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

If you like Derek's writing style and/or true crime stories, you might also grab a copy of "Who Killed Betsy"

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!


5330 Independence Ct.
Punta Gorda, FL 33982
Phone: 1-800-343-4261

Convertible Service
5126 Walnut Grove Ave.
San Gabriel, CA 91776
Phone: 1-800-333-1140

Return to Survivial Guide      Return to Caddy Page