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| The two instruments that Corvette drivers pay the most attention to are
the tachometer and the speedometer. The tachometer lets you know how
the engine is performing. An inaccurate reading can lead to major
problems such as rods and pistons exiting out of the side of the block.
A speedometer that isn't working correctly can lead to an up close and
personal visit with a Highway Patrolman. Lets face it, neither of these
two problems are an enjoyable experience. The symptoms of tachometer
problems are: A sticking needle, The tach will not register above a
certain RPM, Sometimes the tach works correctly, sometimes it does not,
The needle does not return to zero when the ignition is turned on, The
needle jumps radically or fluctuates, or the tach just does not work at
all. The speedometer has similar symptoms: The needle fluctuates,
sticks in one position, Will not return to zero, Will not register past
a certain speed, Reads slow or fast, The odometer does not register or
it also just does not work. All of the problems can be corrected by
making one phone call to Zip Products, Inc.,
8067 Fast Lane, Mechanicsville, VA 23111, (800) 962-9632. They can
supply all of the needed parts to rebuild or replace your tachometer
and speedometer Our Project 77's speedometer and tachometer both needed
repairs. Since this was the case, we decided to do them both. You will
not need any special tools to do the repair but you will need patience.
It should take the average home restorer a weekend to complete the
project. If you have to repair or replace one of the instruments, it is
a good idea to make sure you will not have to do the other one. The
reason I say this is because you will have to take the entire dash and
driver's instrument cluster out of the car, so you want to do this
repair once .... not again at a later date. Now let's get started! |
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01: Here are the items we received from Zip for our
project: The
tachometer, speedometer, a tachometer circuit board, speedometer cable
retaining clip, gauge needle, gauge lenses, gauge seal and a trip
odometer reset
knob. You will also need some hand tools, semi-gloss black spray paint,
1/8"masking tape, and a shop manual for your year Corvette. |
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02: Start by disconnecting the battery. Now remove all
of the windshield garnish mouldings. Remove the console side panels and
loosen the center console gauge cluster. If your car has air
conditioning, remove the driver's side lower A/C duct. Lie on the
driver's side floor and look up behind the instrument cluster.
Disconnect the tachometer and speedometer cables. Now disconnect all of
the light sockets that you can reach. |
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03: Loosen the right lower dash cover retaining screws
and then carefully remove it from the car. The A/C duct, if so
equipped, will also have to be removed. |
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04: The upper dash pad retaining screws should be removed. Now carefully remove the upper pad.
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05: The steering column retaining nuts can now be removed. This will
allow you to push the column down. Remove the left lower dash pad retaining
screws and carefully pull the pad out away from the dash. Tilt the pad so that
you can disconnect any sockets that you could not reach before. Push the
steering column down and then carefully remove the pad and cluster from the car. |
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06: Lay the lower dash face down and remove the
tachometer metal back retaining screws. Mark the location of each screw
because there are different lengths. |
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07: With the metal back removed, you will see the
tachometer circuit board. The tachometer became an electric instrument
in 1975 and its function is controlled by this circuit board. After 20+
years, these boards have a tendency to go bad and usually get hot and
burn in one area. If you look closely in this picture you may be able
to see a dark area. This is where our circuit board had burned which
caused the tach to work erratically. |
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08: This is the new replacement board from Zip. It is
smaller in size but does the same job as the original. If you want to
keep your original tach, just replace the circuit board. This circuit
board is good for both the L82 and L48. Since the face of our Project
77's tach was extremely faded, we decided to replace the whole unit.
Our 77 has an L82 in it but the only new tachometer that is available
is for the L48. The only difference in the two tachs is the redline.
The L48's is lower. Since the owner of our Project 77 shifts at over
6000 RPMs most of the time, the redline didn't make much of a
difference. If your car has the L82 and you want to keep it original,
you will have to replace the circuit board and use the existing tach
face. If it doesn't make any difference, then you can purchase a
complete new L48 tachometer. |
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09: When replacing the circuit board, re-use all of
the original nuts. The nuts will go on the solder side of the circuit
board. The terminals are all marked. Make sure all of the connections
are made correctly. If they are reversed, the tach will be burned out
and that means purchasing another board. The tach needle will need to
be re-zeroed on any 75 or 76 Corvette. If it is not, it will read about
3000 RPMs off. To do this, carefully remove the needle. Set the tach up
so that you can apply 12 volts and a ground to the correct terminals on
the back of the tach. Apply 12 volts to the tach. Now re-install the
needle at 0. Carefully remove the 12 volt connections. Now the tach is
zeroed. On 1977 models, there is no need to re-zero the tach but if you
re-use your old unit, you may want to install a new needle. Carefully
pull the old needle off using your thumb and forefinger. Then install
the new needle as per the instruction for your year Corvette. Sometimes
when removing the needle, the shaft will get stuck and pull up. If this
happens, push the shaft back in a little so it is free. Then re-install
the needle. Note the adjusting screw on the resistor is not for zeroing
the tach. Before re-installing the tachometer you may want to pre-test
it. Connect the tach to the wiring harness. Make sure your three
connections are correct. + is 12 volts. - is negative or ground and
coil. Re-connect your battery. Turn the key to the on position. The
tach needle should point to zero. If it does not or does not move, the
problem could be in the distributor or the wire coming from the
distributor. You may also have the tach connection reversed. If this
happened, then you will need another new board. Now disconnect the tach
and battery and continue with the rebuild. |
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10: The speedo replacement is very simple. Remove the
retaining screws. Again make a note which one goes where and replace
the entire speedometer. It is also a good idea to check the operation
of the trip odometer reset cable. Lube the cable so its movement is
free. Remember, in most Corvettes, this cable has a severe bend and
lubing it will help free its movement. |
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11: We wanted to freshen up our lense bezels so we
removed them. On disassembly you will notice these small rubber pads
between the gauge lense and the bezel. These are small anti-rattle
pads. They keep the lense from rattling against the bezel. To freshen
up the black painted area on the bezels and housing, start by cleaning
them. Now use 600 grit or a fine scuff pad to scuff the black painted
surfaces. Do not remove the black paint. By leaving it and just
scuffing it, you can use it as a primer base. Now take 1/8" masking
tape and tape the chrome outer lip of the bezel. Spray the black
surfaces with a semi-gloss black. When it is done, remove the tape. Now
you have gauge bezels and housings that look new. |
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12: Install the lense gasket, lense and anti-rattle pads onto the housing. |
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13: Now re-install the bezels into the lower dash
assembly. Then place the lense and housing onto the bezels and tighten
into place. Note: You will notice that we marked the bezels R and L.
Even though they are the same, we wanted to re-install them into their
original locations. |
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14: Before you re-install the metal backs onto the tach and
speedometer, make sure you replace the warning light socket gaskets. These are also available from Zip. |
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15: You can now re-install the speedometer and
tachometer into their respective housings. Re-install the metal backs.
Make sure that you put the metal speedometer cable retaining clip onto
the speedometer shaft. Without this clip, the speedometer cable will
not stay attached. |
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16: Connect and attach the trip odometer cable. Make
sure it operates correctly before you move on. We also installed a new
reset rubber knob on the trip odometer cable. |
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17: Here is our instrument cluster assembly ready for installation into the lower dash. |
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18: Lube the speedometer cable before you connect it
to the speedometer. Sometimes an erratic speedometer can be attributed
to a dry speedo cable. The cable can be pulled completely out of the
housing and then greased. Use either white grease or wheel bearing
grease to lube it thoroughly. |
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19: With the lower dash cluster removed, replace all
of the dash bulbs. These are a common auto parts store item and are
also available from Zip. The bulb number is located on the socket end.
It is much easier to replace them now then when the dash cluster is
re-installed. Trust Me! |
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20: The instrument cluster and lower dash is now ready to be re-installed into the car. |
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21: Start the re-installation by tying the dash
harness up as high as possible. This will allow you to slip the
instrument cluster below it as you re-install it. Carefully lower the
dash cluster into place. Remember to push the steering column down to
help installation. Make sure that you get the lower dash pad behind the
center console. |
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22: Now with the dash unit laying in place, connect
all of the sockets and wiring that you can. Again, this is much easier
than lying on your back working from under the dash. Also connect the
speedometer cable. |
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23: When all of the connections are made on the
driver's side, place the lower dash into place and re-install the
screws that hold it in place. |
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24: Now replace the upper dash pad, right lower dash
pad, steering column and cover, console side covers, all windshield
garnish mouldings and re-connect the battery. Start the car and check
the operation of all the gauges. Test drive the car and check the
speedometer. Also, do not forget to turn on the lights and check the
dash bulb connection. If you did everything right, you should be all
set to go for another 20 years! |
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TACH & SPEEDO:
75-77 CORVETTE SPEEDOMETER AND TACHOMETER REBUILD
By: Greg Donahue
Photos by: Author
SOURCE:
Zip Products, Inc.
8067 Fast Lane
Mechanicsville, VA 23111
(800) 962-9632
PARTS LIST:
NOTE:
Some new speedometers and tachometers are no longer available. Zip
Products does offer a rebuild service for your existing gauges. Please
call for details.
The
Author would like to thank Scott Coral of Greg Donahue Collector Car
Restorations, Inc. for his assistance with this article. |
