Line pressure and dropping resistor discovery!

[gl1674]

It seems that everybody is thinking that disconnecting
the dropping resistor gives the full line pressure in transmission.

Well, this is not exactly true. It does give full line pressure
when engine RPMs are below 2000. As soon as RPMs exceed
2000, TCU starts feeding the duty solenoid A with
12 volt pulses (there is another wire directly from TCU to solenoid
A that bypasses the dropping resistor). The pulses are
short, but they are full 12volt pulses, no dropping resistor
involved. Result - line pressure drops.

How do I know this - I hooked up line pressure gauge to transmission and saw the pressure drop from 150+psi to 60psi at 2000 rpm sharp. I scratched my head, looked at wiring diagram and borrowed some electronic tools...

The net result is that the dropping resistor mod is useless - you do not get full line pressure when you really need it, when engine
produces meaningful torque. You should cut the B68 #8 yellow-green wire from TCU harness if you want to force full line pressure.


Now, why the pressure drops all way down to 60psi in my tranny
is a separate story, I don't have an answer yet. If anybody is can reasonably guess whether it's worn Solenoid A or worn valve body (tranny has 152k), or better still, give me replacement parts to try, that would be appreciated...

[svx_commuter]

This is not the result I got with a pressure gauge and the resistor disconnected. The line pressure stayed up and did not change with engine rpm. I leave the resistor plugged in normally so that line pressure increases to the throttle request

[gl1674]

I do have a problem with my transmission - as soon as I have
any input signal on duty Solenoid A, the pressure goes all way to idle pressure (60ps). Initially I suspected that it was an electrical problem, but now I'm thinking it's the solenoid.

TCU feeds the solenoid with two signals - via dropping resistor and directly. Via dropping resistor it provides continuous ~4 volts at idle to 2000rpm, 50% duty cycle at half throttle that quickly goes to 0 as throttle is opened more. The voltage never exceeds 4 volts simply due to Ohm's law and resistance ratios of dropping resistor and solenoid.

Directly TCU does not provide anything until 2000 rpm, then it gives 12 volt pulses with up to 10% duty all way to fully open throttle.
At full throttle duty ratio becomes smaller, probably below 5%, but it is still there.
What I'm seeing on my tranny is that those 5-10% duty pulses are enough to drop the line pressure all way to 60psi...

New solenoid pack is in the plans.

[svx_commuter]

What did you use to look at the duty pulses?

[gl1674]

An oscilloscope.
The pulse frequency is constant 50Hz, which makes me wonder whether the solenoid is designed to close and open with this speed, or it is designed to float in an average position...

[oab_au]

Originally posted by gl1674
An oscilloscope.
The pulse frequency is constant 50Hz, which makes me wonder whether the solenoid is designed to close and open with this speed, or it is designed to float in an average position...

It's designed to float in an average positon.

Good to see you exploring this.

Harvey.