Two obscure car questions for old GM wrenchheads:

Factual Questions
1

Okay; Some of the early to late sixties’ GM marques used a variable torque converter called a “switch pitch”. It was run internally by the tranny’s hydraulics, and controlled externally, electrically.

The Question: How is it controlled? I know it’s simple analog: on or off. But, how is the trans connection (there’s two terminals) wired, and how is it actuated? Full-throttle activation only? Vacuum?

Second question: I’m converting a mid-sixties’ GM from an external-reg Alternator to a modern internal-voltage-regulator alternator.

I actually did most of the conversion almost two years ago (long term project, don’t ask) but I’m redoing the underhood wiring (not completely, but making a lot of changes- the AC is gone now, for example.)

However, I hear of “leakback” through the ALT idiot light keeping the ignition energized and not being able to shut the car off. I can no longer find the instructions I used (they were online, and quite clear) and with the underhood changes I want to make, the routing and conversion is no longer clear.

Has anyone done a similar mod? Have a copy of the instructions on hand?

2

Switch Pitch Trans Detail

3

That same site also provides the alternator conversion details:

http://www.442.com/oldsfaq/ofall.htm#ALL%20Ext.%20Regulated%20to%20Int.%20Regulated%20Alternator%20Conversion

4

To eliminate run-on, install a diode (1A,100V) in series with the wire going to the charging light indicator (anode toward light). This would be the wire at terminal “4” of the external regulator. For internal reglator alternators it would be the smallest wire (usually brown) connected to the alternator.

Disclaimer:
These instructions come from the installation manual of my capacitive discharge ignition system. The manual goes on to say that the diode trick is only needed if the ignition system draws an extremely low current (~uA range). I personally have never needed it.

5

Desmo- There’s no doubt I have the switch-pitch: among other things we have the two-prong electrical connection, plus knowing what the tranny came out of.

The question is, how is it wired? All the manuals I’ve come across for those cars don’t have the kickdown or switch-pitch diagrams (heck, even the factory manjual doesn’t show the AC wiring!)

I can logically presume it’s a matter of applying a full 12 volts to the terminals- IE, no variable voltage since both functions are either on or off.

The question is, when? Kickdown should be when accellerating from a slow speed, but not necessarily full throttle. The switch pitch should engage… when? It changes the stall speed (and thus the torque multiplication) of the converter… so that should be used when? Low vacuum conditions? (Like climbing a hill.)

Third question for the wiring-inclined: GM wiring harnesses of that vintage had a “resistor” wire to reduce the voltage going to the external coil. I’m upgrading to a newer HEI (electronic ignition) and thus don’t need the lower voltage.

I think I found the resistor wire in the harness (it’s an odd braided/fiber cover rather than the usual plastic insulation)- how far into the loom does this stuff usually go? I’m nearly to the firewall already and haven’t seen the end of it.

6

Resistance wires vary by year. Here’s a decent summary:

http://www.tispecialty.com/articles/article5.htm

Re:the switch pitch / kickdown,
I’d wire the switch pitch to a simple toggle switch. “On” (high pitch) for performance, “off” (low pitch) for fuel economy.

The kickdown is going to be trickier. You could just use manifold vacuum, but you really need throttle position too. I’d probably start with a throttle position switch, test, and fine tune with a vacuum activated switch if needed.

7

Interesting summary. Looks like it focuses on the early GM transistorized ignition though (the one all the magazines of the day had great fun calling “the pointless ignition”. :smiley: )

And yes, those amplifiers were VERY temperature and voltage sensitive- there was a component- a large transistor?- that would blow out (literally) if you looked at it crosseyed. And back then, everyone and their brother Bubba would, when having engine troubles, yank a plug wire, jam a screwdriver in there, hold it near some part of the engine and have the other guy crank it so they can check for spark.

This would, invaribly, blow the amp and they’d stop getting spark. :slight_smile:

Anyway, I’ve cut and abandoned the original resistance wire, we’ll be going with a connection to a different switched source. The HEI just needs plain ol’ 12V switched, so I was able to ditch both the resistance wire and it’s starter bypass.

On the torque converter, yes, my plan was, barring finding any details, to have a simple dashboard switch. However, the whole idea of the converter was to essentially add more gearing. I understand that Mopar is/was working on something similar, giving the equivalent of six speeds from an otherwise-standard three-speed automatic.

The battlebarge I’m putting together has a king size engine, but a Queen Mary sized hull. Diff gearing is limited to 2.73:1, which is great for highway (especially with the motor’s torque peak of 510 ft/lb at a leisurely 1,800 RPM) so I could use all the additional gearing help I can get.

Manual switching is workable, but there has to be a way to make it more automatic. The factory apparently just went by throttle position, but I still don’t know how or where.

8

All you needed to do was read the FAQ a bit further.

9

Ah, quite right. I missed that entirely.

Now, I wonder, why high-stall at idle? Less parasitic losses? That’s an aspect I hadn’t considered…

I wonder if I can get MAP sensor and a compact throttle-position sensor and whip up a little programmable controller?

On the alternator, I forgot I don’t have an ALT idiot light, I have a factory ammeter. The wiring diagram isn’t real clear on it, I wonder if I’ll get the “leakback” through it as well. It’s looking like this is one of those cases for empirical testing. :smiley:

Thanks for your help, all.