I am running into a small problem with my 88 superduty 4X4 diesel with C6
trans.this truck was a 5 speed till recently, but I switched it over to
automatic . I am getting low vacuum at the modulator valve, causing a hard
shift at full throttle. there is 17 lbs at idle, but at wot I am only
getting 3 lbs after the vacuum control valve on the injector pump, which is
as far as it will go.any ideas ?? I am not that familiar with automatics,
everything else I have has a manual trans.
Which shift is hard? The modulator is only in play on the
1>2 shift on a C6. If your problem is only 1>2, the
modulator is adjustable. It is probably adjusted in the
ball park if you are getting your 1>2 shift in the 8-10mph
range using light throttle presure. If the shifts are
coming any higher, the modulator is prtobably set too high.
There is an adjusting screw in the hose connection on most
of them. Some factory units did not have this. Most
aftermatket and Ford replacements are adjustable. Are you
getting any fluid in the vacuum line at the modulator? If
so, the diaphram is bad and you can start with a new
modulator. Are you also getting hard shifts on 2>3? If so
you need to be lookng at other things like the line
pressures. Your low vacuum reading at WOT at the modulator
is normal. That is how the modulator determines engine load
and regulates pressure on the shift.
You may want to ping Transurgeon or Gary Glazener here or in
the Ford truck group or possibly the Dodge truck group if
this is no help.
if you go 3/4 throttle it shifts fine . if you go to wot, it will not shift
till you back off a bit then it shifts hard. if you back off to 3/4 or less
it is fine. downshift is fine also, the only problem is low vacuum at wot.
is it possible the vacuum control valve is not calibrated properly to give
enough vacuum when adjusted to full setting??
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