Scissor Top
Troubles
Copyright ©2003 by Derek J. Sherwood
Reproduced with permission courtesy of Derek J. Sherwood (dsherwoo at ycp dot edu)
Version: 26 May 2003
It finally happened -- the unthinkable. You've just dusted off your
pride and joy after a long, hard winter, and are sitting in the
driver's seat in your garage, about to put the top down for the first
time in several months, anticipating good weather ahead. You press the
switch and...nothing. Or perhaps you're out on a cruise with some
friends, and those clouds overhead are starting to look ominous. It's
time to put the top up -- but it doesn't work. Suddenly, that big pile
of glass, canvas, and steel sitting inside your trunk has just become
your worst enemy.
There's nothing like convertible top troubles to take all the joy out
of owning a convertible. Most shops aren't very knowledgeable about
the GM scissor-top, or will want to charge you an arm and a leg to
diagnose or repair it. You might want to attempt fixing it yourself,
but the shop manual offers precious little advice. Perhaps you've
scrounged up a copy of the body manual for your model year, but upon
opening it, you realize it only has about eight pages on the top
mechanism -- seven of which are devoted to removing and reinstalling a
new top, or aligning the existing top, either of which require tools
that appear to have been used for extracting confessions from heretics
during the dark days of the Inquisition.
Before you give up all hope (or a large portion of next month's
take-home pay), you should consider reading this article. For all its
mystique, the GM scissor top is a surprisingly simple assembly, one
that is easy to troubleshoot and repair with tools you may already
have in your garage. The first part of this article deals with the
operation of the top, and the next three parts with some of the common
electrical, mechanical, and other problems that can cause top
inoperability.
Part I: Operation
The scissor-top mechanism on full-size GM convertibles was used,
basically unchanged, from 1971-1976. Unlike previous top mechanisms,
which used a complex system of hydraulics to operate the top, the
scissor top is a completely electro-mechanical system. The system
consists of switches, relays, gears, cables and motors -- there is no
hydraulic fluid in the entire system.
When the top is in the up position and the switch is depressed, power
is transferred through the switch (which is always "hot" and can be
used even with the car off and the key out of the ignition). The power
is transferred to a relay, which trips and activates the motor,
causing the motor to turn and operate a set of nylon gears (the top
transmission). These gears turn shielded cables that run behind the
rear seat area, one to each of the large side gear assemblies that
drive the main rails of the top. The cables transfer the rotation of
the motor to the side rail gears, which then lift the side rails up
and off of the front windshield surround.
As the top nears a point about halfway through the cycle, the small
cables that run the length of the side rails and are attached at the
second "bow" of the top begin to tighten. This pulls a pair of ball
joints mounted in the side rails inward, allowing the second bow to
pivot downward. This, in turn, allows the side rails to move inward
towards each other, giving the top its so-called "scissors" action. The
two side arms cut inwards past each other and have completed folding by
the time the top is fully upright. After this, the entire folded
assembly is pulled, by the gear and cable action, and stacked down
into the trunk well. Springs attached to the rear glass pull it down
safely into the trunk. This happens first, after which the rest of the
assembly folds down over top of it. All of this should happen in the
space of about 30 seconds.
When the time comes to put the top up, this action is reversed, and
the drive cables compel the side gears to push everything up and out
until the fully extended top lands about 4-6" above the windshield
surround. At this point, the driver can pull the top the rest of the
way down and latch it securely.
It's easy to see that, although the top's travel is relatively
straightforward, it performs a complex series of movements in a very
short amount of time with little room for error. Understanding how and
why the top works is essential to proper troubleshooting, so you may
want to read the last section again, perhaps while sitting in your
favorite convertible and operating the switch yourself.
Part II: Electrical Problems
There are several common problems that can cause top inoperability.
First, be sure it's not something simple -- check the fuses. Your
power top (at least on the '76 models) is fused on the same circuit as
the power windows. If your power windows work, the fuse is not the
problem.
Assuming there is no fuse problem, we will start with the switch. If
the top has stopped working, check to see if it has stopped working in
both directions, or only in one. Even if the top is fully retracted or
fully up, hitting the switch should trigger some type of movement as
the cables are turned by the motor. If movement happens in one
direction but not another, you may have a switch problem, which is
addressed below.
The excellent first step to troubleshooting any scissor-top problem is
to get as close to the top's operation as you can. To do this, remove
the rear seat from your car. Position yourself midway in the back seat
area of the car, facing the rear seat. Knee into the horizontal part
of the seat cushion and push up until it comes loose from its mounting
brackets. Remove it from the car. You should be looking at the
seatbelt assemblies at this point. You will need a large set of Torx
sockets and a breaker bar with an extension to remove these. A T50
Torx bit will most likely be the key here.
After removing the rear seatbelt assemblies, you can now lift the top
half of the rear seat up and out of the car. Remove the rear seat
upper cushion from the vehicle. You should now be face to face with the
heart of the scissor top. It consists of a motor about the size of a
small brick, attached to a wiring harness with three wires and a
ground. This harness is routed through a relay, a small rectangular
silver box screwed into the rear bulkhead area. The motor itself is
attached to the rear bulkhead with three cone-shaped rubber nipples.
These allow for quiet operation of the motor, while also allowing the
motor to flex as it torques the cables to operate the top.
First, check the motor for ground. Make sure the ground cable
connection, which is bolted to the rear bulkhead, is clean and tight,
contacting bare metal. If necessary, acquire a new ground screw and
nut and re-attach the ground to be sure. Absolutely nothing will
happen with the top motor if it is not properly and cleanly grounded.
Have a friend operate the top switch in the front seat as you sit in
the rear seat and listen. Do you hear a clicking sound? If so, your
relay is working. A relay can operate, but not allow enough current to
move the top, so even if you hear a clicking, you may have a bad
relay. The best thing to do here is replace the relay with a known good
unit. Relays are available new for about $39-45 from places like
Hydro-E-Lectric and Convertible Service (contact information included
later in this article). It's a good idea to replace the relay anyway
while you are in here, as it is one of the most common causes of
failure, and is a relatively cheap preventative measure. If replacing
the relay works, congratulations! Reassemble everything and go have a
cold beer to celebrate.
A malfunctioning relay can cause problems like slow top operation,
stalling or freezing partway through the cycle, inoperability, or
other odd problems. Before chasing down any mechanical problems, check
the relay to be sure it is up to speed. Remember, the motor can't run
properly if it isn't getting enough power.
If the relay has not fixed the problem, it might be time to check the
motor. It is easily removed from the car. First, gently pull the
rubber nipples out of the rear firewall, being careful not to rip
them. They are available new from the sources at the end of this
article, or you can replace them with appropriately sized bolts if you
wish. Detach the wiring harness at the quick disconnect fitting. Using
a pair of pliers, unscrew the two drive cables where they attach to
the top transmission. You should now be able to remove the motor from
the car.
You could take the motor to a local shop specializing in electric motor
repairs, or you could bench-test it yourself using a 12-volt battery
and a set of leads. Attach the leads to the battery and to the wires
on the harness, being careful to ground properly and not shock
yourself in the process. If the motor is good, it will whirr and come
to life. Have a friend wrap a rag around the output shaft of the motor
and operate it. Be careful, this motor has a surprising amount of
torque! The motor should pull itself out of your friend's hand. If it
does not, your problem may be a weak motor. If the motor does not
operate at all, it is probably bad. Most shops do not have access to
parts to rebuild these, so your best bet is a call to one of the top
shops to see if they have any in stock. It is fairly uncommon for
these motors to die, so if the motor is running and strong, you should
look elsewhere for the cause of the problem.
Now that you have checked the motor and replaced the relay, you can
re-install the motor. To facilitate reinstallation of the rubber
mounting nipples, moisten them with some mineral spirits. This will
make the pliable enough to go back into the holes on the rear
bulkhead. You can then reattach the motor and re-connect the drive
cables in reverse order of the removal procedure. Use a test lamp to
see if you are getting power at the harness or at the relay when the
switch is pressed. This will tell you whether or not the problem is in
the back of the car where you are, or in the front with the switch. You
can ignore this step if you got clicking out of the relay, or if you
replaced it earlier.
If you still have no life when you press the switch, it might be time
to consider a switch replacement. Remember that switches from other
years and models of scissor-top cars will work -- as will power sunroof
switches from 1977 and 1978 Eldorado models. You should be able to
find a top switch for a reasonable price in the local junkyard or
online.
The switch used for these tops is a rocker-style switch. A
spring-action plunger pushes a relay-like see-saw shaped metal
connection inside the switch to make the proper connection to operate
the top. After the switch is released, the spring action returns it to
center, and the "off" position. Time has proven that these switches
were not able to handle the circuit current. Arcing inside the switch
is common, which eventually burns it out. Sometimes, the switch can be
repaired by disassembling it and cleaning the contacts with emery
cloth, but if you have access to a new switch, it would be best to
replace it.
Poor or burnt switch contacts can cause not only inoperability, but
also slow top movement or stopping midway in up or down position, so
before you go after the mechanical source of your problems, consider
replacing the switch. It is a cheaper and easier alternative than
replacing mechanical parts of the top, and it can work wonders. As
mentioned before, the motor cannot be fully effective unless it is
getting all the power it needs to operate as intended.
To get at the switch, you will need to disassemble a good portion of
the dash pad. Disconnect the battery cables, then, starting with the
center crash pad, remove the climate control vents using the small
tabs on the inside of each one. Roll the vent about halfway back and
gently depress the tabs inwards towards the center of the vent, then
pull out. The vent should come out. Do this for all of the vents except
the driver's side center vent, which has no fasteners behind it.
Get a flashlight and peek inside behind the vents. You should see some
mounting nuts and screws. Undo each of these (you will need about a
10" extension and some 8mm and 10mm metric sockets to get them out)
and carefully remove the nuts so as not to drop them down into the
dash. Once you have removed them all, look up underneath the center
crash pad. There should be approximately six screws holding it on from
beneath. Undo these, put the shift lever in LO, and drop the tilt
wheel to its lowest position before lifting up and out. You will need
to disconnect the quick-disconnect (hereafter called QD) connector for
the wiper switch, as well as from the map light, to completely remove
the center crash pad.
After this, unscrew the wood appliqué that runs the length of the dash
from the top switches, behind the radio, and to the passenger side from
behind, reaching up through the glove box. Four screws hold this on.
Remove the radio knobs and springs, and finally the nut over the radio
knob stem, and you can remove the center appliqué. The switch housing
is attached to this. Carefully remove the accessory switch lamps,
mounted in the housing above each switch, and push the switch from the
front, popping it out the back of the housing. Undo the QD connector.
Push your new switch in from behind, and test the top. If everything
operates as it should, reassemble the dash in reverse order as
discussed above. Re-test operation once the dash is assembled to be
sure that you haven't pinched any wires or loosened any connections
during reassembly.
Congratulations! You should now have a fully operational scissor top
electrical system! If your problem is not in any of these areas, you
will need a good copy of the shop manual with a wiring diagram, because
you may have a short, or cut wires, somewhere between the switch and
the relay. Assuming no problems in the wiring along the way, there
should be nothing else electrical standing in the way of you and the
open sky.
Part III: Mechanical Problems
The electrical operation of the scissor top is simple, yet elegant. It
allows a properly tuned top to work flawlessly, quickly, and without
much thought. The feeling you get pressing that switch, and making all
of those gears and cables and relays work for you, is hard to describe
to someone who has never fixed a convertible top problem. But you're
not out of the woods yet. Assuming that you have locked down the
electrical system, there are still a number of mechanical problems
that could be causing you grief when you try to take your baby out of
the garage and into the great wide open.
You may have thought I forgot to tell you to reinstall the rear seat
in the last part of the article. Well, I did that for a reason. Your
best point of access to the mechanical workings of the top is still
found in that rear seat well. If your top is moving slow, or not at
all, or if it binds up, you will still need to be back there to fix
things.
In the case of a slow- or non-moving top, there are a number of
potential problems. This article will focus on the most common: the
drive cables, the transmission, the side gears and side cables, and
the ball joints at the second bow.
The transmission is the least likely culprit, so we will cover that
first. Mounted on the side of the motor, the transmission is turned by
the output shaft. It uses a pair of small nylon gears (one about 1" in
diameter, the other about 3" in diameter) to give the motor the gear
ratio necessary to turn the cables and operate the top. You can remove
the transmission housing and take a look at it. It will most likely be
very greasy inside. Be careful to note the position of the gears as
you disassemble the transmission. They sit inside the housing with no
attachment, so they are easy to remove and reinstall. As long as the
teeth are not stripped, you can rule out the transmission as a
problem. You might consider cleaning out the old grease at this time
and replacing it with new grease. From my experience, it appears to be
regular cosmoline grease, so find some at your local military surplus
store and re-pack the transmission and reattach it.
The drive cables are what transfers power from the motor to the side
gears. They are a small square metal spring cable sheathed inside a
plastic outer cable to allow for easy turning. Disconnect yours and
view them. The driver's side cable is smaller than the passenger side
cable. Try turning the ends of the cable with a pair of pliers. They
should turn smoothly, without binding. If they bind, they may be coming
apart inside, or grinding against the inside of the sheathing under
load. You can disassemble them and spray some white lithium grease
inside them to allow them to turn more freely, or you can replace them
at a cost of about $45 each. Replacement might be the best option as
you will get a long and happy life out of the new cables.
The drive cables screw into the side gears on either side rail. Look
to make sure the square hole where the inner part of the cable
attaches to the side gear isn't stripped or worn. If it is, new side
gears may be in order. If not, you should be OK. The side gears do not
need much in the way of lubrication, and a bit of grease should make
them run more smoothly. Make sure your drive cables are tightly
attached to both the transmission and the side gears.
Operate the top and watch the sheathed cables. Do they twist and flop
around visibly -- so much that they bind? If so, you can use some small
cable clamps and self-tapping screws to attach them securely to the
rear bulkhead area from inside the trunk. This way, the cables can
still turn, but not so wildly that they bind themselves. This
controlled turn can transmit more power to the side gears and allow
for better top operation.
The side cables are another area of concern. Often, these cables break
or wear with age. They are attached to the side of the body in the
rear, and are what keeps the top tight against the frame while it is
operated. They terminate at the header bow, and they should be screwed
into the second bow and have a little pigtail at this bow. If they are
not, this may be causing your top to function improperly, or not at
all. You can easily cause damage to the top mechanism if these cables
are broken or improperly attached. Make sure these cables are tightly
secured all along each side rail and at the second bow ball joints and
header, as well as at the rear on the body hooks.
The second bow ball joints should be inspected at this time as well.
They are attached to two small arms, which pivot inwards as the top is
retracted. If the ball joints, or the pieces they join into, are worn,
the second bow may not be dropping back at the right time, causing the
top to retract improperly or not at all. The second bow ball joints
are often very worn from years of use. These can be purchased new, as
can a reconditioned second bow assembly, although this option is very
expensive. Replacement of these parts is easy, and can be done in
15-20 minutes by unscrewing the old joints. Be sure your bow or the
ball joints are really the problem before going this route. You may
also wish to lube the ball joints lightly with WD-40 or white lithium
spray to see if that helps them move better.
Once you have done all of this and tested your top, you can reassemble
your back seat and congratulate yourself on having saved probably enough
money to buy an entire parts convertible, or another project car! If
everything is working as it should be, great going! If not, re-read
the article to see what you might have missed, or consider consulting
a professional for further assistance.
Part IV: Other Problems
Some common problems with top operation have been discussed above, but
one or two problems can arise that did not lend themselves to
discussion earlier. Two of the most common "other" problems are
mentioned here.
The first is improper alignment of the side rails, causing the two
side arms to strike each other as the top scissors inwards about
halfway up. You can easily see if your top is doing this by turning
around and watching it as you put it down. If the large side rails are
hitting each other, you will also see scratch marks, most likely on the
driver's side rail, as it is the one that stays out while the passenger
rail folds down in front of it.
To correct this problem, you will need to tighten the large side rail
nut that holds the side rails and gear assemblies on. This requires
considerable removal of rear trim on and is best done with the
assistance of the body manual. Tightening this nut can do wonders for
the operation of the top, and can also prevent problems caused by the
side rails hitting each other on the way down.
Another problem is the so-called "rub rail" on the third bow. This
rail, which is plastic, prevents metal-on-metal contact as the top is
retracted. If this rail has become worn, the scratches and pits in it
can cause the top to stick as it comes down. You can unscrew the rail
and sand it lightly to make it smooth, or if it is too far gone, you
can replace it entirely -- the rub rail is available new.
I hope this article has been of some assistance to you in learning how
your convertible top works, and what problems can arise in the
operation of the top. By understanding and repairing your own scissor
top, you will take a giant step beyond many convertible owners who do
not take the time to learn how their machine operates, and as a
consequence spend more time waiting for phone calls from a top shop
than they do enjoying the warm summer breeze. Many of these repairs
are easily done during the winter months, so when summer comes, all
that's left is to go out, drop the top, and take a drive!
Contacts:
Hydro-E-Lectric
5330 Independence Ct.
Punta Gorda, FL 33982
Phone: 1-800-343-4261
http://www.hydroe.com
Convertible Service
5126 Walnut Grove Ave.
San Gabriel, CA 91776
Phone: 1-800-333-1140
http://www.convertibleparts.com