Automatic gearbox shift issue
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Automatic gearbox shift issue
I have an issue with the automatic gearbox in my 86.5. The shift from 2 to 3 is very firm when the engine speed is low ( below 1000-1200 rpm/ very low throttle).The downshift from 4 to 3 can also be very firm out of e.g. roundabouts with low engine speed/throttle. Oil, filter, modulation valve and K1 spring kit are all new. Oil level is correct. The correct vacuum is also present at the modulation valve. In all other situations the shifts are fine. To me it seems that a valve or something are sticking and then suddenly opens up. Any ideas?
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From: Boulder Creek, CA
I have an issue with the automatic gearbox in my 86.5. The shift from 2 to 3 is very firm when the engine speed is low ( below 1000-1200 rpm/ very low throttle).The downshift from 4 to 3 can also be very firm out of e.g. roundabouts with low engine speed/throttle. Oil, filter, modulation valve and K1 spring kit are all new. Oil level is correct. The correct vacuum is also present at the modulation valve. In all other situations the shifts are fine. To me it seems that a valve or something are sticking and then suddenly opens up. Any ideas?
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Chronic Tool Dropper
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The TV (loosely: Throttle Valve) cable (perhaps poorly described as the "Bowden cable") modulates control pressure to help determine -when- the transmission will upshift or downshift. The vacuum modulator helps determine how firm those shifts will be by deducing engine load from manifold pressure (vacuum). The other deciding factor is the governor, which effectively factors in engine speed.
You can adjust the modulator to make the shifts softer or firmer, remembering that a firmer shift risks impact abuse. At the same time, a softer shift has more slippage, therefore more wear to clutches and metals at every shift.
On my slightly later car (with the same transmission...) there's some "protection" built in via a circuit that momentarily backs off ignition timing during upshifts. That reduces the power so reduces the strain on the gearbox during upshifts. IIRC there's a control pressure switch that does the primary duty, but I need to spend time with some wiring drawings to confirm the actual pieces and function. Others may be able to tell us immediately if your car has that feature and how it works.
Bottom line though is that the timing and firmness of shifts is interactive between the TV cable adjustment and the modulator, in that they both manage control pressure. Further, the kickdown solenoid operates a little valve that dumps part of the control pressure flow when opened., forcing a downshift if something else hasn't done so already. Start by adjusting the TV cable so that the car shifts at the right times. Then adjust the modulator if needed to take care of harshness.
FWIW, I have my TV cable set up a few turns "loose", so the car shifts up earlier. The payback for that is that the transmission shifts up a little harshly, especially if I momentarily relax pressure on the throttle to coax an even earlier upshift. The other payback is that the trans downshifts a lot "later" than one might like, staying in a higher gear longer under increasing load. This is somewhat a carryover from the days in L.A. traffic, when it's better to just leave the car in a higher gear and let the low-end engine torque do the work of slushing through the slow-and-go stuff. Know also that you can dramatically change the personality of a 928 with that TV cable adjustment. Always-sleepy to screaming-demon-possession is a just a few turns of the TV cable adjustment tighter.
You can adjust the modulator to make the shifts softer or firmer, remembering that a firmer shift risks impact abuse. At the same time, a softer shift has more slippage, therefore more wear to clutches and metals at every shift.
On my slightly later car (with the same transmission...) there's some "protection" built in via a circuit that momentarily backs off ignition timing during upshifts. That reduces the power so reduces the strain on the gearbox during upshifts. IIRC there's a control pressure switch that does the primary duty, but I need to spend time with some wiring drawings to confirm the actual pieces and function. Others may be able to tell us immediately if your car has that feature and how it works.
Bottom line though is that the timing and firmness of shifts is interactive between the TV cable adjustment and the modulator, in that they both manage control pressure. Further, the kickdown solenoid operates a little valve that dumps part of the control pressure flow when opened., forcing a downshift if something else hasn't done so already. Start by adjusting the TV cable so that the car shifts at the right times. Then adjust the modulator if needed to take care of harshness.
FWIW, I have my TV cable set up a few turns "loose", so the car shifts up earlier. The payback for that is that the transmission shifts up a little harshly, especially if I momentarily relax pressure on the throttle to coax an even earlier upshift. The other payback is that the trans downshifts a lot "later" than one might like, staying in a higher gear longer under increasing load. This is somewhat a carryover from the days in L.A. traffic, when it's better to just leave the car in a higher gear and let the low-end engine torque do the work of slushing through the slow-and-go stuff. Know also that you can dramatically change the personality of a 928 with that TV cable adjustment. Always-sleepy to screaming-demon-possession is a just a few turns of the TV cable adjustment tighter.
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The TV (loosely: Throttle Valve) cable (perhaps poorly described as the "Bowden cable") modulates control pressure to help determine -when- the transmission will upshift or downshift. The vacuum modulator helps determine how firm those shifts will be by deducing engine load from manifold pressure (vacuum). The other deciding factor is the governor, which effectively factors in engine speed.
You can adjust the modulator to make the shifts softer or firmer, remembering that a firmer shift risks impact abuse. At the same time, a softer shift has more slippage, therefore more wear to clutches and metals at every shift.
On my slightly later car (with the same transmission...) there's some "protection" built in via a circuit that momentarily backs off ignition timing during upshifts. That reduces the power so reduces the strain on the gearbox during upshifts. IIRC there's a control pressure switch that does the primary duty, but I need to spend time with some wiring drawings to confirm the actual pieces and function. Others may be able to tell us immediately if your car has that feature and how it works.
Bottom line though is that the timing and firmness of shifts is interactive between the TV cable adjustment and tyhe modulator, in that they both manege control pressure. Further, the kickdown solenoid operates a little valve that dumps part of the control pressure flow when opened., forcing a downshift if something else hasn't done so already. Start by adjusting the TV cable so that the car shifts at the right times. Then adjust the modulator if needed to take care of harshness.
FWIW, I have my TV cable set up a few turns "loose", so the car shifts up earlier. The payback for that is that the transmission shifts up a little harshly, especially if I momentarily relax pressure on the throttle to coax an even earlier upshift. The other payback is that the trans downshifts a lot "later" than one might like, staying in a higher gear longer under increasing load. This is somewhat a carryover from the days in L.A. traffic, when it's better to just leave the car in a higher gear and let the low-end engine torque do the work of slushing through the slow-and-go stuff. Know also that you can dramatically change the personality of a 928 with that TV cable adjustment. Always-sleepy to screaming-demon-possession is a just a few turns of the TV cable adjustment tighter.
You can adjust the modulator to make the shifts softer or firmer, remembering that a firmer shift risks impact abuse. At the same time, a softer shift has more slippage, therefore more wear to clutches and metals at every shift.
On my slightly later car (with the same transmission...) there's some "protection" built in via a circuit that momentarily backs off ignition timing during upshifts. That reduces the power so reduces the strain on the gearbox during upshifts. IIRC there's a control pressure switch that does the primary duty, but I need to spend time with some wiring drawings to confirm the actual pieces and function. Others may be able to tell us immediately if your car has that feature and how it works.
Bottom line though is that the timing and firmness of shifts is interactive between the TV cable adjustment and tyhe modulator, in that they both manege control pressure. Further, the kickdown solenoid operates a little valve that dumps part of the control pressure flow when opened., forcing a downshift if something else hasn't done so already. Start by adjusting the TV cable so that the car shifts at the right times. Then adjust the modulator if needed to take care of harshness.
FWIW, I have my TV cable set up a few turns "loose", so the car shifts up earlier. The payback for that is that the transmission shifts up a little harshly, especially if I momentarily relax pressure on the throttle to coax an even earlier upshift. The other payback is that the trans downshifts a lot "later" than one might like, staying in a higher gear longer under increasing load. This is somewhat a carryover from the days in L.A. traffic, when it's better to just leave the car in a higher gear and let the low-end engine torque do the work of slushing through the slow-and-go stuff. Know also that you can dramatically change the personality of a 928 with that TV cable adjustment. Always-sleepy to screaming-demon-possession is a just a few turns of the TV cable adjustment tighter.
Chronic Tool Dropper
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You can of course "soften" those particular shifts with a modulator adjustment. You also adjust all the other shifts at the same time. Buried in all that stuff I shared in my previous post is the idea that you can effectively "soften" the shifts you describe by -tightening- the TV cable. That will cause the upshifts to happen "later", with more engine RPM, and therefore have more power with more slip, less clunk, as it upshifts. The Good News is that the TV cable adjustments are easy and convenient. I keep a logbook of all adjustments made to the car so I can always have the option of going back to where I started if something doesn't work the way I want it to.
I'll throw this out too -- The cable between the quadrant/bellcrank assembly to the actual throttle body is known to stretch over time and use, as is the cable from the pedal to the bellcrank. Before you get too deep into TV cable and modulator adjustments, adjust the throttle cables so that the throttle is just at full-open as the little arm for the downshift switch actuates the switch. That switch actuation (switch closes, well after initial engagement) should coincide with full throttle opening but without compression of the pedal cable spring.
I'll throw this out too -- The cable between the quadrant/bellcrank assembly to the actual throttle body is known to stretch over time and use, as is the cable from the pedal to the bellcrank. Before you get too deep into TV cable and modulator adjustments, adjust the throttle cables so that the throttle is just at full-open as the little arm for the downshift switch actuates the switch. That switch actuation (switch closes, well after initial engagement) should coincide with full throttle opening but without compression of the pedal cable spring.
