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.
Codes:
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 Manager Richard Merriman Vancouver, Washington, USA
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