Bad TPS?

I've searched, and gotten a lot of great info on testing the TPS, but would like to know what, if anything, can be made of my results:

I tested my TPS (trans will not shift on it's own, stays in 1st when shifter is in D) and got about 4.2v on the A side, and nothing on the C side. The C side will gradually increase from 0v to near 4.2v with throttle input. The A wire does not change with throttle position, 4.2v constant.

This is an '88 Renix with AT, any ideas?

The four pin connector is for the TCU (tranny control). "A" is input voltage, "B" is output voltage (throttle closed), "D" is the ground (through the TCU). Test with the plug plugged together.
"A" to "D" is typically 4.65 volts (book says it should be 5 V). "B" to "D" is typically 3.85 volts. though anything up to 4 volts or so works just fine.
You may want to first test the "D" key in run position and connector connected, to vehicle ground, with the volt meter on DC volts. There will usually be a fraction of a volt, this is the standing voltage (resistance). If it is very high, there is a poor ground or excessive resistance, usually at a connector. You can also separate the plug and ohm test from the "D" pin to vehicle ground. There is typically a small resistance but excessive resistance is an indicator of poor continuity.
TCU grounds to the dipstick holder, the wire to ground ring connection is often corroded and not soldered. The ground goes through the C-101 connector on some models, which can also cause excessive resistance. The TCU has some built in resistance as the ground goes through it, but is minimal.
The three pin connector at the TPS is the input for the ECU (motor).
Trouble is often excessive resistance at the C-101 connector, the TAPS connector or the ground. Or water, solvent or oil that has seeped into the TAPS. Sometimes there is a dead spot in the TAPS, as the throttle is opened.
Is this your MJ? What happens when you pull the TCU fuse (under the dash)? Will it then manually shift?

Thanks for your help so far. I tested the 4-pin connector and here's what I found:
A 3.9v steady
B 3.3v decreases smoothly as throttle is opened

These readings aren't too precise as I'm not using a digital multimeter.

The grounds appear to have checked out fine, OHM to 0.

Do you think this TPS is good or bad? Any idea why input volts are so low?

This is on my '88 MJ, I need to pull the fuse out and see if it will manually shift.

Follow the input wire to the TCU side of the C-101 connector on the fire wall, make a small cut lengthwise on the wire and voltage test from there to the TPS ground. The wire there is a straight shot from the TCU, so the only possible resistance or faulty contact is the plug at the TCU.
Might want to also test the voltage "in" for the three wire connector also, just for reference. The voltage in for the three pin connector, comes form the ECU and for the four pin connector from the TCU. About 90% sure I´m right, but it has been awhile since I picked my way through the harness.
Mine had a low input voltage that turned out to be WD-40 in the TPS connector, I tested for voltage on both sides of the TPS connector and was loosing a volt at the connector.
I use an Exacto knife, and make a small cut lengthwise in the wire, then seal it back up with a dab of silicon. Trying to get a good reliable contact for the meter leads, other ways, is iffy. I also run the TPS wires (both sets) over the top of the TB rubber intake elbow, makes testing really easy.
I doubt the TCU is putting out low voltage, but anything is possible. I scrubbed all the connectors at the TCU and ECU a couple of years ago. I did find a couple of contacts (pins) that had backed out of the holder, the locking tabs for the contacts (pins) aren't real reliable.
Sorry about the typos, my spell checker gets kind of pushy and doesn't know what a TPS is and keeps wanting to change it to a TAPS.
There is usually some resistance were the ground wires for both sides of the TPS, go through the TCU and the ECU (I've measured 2 of each and all had some resistance in the module for the ground circuit, but not much). I think there is some kind of primitive surge protection in the circuit. A reliable, fairly accurate digital meter is just about a must, to get good readings. Mine cost about $25 at Home Depot.

So does it sound like this TPS tests out okay, except for a voltage loss somewhere before the TPS?

The only way to know for sure is to swap in a known good one. But your "in" voltage sounds low to me and actually the system seems to work better with close to 4 volts out.
A smooth sweep of the needle on your meter is a good sign, I had one TPS that had a dead spot at around 1/3 rd throttle. Had another that I covered with plastic, then sprayed out my TB with carb. cleaner. And someway or the other got some solvent in the TPS anyway and was getting questionable readings (and shifts). Water, solvent and oil seems to find It's way inside the TPS and screws things up. I learned my lesson and now strip everything off of the TB before cleaning.
The TPS voltage may not be your whole problem. But it sure can't be helpful.

I had the same problem with my tranny not shifting. My TPS tested ok, the results seemed to be a little jumpy as I moved the throttle. I installed a new one and the problem was gone. That was about 3 years ago.