Mechanics

AUTO TRANS SHIFTS LATE

1993 Nissan 300ZX

Transmission problem
1993 Nissan 300ZX 6 cyl Two Wheel Drive Automatic 140000 miles

I've got a 1993 Nissan 300zx N/A auto that won't upshift until around 4000RPM under normal throttle. The transmission was just replaced with a 240sx auto trans, if that changes anything (it shouldn't, since they're the same).

What I know is:
The voltage at the TPS sensor connector gave 0.5V - 4.1V (spec is 0.5-5) and at the transmission control unit harness was 0.33V - 3.1V. It's slightly different, but if anything, I'm thinking having a lower voltage would make it shift earlier, since it would think you're giving it less throttle.
The revolution sensor tested fine, as did the vehicle speed sensor (and the speedometer is accurate).
The gear ratios between the 300zx and 240sx transmissions are the same (since it's the same trans), the tire/rim size is stock, and the differential gearing is untouched.
The kickdown switch and wide open throttle switch aren't malfunctioning.
Hooking up the 240sx TCU instantly changes the shift pattern/schedule, which then instantly changes back when the 300zx TCU is reconnected, making me think it's a problem with the electronic controlling end, not the transmission itself. Thinking the TCU was at fault, I replaced it and the problem stayed exactly the same. Some cars around this era have mode switches (like sport and regular) which change the shift pattern, but as far as I know, the 300zx never did.

Something else I'm confused about is the overrun clutch solenoid valve. It's one of the outputs controlled by the TCU and according to the factory service manual, it should be on when the timing solenoid is operating (in gears 1 and 4) and off when the timing solenoid isn't operating (in 2 and 3). When I checked it, it had voltage in all four gears. I've never even heard of the timing solenoid and if it's in the transmission, it's not something controlled by the TCU
(the TCU has no output for it). Do you know anything about the timing solenoid of the overrun clutch solenoid valve and whether it'd have anything to do with when the trans shifts? . Or any suggestions as to what this could be? It's got me pretty confused.
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Eldewen
March 20, 2008.



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