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.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.