1990 928 S4 Auto Modulating Pressure Adjustment
03-09-2008, 04:18 AM
#1
Burning Brakes
Thread Starter
1990 928 S4 Auto Modulating Pressure Adjustment
A28-16 Auto Gear Box.
Checked out for leaks, slight leak at "O" ring nut from reservoir to sump pan. Nipped up and now tight. ATF level checked cold and hot, spot on.
Been haveing some problems with firm shifts, and I have been gradually moving the "T" piece on the modulating valve anti clockwise, but this was hit and miss approach.
I have a small pressure gauge and got on my lathe and turned up a nipple to fit in the modulation valve test point to take my pressure gauge. Run gear box up to temperature and found that the pressure was 92 psi or 6.34 bar and it should be 4.2 bar +/- 0.05 bar. Turned out the pressure adjustment around 5 turns and achieved around 61 psi or 4.2 bar as per specification. This pressure was measured with the vacuum line removed from the modulating valve. I measured the vacuum at idle and 50 km/hr and the vacuum was minus 58 kpa or -0.58 bar which was considered excellent. When I took the motor RPMs up and back to idle the vacuum went from -0.4 bar to .64 bar.
Checked bowden cable for control pressure adjustment, OK as per specifications. There is no adjustment for the control pressure as it is controled by the modulation pressure.
Whilst I was in there I checked the starter interlock switch with a 4mm drill shank and checked the gear selector cable in "N", however, I needed to remover the acutating lever and when I reassembled I missed getting the starter lockout switch lever correctly into the hole in the acutating lever. To cut a long story short I broke the starter lockout lever inside the switch. I sourced a 2nd hand part and I drilled out the 5 rivets on the old switch to check out the internals. I maybe able to repair it at a later date using two pack epoxy glue with suitable wire strengthening pieces. Set up new switch in "N" as per the WSM and check gear change bowden cable, no adjustment necessary.
Took car for a run and totally different car. Gear changes up and down the range "smooth as", When car up to temperature carried out a TC stall test, excellent result around the lower stall speed of 1750 rpm. I now need the check shift points, up and down, for the A28/16 transmission as per the WSM.
I will make up connections for the 3 test points on the A/T, so that I can check the modulating pressure, the control pressure and the oil pressure all at once whilst driving and I will also install my vacuum gauge to fully see what is happening.
Planned to do a 850 kilometre trip on Saturday with my wife, which lasted around the block. Unfortunatley she thinks that the 928 is a heap of "SH one T" even though she learned to drive in my 912 Porsche over 40 year ago. Changed car to one of GMs, repacked luggage and made the trip. Arrived safely, but not as exciting for me.
Tails 1990 928S4 Auto
Checked out for leaks, slight leak at "O" ring nut from reservoir to sump pan. Nipped up and now tight. ATF level checked cold and hot, spot on.
Been haveing some problems with firm shifts, and I have been gradually moving the "T" piece on the modulating valve anti clockwise, but this was hit and miss approach.
I have a small pressure gauge and got on my lathe and turned up a nipple to fit in the modulation valve test point to take my pressure gauge. Run gear box up to temperature and found that the pressure was 92 psi or 6.34 bar and it should be 4.2 bar +/- 0.05 bar. Turned out the pressure adjustment around 5 turns and achieved around 61 psi or 4.2 bar as per specification. This pressure was measured with the vacuum line removed from the modulating valve. I measured the vacuum at idle and 50 km/hr and the vacuum was minus 58 kpa or -0.58 bar which was considered excellent. When I took the motor RPMs up and back to idle the vacuum went from -0.4 bar to .64 bar.
Checked bowden cable for control pressure adjustment, OK as per specifications. There is no adjustment for the control pressure as it is controled by the modulation pressure.
Whilst I was in there I checked the starter interlock switch with a 4mm drill shank and checked the gear selector cable in "N", however, I needed to remover the acutating lever and when I reassembled I missed getting the starter lockout switch lever correctly into the hole in the acutating lever. To cut a long story short I broke the starter lockout lever inside the switch. I sourced a 2nd hand part and I drilled out the 5 rivets on the old switch to check out the internals. I maybe able to repair it at a later date using two pack epoxy glue with suitable wire strengthening pieces. Set up new switch in "N" as per the WSM and check gear change bowden cable, no adjustment necessary.
Took car for a run and totally different car. Gear changes up and down the range "smooth as", When car up to temperature carried out a TC stall test, excellent result around the lower stall speed of 1750 rpm. I now need the check shift points, up and down, for the A28/16 transmission as per the WSM.
I will make up connections for the 3 test points on the A/T, so that I can check the modulating pressure, the control pressure and the oil pressure all at once whilst driving and I will also install my vacuum gauge to fully see what is happening.
Planned to do a 850 kilometre trip on Saturday with my wife, which lasted around the block. Unfortunatley she thinks that the 928 is a heap of "SH one T" even though she learned to drive in my 912 Porsche over 40 year ago. Changed car to one of GMs, repacked luggage and made the trip. Arrived safely, but not as exciting for me.
Tails 1990 928S4 Auto
