Those are indeed the pinouts, nice find! But if it's down to jumping circuits because the wires aren't even there, you're probably right it's not too helpful. Well, I have a few spare capacitors if anyone needs them lol.

 

My DLC1 doesn't have any terminals but for the jumper pin either.

I'm not an electronics expert so I wouldn't try the following without some kind of expert confirmation. I went through the following to clarify to myself what is actually being done at the DLC1.

In the original post:

• OP1 = Engine Temperature (coolant) Sensor
• E1 = The signal ground of the ECU

According to the 1998 electrical wiring diagram, here are the engine coolant temp sensor and the ECU signal ground:


Coolant temperature sensor (OP1 = THW) ??


ECU Signal Ground

You can probably measure the ripple voltage the same way as is done at the DLC1 by back-probing the THW and E1 pinouts at the ECU with the capacitor attach to one of the DVM test leads. But you're still going to have to get to the ECU and the wiring harness. Is the effort worth the time? Might as well just go ahead, unplug the ECU and swap the capacitors.

 

I've been experiencing some herky-jerky shifting in my 99 LS400 ever since I got it. It's most noticeable at 5-10% throttle in slow/stop-n-go traffic, but I recall a couple of instances where I had to WOT and the transmission/ECU didn't do a thing for a solid 2-3 seconds; I would not expect Toyota/Lexus to ship a car with kind of programming behavior. Due to all the known issues, I'm assuming my ECU capacitors are going bad, but I wanted to at least get some sort of confirmation before I immobilized my daily driver, so I gave all this a shot.
Unfortunately for me, my DLC1 connector doesn't have the OP1 coolant sensor terminal, but it did have the E1 ECU signal ground terminal. I looked at the parts location in the EWD, and this is where it shows the connector for the ECT sensor is:
https://i.imgur.com/QOWzBeI.png
Per paulo's screenshot above, the wire I'm looking for in place of that OP1 terminal should be colored red with a light blue stripe. Knowing the wire color pigmentation in the engine bay is subject to change over time due to the heat, I was confident I found it here just behind the TPS sensor based on the other brown ground wire; red with light blue turned into darker brown with gray stripe in 25 years:

Since I was here (and to confirm I had a good connection), I checked the resistance of the ECT sensor to ground per these specifications, and it appeared to be in good health: https://imgur.com/5mizVuA
Once confirmed, I got my meter wired in series with a .47 uf film capacitor like so, sticking the ground probe in the E1 port of the DLC1 connector, and started it up. With a readout of around 1.47 mV AC, should I be looking elsewhere? By the time I had everything figured out and set up, the engine was just about warmed up when I took this: