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1972 454 Corvette tachometer

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Old 08-15-2017, 12:10 PM
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jimmyhall
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Default 1972 454 Corvette tachometer

New member to corvette forum. Well actually a new member to any forum. I have a 1972 corvette 454 convertible. I have two issues and was wondering if the are connected. My wiper door has a mind of its own opens and closes at will. I checked the filter and seams to be ok with lots of vacuum. My tachometer also does not work. I read that the solenoid for the wiper door is attached to the tach. Would the solenoid cause a tach issue as well?


Very new to online so if I posted incorrectly sorry.
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Old 08-15-2017, 01:20 PM
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Alan 71
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Hi jh,
Welcome!
A 72 convertible big block!!!
Is that it in your avatar?
VERY nice.

The tachometer is connected to the distributor by a simple sheathed cable. The problem with your tachometer is unlikely to have anything to do with the car's vacuum systems.
You might begin by checking the connection of the cable to the distributor and the condition of the end of the cable in the sheath. The end can sometimes been 'rounded off' so the distributor gear doesn't spin it.

The wiper door problem….
Is this a new problem?
Do the wipers work when the door is open?
Does the door open when the wiper switch isn't pushed?
Does the door close when the wipers are running?
Does the door open at start up?
Does it open when you shut the engine off?

Do you happen to have an Assembly Instruction Manual' (AIM) or a 'GM Chassis Service Manual' for your car.
Regards,
Alan

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Old 08-15-2017, 01:22 PM
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Originally Posted by jimmyhall View Post
New member to corvette forum. Well actually a new member to any forum. I have a 1972 corvette 454 convertible. I have two issues and was wondering if the are connected. My wiper door has a mind of its own opens and closes at will. I checked the filter and seams to be ok with lots of vacuum. My tachometer also does not work. I read that the solenoid for the wiper door is attached to the tach. Would the solenoid cause a tach issue as well?


Very new to online so if I posted incorrectly sorry.
You have two independent issues. The "tach solenoid" is actually a blocking solenoid for the wiper door system that just happens to mount on the back of the tach (and thus the name). This solenoid has no function to the tachometer.

So, for the tachometer, first I'd pull the tach cable and put a drill on it in reverse... this should make your tach operate. If it does then your issue is in the distributor (and common). The factory distributors have a history of taking out the cross gears that drive the tachometer. In most cases the teflon button inside the distributor has failed (in older cars the wall of the distributor housing has failed). In your case it might be an easy fix... but you'd need to pull the distributor and investigate.

In the picture below you'll see a brass button... but this is a distributor housing that has been repaired. In your case there would be a teflon button where the brass is. You should also check the upper and lower busing clearance.



Another item that can fail... The coupler for the cross gear.


If you look at the picture below, you'll see a tit marked "Here".. Your distributor should have a hole there.. (this is an early distributor) and inside the hole is where the teflon bushing is inserted.



Teflon button.


The wiper door raising at will... If I had to guess... and I'm only guessing...
Two things... the wiper switch may have failed or you have a small hose loose somewhere. It takes vacuum on the small hose side to hold the wiper door closed... so if you lose vacuum the door will open. The wiper switch controls the power that runs to the tach solenoid and the job of this solenoid is to block vacuum on the small hose side... So I'd check your small hose's at the relay, the over-ride first... I've pasted a diagram below to help you. It's also possible the relay is going bad too...

Check the hoses at A E C and H in the diagram below.

Willcox




Last edited by Willcox Corvette; 08-15-2017 at 01:27 PM.
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Old 08-15-2017, 01:22 PM
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The tach and wiper door are two separate problems.

The tach has a mechanical cable that connects to the distributor. Do you still have the original distributor in the car? Is the tach cable attached to the distributor?

The wiper door issue can be a few things. A better description of what it is actually doing would help diagnose.
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Old 08-15-2017, 01:39 PM
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Originally Posted by Alan 71 View Post
Hi jh,
Welcome!
A 72 convertible big block!!!
Is that it in your avatar?
VERY nice.

The tachometer is connected to the distributor by a simple sheathed cable. The problem with your tachometer is unlikely to have anything to do with the car's vacuum systems.
You might begin by checking the connection of the cable to the distributor and the condition of the end of the cable in the sheath. The end can sometimes been 'rounded off' so the distributor gear doesn't spin it.

The wiper door problem….
Is this a new problem?
Do the wipers work when the door is open?
Does the door open when the wiper switch isn't pushed?
Does the door close when the wipers are running?
Does the door open at start up?
Does it open when you shut the engine off?

Do you happen to have an Assembly Instruction Manual' (AIM) or a 'GM Chassis Service Manual' for your car.
Regards,
Alan
Thank you for your help.


Yes Wipers work fine when you hit the button.
Yes the doors opens and closes while driving.
Yes door opens at start up. (I will have to check and make sure it does this all the time at start up).
Yes it opens when you shut the engine down. then a few seconds later you can push the door down manually.
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Old 08-15-2017, 01:44 PM
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Originally Posted by Willcox Corvette View Post
You have two independent issues. The "tach solenoid" is actually a blocking solenoid for the wiper door system that just happens to mount on the back of the tach (and thus the name). This solenoid has no function to the tachometer.

So, for the tachometer, first I'd pull the tach cable and put a drill on it in reverse... this should make your tach operate. If it does then your issue is in the distributor (and common). The factory distributors have a history of taking out the cross gears that drive the tachometer. In most cases the teflon button inside the distributor has failed (in older cars the wall of the distributor housing has failed). In your case it might be an easy fix... but you'd need to pull the distributor and investigate.

In the picture below you'll see a brass button... but this is a distributor housing that has been repaired. In your case there would be a teflon button where the brass is. You should also check the upper and lower busing clearance.



Another item that can fail... The coupler for the cross gear.


If you look at the picture below, you'll see a tit marked "Here".. Your distributor should have a hole there.. (this is an early distributor) and inside the hole is where the teflon bushing is inserted.



Teflon button.


The wiper door raising at will... If I had to guess... and I'm only guessing...
Two things... the wiper switch may have failed or you have a small hose loose somewhere. It takes vacuum on the small hose side to hold the wiper door closed... so if you lose vacuum the door will open. The wiper switch controls the power that runs to the tach solenoid and the job of this solenoid is to block vacuum on the small hose side... So I'd check your small hose's at the relay, the over-ride first... I've pasted a diagram below to help you. It's also possible the relay is going bad too...

Check the hoses at A E C and H in the diagram below.

Willcox



Just purchased the car, but the distributer is not the original to the car. I will look tonight and see if I can see this issue.
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Old 08-15-2017, 01:54 PM
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Alan 71
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Hi jh,
Take a look at some of the Willcox suggestions for the wipers.

It helps to understand the wiper and headlight system to figure out why they're acting up.

There are 2 'sides' to the vacuum systems.

The 'control' side is made up of the vacuum switches, relays, and the small hoses. The 'control' side sends a vacuum 'signal' to the actuators to open or close.

The 'servo' side is made up of the vacuum reservoir, the actuators, and the large hoses. The 'servo' side uses vacuum to open or close the doors when they receive the 'control' signal.

In addition there is a check-valve (gray metal, one hose in, two hoses out; one small ('control' vacuum) and one large ('servo' vacuum);and a filter (white plastic).
This should be your FIRST test.
Check the valve it is working when you can blow in to the 2 port side but can't 'suck' from the 2 port side.

Start there! It gets crazier after that test!
Regards,
Alan

This is the typical connection for the tachometer cable to the distributor body.

Last edited by Alan 71; 08-15-2017 at 01:57 PM.
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