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PCM or not PCM, that is the question
Posted to Transmission Forum on 9/17/2018 22 Replies

I got this 04 T-bird in with a slam shift in reverse and third with no slippage. Another shop was trying to sell him a new transmission for $7+ Gs. When he called me about it, I told him it sounded odd, and he should bring it over for a diagnostic.


u1040 SCP(J1850) Missing vehicle speed data. (this code comes up in: Front electronics, GEM/central timer, Rear electronics, and Instrument cluster.

u1040 SCP(J1850) Communication bus fault. This one is under Instrument cluster.

B1875 Turn signal/switch circuit fault. (included for full disclosure...)

Clearing codes does not clear any of them.

Fluid pressures with tests:

Reverse pressure test - A + B (line) pressure in reverse - spikes 400-500 psi at point of shift to reverse, then it levels out at 180-220 psi. Gives good diff. clunk. (I though my gauge would pop for a second...) (This is the original customer complaint)

Pressure control tap (solenoid C) - 80-90 constant, all gears. The ATSG books says park/neutral should be between 0-15 psi, and drive, 3, 2, 1, should be also between 0-15 psi. However, the ATSG guys seem to discount these numbers when I told him these results.

Reverse pressure test - sol. A + B (line) pressure- Controlled by scan tool 220 or 120 psi, But nothing in between.

Pressure control tap solenoid - Controlled by scan tool 25 or 120, But also nothing in between.

Reverse pressure test - A + B (line) pressure - Grounding A + B solenoid wires at transmission harness - Dropped to below normal - 65 psi

I also checked A, B and C Pressure control solenoid resistance at solenoid plug - all 5.2 ohms.

The ATSG book says reverse pressure should be 100-140 psi.

I called the ATSG guys and they said to ground A and B solenoids, the pressure should drop to within specifications. Since it dropped significantly below (65 psi), the solenoids are capable of reducing the pressure to the specifications.

The ATSG guy said that if the pressure did drop that far, the computer is incapable of grounding the solenoids enough to drop the pressure.

So, are the ATSG guys correct? I can't help thinking there could be another input that could cause the computer to run the fluid pressure to high. But maybe not since it sometimes spikes all the way up to between 400-500 psi when first entering reverse.

I checked all the grounds for the PCM, they were okay.

What say you?

Richard Merriman
Richard Merriman
Vancouver, Washington, USA

22 Replies Received (View Replies)


car Vehicle Data

2004 Ford Thunderbird 3.9L

Engine3.9 L
Trans5-speed Automatic (Electronic)

car Vehicle Data

2004 Ford Thunderbird 3.9L

Engine3.9 L
Trans5-speed Automatic (Electronic)