Well, when I ordered the 5.6 & 6.0, I skipped the 5.2, which I think I should have gotten it.
I think I was wrong in my posting earlier. I tested it again right now and just the 5.1v ZD is pretty close: neutral is 4.44v, 6th is 4.34v, and 5th is 4.19v. With 330 Ohm resistor, neutral is 4.57v, 6th is 4.42, and 5th is 4.24v.
I think the 5.1v ZD may be perfect. The 5.6v did nothing because initial voltage is 5.14v (with that, I didn't bother with the 6.0). If I get a 5.2v ZD, it may or may not do anything. If I order it Sunday, it should be here Tuesday. I could also try a lower wattage ZD. The ones I got were 0.5 watt, but 0.35W is available. Maybe I could try a 0.35w 5.1v ZD and a 0.5w 5.2v ZD to see what the results are.
The difference, however, between 4.34 and 4.38 is less than 1%. I think I may just roll with that. All you'd have to do is tap into the red & black wire and attach the ZD in between them. All other gears worked perfectly and only 5th, 6th, and N are affected.
With only the 5.1v ZD:
1st through 4th were right on
5th = 4.53% less than stock measurement of 5th gear (4.19 vs 4.38)
6th = 0.92% less than 5th gear measurement (4.34 vs 4.38)
Neutral = 15.77% less than stock Neutral measurement (4.44 vs 5.14)
Now, since it's above 0.6v (per the FSM), it shouldn't throw a code. If the Neutral wire grounds, we should be okay. I need to find a micro/mini relay so I can open the ZD circuit when the blue wire is grounded to overcome this.
With the 5.1v ZD & 330 Ohm:
5th = 3.30% less than stock measured (4.24 v 4.38)
6th = 0.91% more than stock 5th gear (4.42 v 4.38)
It may be better to have it below because of any extra resistance that may occur.