The late-version convertibles ('71 to '75 [or '76 Eldorado]) have a different type of top mechanism and control than all the others. Instead of a hydraulic pump, cylinders on either side of the seat, and a pantograph mechanism, they have a high-speed electric motor with cable drive, located behind the seat back, and the famous "scissors" mechanism (the Corvair, of all things, has the same top motor system).

I won't go into the top itself, as I haven't taken that apart yet, but, partly in order to assure myself that it works, I have modified the electrical control for the motor, and thought the information might be useful to someone else down the line.

The top motor in these cars is a slightly larger version of the motors used in GM power seats. It has a three-wire input system controlled by the same odd relay that the seats use. These relays are prone to failure, hard to find, and often expensive when you do. Prices range from $5 (or just-stick-it-in-your-pocket) at the local junkyard, if you can find one, to about $60 from the convertible parts suppliers. In the middle ground, you'll find people selling used ones (not the best idea to use) on eBay, etc. for $20-30.

I was able to disassemble my relay and clean it, and it worked fine afterward, but I discovered that I still had a problem with my top motor, and this is where my solution comes in.

The reason why the top motor in these cars is unusual is because instead of having the normal "reverse polarity to reverse direction" system that most simple DC motors use, it has a system involving a main armature coil and two field coils, A and B (one for "top up" or "seat forward," one for "top down" or "seat backward").
The top control switch sends current to either an "Up" or "Down" wire, and these two wires, along with an always-hot "Batt" wire, go together to the top relay behind the back seat. When you press the top switch, the wire you energize pulls in the relay, which connects that wire to its appropriate field wire, while at the same time connecting the Batt wire to the main armature wire. With +12v on both a single field wire and the armature wire, the motor turns in the desired direction.

In my car, after I cleaned and adjusted the relay, disassembled, cleaned and lubricated the motor and the gear drive (and removed a wave-washer that was jammed between the rotor and the stator), and cleaned all the contacts, I tested the motor directly off the battery, up front, sitting on the radiator support, and it practically jumped off the car when I connected it. I installed it behind the seat, and connected the relay and all the wires, but I found that when I hit the switch, the relay clicked and the lights dimmed, but the motor didn't move. I bypassed the relay with jumpers, but still couldn't get the motor (disconnected from the top) to budge. Finally, I ran a couple of heavy jumper wires from the battery straight to the motor, and it jumped immediately when I touched the wires together.

The fact that the motor ran fine with my own wires, but not with the car's wires meant that the car's wiring was not passing enough current, because of resistance somewhere (probably everywhere). I decided that it would be far easier to run my own wires than to try to fix the problems with the car's wires, especially since I really only had to run a single hot wire to the motor, as I could use the original top switch wires as signal wires for my own relay system, even though they couldn't deliver enough current to run the motor.

So, here's the schematic I developed, using a pair of DPDT relays (I really only needed DPST, but try finding those...). I chose to use a pair of Tyco T92P11D22-12's for $12 each from Mouser, because of their dust and weather resistance, and general heavy-duty nature (10,000,000 cycles ought to be enough, don't you think?), and I connected them as indicated in the schematic below (I've posted this same schematic somewhere on here in the past, but I'll be darned if I can remember where).



Here's a fancier schematic:



Here's a picture of the OEM relay:



Here are a couple of pictures of my installation, before and after wires:





I ran an 8 gauge wire (Home Depot, not expensive, about 25 feet was more than enough) directly from the battery (through a fuse) to a terminal strip behind the back seat. I went along the top of the fender liner and over the brake booster, following the harness, then down and in through the speedo cable grommet, and under the rug along the tunnel, to the back. The wire was kind of stiff, so I didn't try fishing it through the original wire chase. From the terminal strip, I fed the current to the motor through the two grey relays, using the original switch wires as a control input. I ignored the original Batt wire, and just left it sitting in the Packard 56 connector where it can't hurt anything.

As you can see, there's plenty of space back there.
I used the indentation in the center to house everything, but I could probably just have put everything right where the original relay went.

If you look, you can see the terminal strip, the two grey industrial relays down low, a single black relay mounted higher, just left of the terminal strip (which I used for my trunk release, and which is not involved with the top), the top motor to the left of those, and, not pictured, the original top relay is to the left of that. I left myself some extra wire (better too long than too short), and just stuffed it, along with the original wiring, into the dead space between the bulkheads

Soldering all the connectors was, as usual, a bit of a pain, but after assembly, the whole thing worked great.

In closing, I know that this will be of interest to only a very small group of people, but it may save a few of them some trouble, time, and money, so I figured I should post it on up .

- Eric

note: this subject is also discussed here.