the board is a signal amp to bring the tach signal up high enough to drive the gauge (and smooth it out).
My experience probably was a full day, too ...but it was spread out over several days as I cleaned the interior. I realized during the process that the interior was painted (who'd have thunk it?).
Was your experience similar?
Heel & Toe
Joined: Aug 2004
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Yes, if I push on the plug more, the tach faceplate definitely flexes, so I do not go full strength (this I do while facing the dash and reaching up under the dash to the plug). So I assume the board is flexing, but it should not break as it is attached at three points. The Nordskog after-market board required the three washers to be put on the contacts between the board and the tach. Was yours the same? I would not think you shorted the board if you had disconnected the battery, but maybe the tach still carried a charge??
I did the entire job in literally one day, the worst part being getting the dash up over the steering column, even though I had dropped the two bolts that secure the steering column to the lower dash frame, it gave just enough play on the column to wedge the dash pad up over the column...miserable experience...
I did the entire job in literally one day, the worst part being getting the dash up over the steering column, even though I had dropped the two bolts that secure the steering column to the lower dash frame, it gave just enough play on the column to wedge the dash pad up over the column...miserable experience...
Thread Starter
Pro
Joined: Apr 2005
Posts: 603
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From: Illinois, USA
Originally Posted by wmrvette1976
Yes, if I push on the plug more, the tach faceplate definitely flexes, so I do not go full strength (this I do while facing the dash and reaching up under the dash to the plug). So I assume the board is flexing, but it should not break as it is attached at three points. The Nordskog after-market board required the three washers to be put on the contacts between the board and the tach. Was yours the same? I would not think you shorted the board if you had disconnected the battery, but maybe the tach still carried a charge??
I did the entire job in literally one day, the worst part being getting the dash up over the steering column, even though I had dropped the two bolts that secure the steering column to the lower dash frame, it gave just enough play on the column to wedge the dash pad up over the column...miserable experience...
I did the entire job in literally one day, the worst part being getting the dash up over the steering column, even though I had dropped the two bolts that secure the steering column to the lower dash frame, it gave just enough play on the column to wedge the dash pad up over the column...miserable experience...
The battery was not disconnected, but during key off, there should be no power to the tach, so it should have the same effect. There really isn't anything to store power at the tach, so I don't think that was an issue.
It seems hard to believe that we are the only two that had additional tach problems during a repair.
miserable experience doesn't even cut it. I basically had to pull all the dash/trim off in front of the driver seat ....took the chance to clean it up, though. I guess the interior had been painted before -- is that unique to vette owners? Thats' the first time I had seem that.
Brian.
Racer
Joined: Jun 2004
Posts: 252
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From: Shawnee KS
I just installed a tach board in my 75 although I am not sure who made it. It was also very small and had the three insulating washers. When I hooked mine up it stayed at 3500 RPM and would rise wbut would not drop below 3500 RPM. I tried to adjust it by turning the potentiometer but couldn't get it to go any lower than around 2500 RPM. I sent it off to a forum member (Mike Mercury) and he was able to calibrate it for a reasonable cost. There were some other things that he offered to do as the internal workings were a little off, but it seemed close enough for my casual driving needs. He sent along pictures at various RPM settings and even a video. High quality work, I'd say. He had to replace the potentiometer that was on it as the one that is on there was way too sensitive and would change just by vibration. He recommended tach boards made by Gardner. On my board, the receptacle does not seat all the way down on the posts. There is some post visible at the bottom so I think that's pretty normal. Good luck!
Last edited by brystheguy; May 24, 2005 at 01:18 PM.
Thread Starter
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Joined: Apr 2005
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From: Illinois, USA
I checked the box and mine says "Corvette America" as the vendor for the part. I do recall the potentiometer on the board, but did not adjust it (it was curiously "horizontal" which suggests just they just install them.
Did Mike recommend where I could find a "Gardner" board for the vette? I made my purchase from Eckler's, if that helps.
Did Mike recommend where I could find a "Gardner" board for the vette? I made my purchase from Eckler's, if that helps.
Racer
Joined: Jun 2004
Posts: 252
Likes: 0
From: Shawnee KS
Originally Posted by C3nMe
I checked the box and mine says "Corvette America" as the vendor for the part. I do recall the potentiometer on the board, but did not adjust it (it was curiously "horizontal" which suggests just they just install them.
Did Mike recommend where I could find a "Gardner" board for the vette? I made my purchase from Eckler's, if that helps.
Did Mike recommend where I could find a "Gardner" board for the vette? I made my purchase from Eckler's, if that helps.
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Heel & Toe
Joined: Aug 2004
Posts: 21
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Brian,
Someone posted this link in my post for my 76 tach problem. I have not read it yet.
http://www.corvettefaq.com/c3/RebuildSpeedo.htm
Bill
Someone posted this link in my post for my 76 tach problem. I have not read it yet.
http://www.corvettefaq.com/c3/RebuildSpeedo.htm
Bill
Pro
Joined: Nov 2004
Posts: 659
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From: Flower Mound Texas
I went through this a few months ago with my 75. There are two problems here. First, you have to use the insulating washers. Without these washers installed, my tach would barely move, even on heavy throttle.
Once I figured out the washer problem (OK, my buddy actually figured it out!) the second problem was with the POS tach board that had the potentiometer. My symptom was a little different - I could rev up to 3500 RMP and then the tach would drop to zero. I finally trashed the board and bought a complete tach assembly from Corvette Clocks by Roger. For grins and giggles, I put the tach board from Roger on my old tach and it worked fine, which proved to me at least that the tach board was the cause of the second problem. I have had dialogue with multiple members on this forum who have had the same problems with these circuit boards. In my case, I bought it from Mid-America.
Also during this ordeal I bought a tach filter and installed it. In my case, it did not make any difference other than cause confusion during the troubleshooting process. I've seen on this forum where lack of a tach filter can cause some symptoms similar to what you're describing, but in my case I think I pretty much wasted my time even considering it. Good Luck!
Once I figured out the washer problem (OK, my buddy actually figured it out!) the second problem was with the POS tach board that had the potentiometer. My symptom was a little different - I could rev up to 3500 RMP and then the tach would drop to zero. I finally trashed the board and bought a complete tach assembly from Corvette Clocks by Roger. For grins and giggles, I put the tach board from Roger on my old tach and it worked fine, which proved to me at least that the tach board was the cause of the second problem. I have had dialogue with multiple members on this forum who have had the same problems with these circuit boards. In my case, I bought it from Mid-America.
Also during this ordeal I bought a tach filter and installed it. In my case, it did not make any difference other than cause confusion during the troubleshooting process. I've seen on this forum where lack of a tach filter can cause some symptoms similar to what you're describing, but in my case I think I pretty much wasted my time even considering it. Good Luck!
