Hello all,

I've done the obligatory search (searched the word "convertible" within the Electrical section, can't get more general and inclusive than that), but have found nothing about this.

Does anyone have any experience with fixing the odd cable-drive convertible top motors for the '71 to '76 full size models?

I've had mine apart a few times now. The first time, it actually worked, but then stopped when I connected it to the top, and I haven't been able to get it to work again.

Power is there (hot all the time at the Orange, hot on the other 2 when the switch is in the appropriate position), Ground is connected, Relay is working (passes control power through to Up and Down coils, closes relay for Orange Hot connection from body to Red Hot of motor when switch activated in either direction).

Up and Down directional coils both have equal continuity to ground (a few ohms, I forget exact values, but single digits, as you'd expect), bimetallic overheat breaker points are making contact, as they should, Continuity between segments of armature is equal, except for one segment, which is a bit higher, and may be dead (but I expect it would still turn, unless it started on that segment), brushes and commutator clean and making good contact, no metal bridging commutator segments, bearings in good shape, well lubed, and moving freely when assembled.

Basically, everything looks fine, but the motor won't go, either in or out of the car. I hate replacing things that work - any ideas or experience to help me?

Thanks,

- Eric

 

Have you tried straight 12V to the motor, without the relay? Those relays get unreliable with age.

How about the transmission part of the motor? Does it turn free, no binding?

Do the cables and gearboxes operate freely? Have you tried operating them with an electric drill from the transmission end?

Is the switch any good? They were notorious failure items too. Just because it will direct current to the motor does not mean it's good; I used to have to take mine apart about once a year and clean it inside. Even the last NOS switch in the GM parts system started acting up after about three years. I ended up putting in a heavy duty SPDT spring-return switch from Radio Shack and I didn't have near the top control issues once I got rid of the factory stuff.

More and more I realise why I stay away from 71-75 GM full-size ragtops. That damn scissor top is more than I want to fool with again.

 

Yup. Relay works fine, passes full voltage through. I did not try to hook it up directly to the wires the second time I tested it, though, now that I think of it - I should try that for the sake of thoroughness.

Completely fine. Well greased. Good teeth, no obvious wear.

Haven't even gotten to that part yet. This is all with the cables disconnected. I know my driver's side cable is bad (cracked outer and broken-off screw collar), but I'm not buying a new one until I go to a junkyard that's rumored to have some of these and see what they've got.

I did not know that I could try them with an electric drill, though. Thanks for that.

Seems to be just fine - push switch, get power on appropriate wire.

Hmmmm... Interesting. I hadn't realized those were so bad. Never had a problem with the ones on my Cutlass and Skylark. That'd be easy enough to do, though - I was all through the dash a couple of days ago soldering up previous butchery. Car must be fireproof if it didn't burn up with all the stupidity inflicted on its wiring by earlier owners.

Yeah, can't argue with you there, but beggars can't be choosers, and this car was so cheap, I just couldn't pass it up.

Thanks for the advice - I'll look into all of it (maybe not for a week or so , though - work schedule) and get back to you.

- Eric

ps: As you seem to be experienced with these tops, I also posted a question under "Interiors" about what's normal with the bows.
Is it normal for them to be so loose when the top is down that it all feels like a "bag of parts" when you grab part of it and wiggle it? A-body tops feel solid in all positions.

 

See if you can measure voltage right at the motor with everything connected as it should be. It might even require meter leads with the needlepoints that can be inserted into a connector or even pierced through the insulation.
If there is over 11.8V or so there with the switch activated, I suspect a motor issue.
If it is way less and the dome lights do NOT dim much (or any), then I suspect a fault in the switch / relay / connectors.
If the voltage is way less and the dome light dims very noticably, then suspect either a transmission fault, binding, or a possible motor issue.
If you have an ammeter (up to 20A), that would be very handy right now. A cheapo car ammeter could even be used for testing current draw of the motor.

Hope this points you in the right direction.

As for looseness, I would think it should be tight. Has the top been replaced?
The A bodies have lots of shim washers and spring washers at many joints to help keep them tight. Not sure if the others had those (but would assume so.)

 

Rob, these tops are much different that the A-body or pre-71 top. They are ALL electric- no hydraulics at all.

Where a hydraulic top frame folds back onto itself, the electric top frame siderails fold inward at their centerpoint when top is lowered, making an "X" as it retracts- hence "scissor top". The bows are attached to the rails with ball joints that are often never lubed and wear badly as a result, causing the looseness described. I've seen them get rotated clear around to the opposite side of where they're supposed to be- if you're lucky, when that happens it will just rip the cloth bowdrill off the roof; if not it will rip the top itself.

There are gearboxes at each side of the top well that drive the top frame up or down. These are driven by cables from the top transmission on the motor. Cables are similar to speedometer cables and color coded right and left. The gearboxes need to be lubed every year too and they often are not, so you can imagine what happens to the drive cables.

The scissor top was originally designed for Corvairs to keep hydraulic fluid out of their engine compartments. It was adapted for the B and E car convertibles in 1971, ostensibly to allow a full-width rear seat, but I figure it was a cost thing so they could use the rear seats from the closed cars and not have a special convertible rear seat. The cost of it had already been amortized with the Corvair, so they got rid of the hydraulic system and convertible-specific rear seat at the same time.

MD, make sure to check the drain gutter frequently. Older cars used a metal or plastic drain trough to direct water to the drain tubes in the wheelwells. These tops have a piece of top fabric attached to the car body and the top frame for the drain gutter, which can be seen from inside the trunk. It still directs water to the drains ahead of the wheelwells, but it's subject to getting pinched which will render it useless. It's also bad about tearing after it gets some age and a hole in it will dump water straight into the trunk. It seems most trim shops forget about the gutter when they replace a top cover, and every trim shop I know hates to see one of those tops coming.

 

Well aware of the lack of hydra power in the big cars; motor testing should still be similar to the pump motor in the A bodies.

Good point to check the mysterious, hidden rain gutter...
The shop that replaced my top (well for the previous owner) must have realized that it could channel water into the quarters (like its supposed to!) Therefore, he sealed the two drain ends with tons of masking tape.
Guess he wanted it to all drain into the trunk instead. Geez.......

 

No worries about the drain gutters!

This car is a rat and has none of that stuff at all.

The huge holes in the trunk and under the back seat have all been very unprofessionally patched with sheet metal, so I expect I'll make some "drain holes" with an electric drill when I see where the water goes.

I'll reinstall it all when I get around to doing the top, but for now, there's nothing at all between the folded roof and the trunk.

I'm very fond of rat convertibles, because, so long as you can get 'em cheap, you can enjoy them without having to worry about them, and if they're running, they never lose their value.

- Eric

 

Thanks, RocketRaider and Rob!

Your advice was exactly right.

Based on a quick review of the wiring digram, I would never have suspected the switch as a current choke-point, especially since I'd never had a problem with an A-body top switch, and I was getting about 12 volts at the motor with definite dome light dimming, but I took your word for it, and removed the motor from the circuit and tested it.

I made a quick pigtail by soldering up some 10 gauge wire with two spade connectors on one end, connected them one to the motor hot and one to one of the directional windings, grounded the ground and touched the hot to the hot post right on top of the battery, and the thing spun like a champ. Switched the directional connection and it spun right up the other way.

Clearly, the problem is in the power supply to the motor, so I will simply eliminate GM's stupid system, and replace it with a better one (Why the f*@k would you run high amps through a tiny dashboard switch and then all the way to the back of the car, when you're already using a weird custom relay back there? Why not just shorten the current path, use less large-gauge wire, and do the switching right next to the motor?!?). I'll use the switch for a low-current (~140mA) signal to relays mounted where the original relay was, and may run a separate high-current line straight from the battery. That way, the switch will last forever, and maximum current will get to the motor. If I want to, it will also let me wire a momentary toggle switch in each rear wheel well, or in the top well, so that I can move the top while standing next to it and help it fold. I'll use two of these $10 DPDT relays (though I really only need DPST), which will switch 30A each, and shouldn't even break a sweat. I've included a copy of my rough schematic, in case anyone wants to see how to do it. Incidentally, this is also a way for people with bad top relays to avoid buying a $50 replacement.

- Eric