1-2 shift too soft

[81dmc]

So after my 3.0 install, I've noticed the 1-2 shift is rather soft while the 2-3 is harder. This translates to violent slipping 1-2 under mild-heavy part throttle acceleration. I don't want to glaze the clutches.

Does this sound like a symptom of different power under vacuum, or some other problem?

Could I just turn up the vacuum modulator to compensate?

[DMCMW Dave]

So after my 3.0 install, I've noticed the 1-2 shift is rather soft while the 2-3 is harder. This translates to violent slipping 1-2 under mild-heavy part throttle acceleration. I don't want to glaze the clutches.

Does this sound like a symptom of different power under vacuum, or some other problem?

Could I just turn up the vacuum modulator to compensate?

Before you change the modulator setting, put a pressure gauge on the trans (instructions in the manual). Then you can change the pressure if needed without flying blind on it. You should also have a vacuum gauge teed in at the modulator to see what it's trying to respond to. The modulator doesn't "care" what gear you are in; it only responds to engine vacuum. Hight vacuum = soft shift, low vacuum = hard shift. If you disconnect the vacuum line, it will shift "hard" all the time. The shift point (RPM/speed) is not impacted by the modulator.

I kind of suspect other issues, but I'd start here. If the pressures seem per spec, you probably are about to have the clutches fail anyway.

[81dmc]

Before you change the modulator setting, put a pressure gauge on the trans (instructions in the manual). Then you can change the pressure if needed without flying blind on it. You should also have a vacuum gauge teed in at the modulator to see what it's trying to respond to. The modulator doesn't "care" what gear you are in; it only responds to engine vacuum. Hight vacuum = soft shift, low vacuum = hard shift. If you disconnect the vacuum line, it will shift "hard" all the time. The shift point (RPM/speed) is not impacted by the modulator.

I know pressure is supposed to be 113psi, but is there supposed to be a vacuum spec? I was thinking I could use some sort of vacuum restrictor like old Mercedes diesels use.

I kind of suspect other issues, but I'd start here. If the pressures seem per spec, you probably are about to have the clutches fail anyway.

Not what I want to hear obviously. What another issues do you think could be suspect?

[David T]

Obviously check the line pressure and the vacuum but I found the shifts too soft also. That's why when I rebuild a transmission I use stiffer springs in the clutches for faster, harder shifts. That way the clutches don't slip as much and will last much longer too.

[81dmc]

Obviously check the line pressure and the vacuum but I found the shifts too soft also. That's why when I rebuild a transmission I use stiffer springs in the clutches for faster, harder shifts. That way the clutches don't slip as much and will last much longer too.

Stiffer springs in clutches? Usually that would mean increased pressure is needed for piston application.

Or are you perhaps talking about something in the valve body?

I haven?t had a DeLorean trans apart, so I don?t know too much about it.

[David T]

Stiffer springs in clutches? Usually that would mean increased pressure is needed for piston application.

Or are you perhaps talking about something in the valve body?

I haven?t had a DeLorean trans apart, so I don?t know too much about it.

The springs in the clutch packs hold the clutches. The hydraulic pressure is what releases them. As long as your line pressure is within specs, stiffer springs just give you faster, firmer shifts. The vacuum modulator REDUCES line pressure to give you softer shifts. The valve body also has built-in delays so as to let the clutch packs slip so when one clutch pack is engaging the other is releasing. The more the clutch packs slip the more they wear and the more heat gets generated. Faster, firmer shifts will add some more shock load but the clutch packs will last a lot longer. Think of it like a "Shift Kit" for a Renault automatic transmission. To get faster, firmer shifts without changing the springs, just block off the vacuum line to the vacuum modulator. It will give you slightly firmer shifts. The most important things are to have clean hydraulic fluid at the correct level and the correct line pressure. You can also experiment with the cable from the throttle spool to the shift governor (aka shift computer) to affect the timing of the shifts.

[Ron]

The springs in a clutch pak are for piston return -- Hydraulic pressure against the piston(s) is what makes the clutches hold.

FWIW- For most transmissions, you turn the screw in inside the modulaor's vacuum port:
(R)ight to (R)ise) the RPM shift points.
(L)eft for (S)ooner and (S)ofter.

+1 on checking line pressure.
Check fluid color -- Dark means clutch wear=>slippage.

[81dmc]

The springs in a clutch pak are for piston return -- Hydraulic pressure against the piston(s) is what makes the clutches hold.

Yep, that's what I thought...

Under medium throttle, my 1-2 feels like it's almost hanging before it finally clunks into 2nd.

[Ron]

Yep, that's what I thought...

Under medium throttle, my 1-2 feels like it's almost hanging before it finally clunks into 2nd.

I think I may have edited my previous post while you were replying...

If it seems like it shifts a little late, you might try confirming good line pressure then turning the screw (~1/8 turn at a time) to the right (clockwise) to make it shift later and firmer.

[81dmc]

I think I may have edited my previous post while you were replying...

If it seems like it shifts a little late, you might try confirming good line pressure then turning the screw (~1/8 turn at a time) to the right (clockwise) to make it shift later and firmer.

I lost my old pressure gauge, so I tried turning it maybe 10 teeth (15psi), and it gave me a very bad slipping 3rd gear shift and really only marginal improvements with 1>2.