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BJ8 Overdrive Pressure Loss

AUSMHLY

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Why does my overdrive lose all pressure so quickly, when it seems it should not?

This was copied from The Healey Technical Archives:

Start the engine in neutral and adjust the idle to about 1500 rpm. Without feeding any gas, place the gearshift into 4th (OD off) gear while watching the gage. The pressure should increase from 0 to 300psi in TWO TO THREE SECONDS. In another 10 seconds, the pressure should have gone from 300 to 470psi, where it should remain constant. Keep the car running at 1500 rpm while watching the gage - it should stay rock steady at 470 to 490psi.

Now engage OD while watching the gage - it should drop about 100 psi when OD kicks in, then climb back up to 470-490 in about 5 seconds. While watching the gage, place the gearbox into neutral, hit the brakes, turn off the engine. When the rear wheels stop turning, the pressure will drop slowly into the low 400's, then continue dropping at a rate of about 10 psi per minute down to about 250-300 psi where it should stay for several hours. These numbers are not absolute, simply my records over the past 30 years. However, they will give you a benchmark of how well your OD hydraulic system is functioning and if further repairs are necessary.

Here's how long it takes for the pressure drop:

Car on jack stands.
4th gear, 1500 rpm, OD off, 450 psi.
4th gear, 1500 rpm, OD engaged, drops immediately to 200 psi, 5 seconds to 450 psi.
4th gear, 1500 rpm, OD off, goes/stays at 410 psi.
Into neutral, brakes on, ignition off, 410-300psi takes 3 seconds, then 7 seconds to 0 psi.


What do I need to look into why the pressure drops so fast to 0?

Would the O-Ring conversion accumulator housing/pistion from AH Spares fix this?
> O-RING CONVERSION-accumulator piston <
Would replacing the accumulator pressure spring help? (Mine has a washer, but I believe there was a Healey tech notice that recommended adding a washer.)
Would replacing the spring/plunger/ball bearing for both the operating valve and non-return valve help?
Probable issue of the non-return valve seat not smooth enough to make a seal?

Thoughts anyone...
3ADA0079-921D-43B0-BA91-7CC2323DEEE5.jpeg
 

Bob_Spidell

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re: "Would the O-Ring conversion accumulator housing/pistion from AHSpares fix this?"

Possibly, but not for the reason you may think: If your accumulator bore is badly scored, the rings worn, or both, fluid could escape. I don't know where it would go, but if you pull the accumulator cover off and it's full of fluid you'd have your answer; replacing the accumulator with either rings or O-ring type would (inadvertently) fix this. Another possibility is the O-ring on the housing, but these shouldn't be subject to wear (shrinking or hardening maybe).

I think the O-ring conversion is designed to reduce/eliminate bore wear in the accumulator (likely the same reason the manufacturer went from steel rings to O-rings in the operating pistons; other possibility: cost reduction).

re: "Would replacing the spring/plunger/ball bearing for both the operating valve and non-return valve help?
Probable issue of the non-return valve seat not smooth enough to make a seal?"

Very possibly/likely. Your accumulator appears to be bleeding down quickly, but I'm skeptical even a perfect system could hold pressure indefinitely; it's designed to (mostly) hold pressure from a constantly operating pump. I don't think the operating valve is your issue, but worn valve ball and/or seat could certainly cause problems (though if it was the non-return valve on the pump you probably wouldn't build good pressure in the first place). The non-return valve functions up to thousands of times per minute, but the operating valve (probably) only a few times/hour.

Edit: If your operating valve got stuck in the open position, I think it would allow pressure to bleed down quickly but would probably cause the O/D to stay engaged longer after you 'threw the switch.'
 
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Healey Nut

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At some point your going to have to commit to pulling the OD , pull it apart and investigate . If the car has a lot of miles then chances are it’s due for overhaul . They are not that complicated .
Patience ,some basic hand tools , a vernier caliper a clean work surface to lay out parts a phone to take pics and of course the spare parts once it’s apart .
Take your time , replace anything that wears , piston rings , o rings , the little ball bearings . .
The OD is a small hydraulic pump that runs continually when the car is running . When you flick the switch , oil is diverted through the operating valve and the hydraulic pressure is greater than the spring pressure holding the clutch open and it engages . Open the switch , hydraulic pressure drops , spring pressure overcomes hydraulic pressure and the clutch disengages .
The main thing you have to do when setting it up is DONT follow the instructions in the manual about the drill bit and the hole alignment . I guarantee that won’t work .
you have to measure how far the operating ball rises . Hence the vernier caliper . It has to lift the ball and this is from memory but I think it’s 5/64“ .
If you get the articles of the Healey Technical archives there’s a great article in there about how to set up the O/D so it works correctly .

 

Bob_Spidell

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I'm not trying to start an argument, and maybe I'm misreading Nut's comment, but my understanding is a little different so maybe someone can straighten me out:

- The O/D pump is driven by a cam on the gearbox output (main) shaft, so if the engine clutch is disengaged--pedal in--and/or the car is in neutral the pump isn't operating.

- The 8 very strong springs, in two lengths, in the front section of the O/D tightly bind the O/D's cone clutch's forward section ('sliding member') to its rearward/inner section ('annulus'). If not slipping the O/D is effectively a solid shaft when O/D is disengaged/off (i.e. the cone clutch, sun gear, planetary gears and annulus are essentially a solid unit driving the output shaft).

- The operating valve's ball bearing is held down by both a spring and oil pressure when O/D is not engaged. When energized the solenoid rotates a cross-shaft--it has an arm on the opposite side for 'adjustment'--which has a cam on it that lifts the shaft and ball and allows pressurized fluid to flow to the operating pistons. The two operating pistons overcome the heavy springs and pull the sliding member--with the wet clutch lining--against the brake ring, locking the sun gear in place, and allowing the planetary gear carrier to rotate the planetary gears about the stationary sun gear. The planetary gears, being both rotated by the carrier and about the sun gear, drive the annulus to overrun the gearbox output shaft's rotation; hence 'overdrive.'

The shop manual is insistent that the 8 heavy springs be replaced on overhaul, and I wondered if it's necessary--my BJ8's O/D was still operating fine at 200K miles--but given the work required to R&R the gearbox and O/D, and the hassle of getting the O/D mated back up to the gearbox I did as ordered.

Now, can someone explain how my O/D worked at all given the state of the rear thrust washer (note fretting inside the annulus):
 

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AUSMHLY

AUSMHLY

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Maybe I'm chasing my tail.
Question: Should there be any PSI when in neutral after the cars been in gear?

Shop manuals say should have 470-490psi when in gear, I didn't read in the shop manuals what the pressure should be and for how long once put in neutral and turn the car off.

Please see my first post about why I'm pursuing why I have no PSI when in neutral after being in gear.

I understand starting the car, not in gear, no psi in the tranny. Once in gear, pump creates pressure. Above article points out pressure drops to 250-300 and stays there for a couple hours once in neutral and cars turn off.

True or not?
 
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Bob_Spidell

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re: "Maybe I'm chasing my tail."

Probably. If any of your pressure-holding devices--non-return valves, accumulator--were faulty your O/D wouldn't be able to develop sufficient pressure (350PSI+) to operate in the first place. The only thing I can think of would be your O/D throttle switch being stuck in the on position, allowing pressure to bleed past the operating pistons' seals--O-rings or steel rings--but, you wouldn't be able to drop out of O/D with the dash switch (in the expected second or two).

If you really need to know, do as 'Nut suggested and go through the O/D, though a definitive answer isn't guaranteed.
 

CraigC

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Maybe I'm chasing my tail.
Question: Should there be any PSI when in neutral after the cars been in gear?

Shop manuals say should have 470-490psi when in gear, I didn't read in the shop manuals what the pressure should be and for how long once put in neutral and turn the car off.

Please see my first post about why I'm pursuing why I have no PSI when in neutral after being in gear.

I understand starting the car, not in gear, no psi in the tranny. Once in gear, pump creates pressure. Above article points out pressure drops to 250-300 and stays there for a couple hours once in neutral and cars turn off.

True or not?

For the "harsh disengagement" problem you indicated earlier in this thread, you are ABSOLUTELY chasing your tail. The rapid drop off of the pressure indicates, as Bob Spidell noted, that somewhere there is a sealing point that is no longer sealing as intended. It may also play into the lower than ideal, but still more than needed to operate, system pressure.

Again, harsh disengagement can usually be attributed to an incorrectly adjusted throttle switch or an improper downshift technique by driver. Treat the throttle the same way you would on a standard 4-3 downshift. The better job you do at "rpm matching", the smoother the downshift. That is the point of the throttle switch but it does require proper driver input to achieve.

re: "Maybe I'm chasing my tail."

Probably. If any of your pressure-holding devices--non-return valves, accumulator--were faulty your O/D wouldn't be able to develop sufficient pressure (350PSI+) to operate in the first place. The only thing I can think of would be your O/D throttle switch being stuck in the on position, allowing pressure to bleed past the operating pistons' seals--O-rings or steel rings--but, you wouldn't be able to drop out of O/D with the dash switch (in the expected second or two).

If you really need to know, do as 'Nut suggested and go through the O/D, though a definitive answer isn't guaranteed.
Bob, the throttle switch is completely out of play, as far as solenoid operation is concerned, once you shift into neutral. The inhibitor switch has gone open and cut power to the solenoid once out of the 3-4 gate, so pressure/flow to operating pistons has been isolated from system/accumulator pressure.

Your thrust washer in an earlier post looked pretty well beat up. I'm not too surprised the OD still worked. Was it "a bit" noisy when in OD?

As for going through the whole OD, I'd save that for when you encounter a true low pressure-related issue. Doing it now would put it in the category of a "blanket repair" where everything gets replaced, but you may never know the actual cause. Although it would not likely apply in this case, I did watch former coworkers get burned by this method. Do a costly "repair", and still have the same problem, because they failed to do sufficient diagnosis first.
 

Bob_Spidell

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re: "Bob, the throttle switch is completely out of play, as far as solenoid operation is concerned, once you shift into neutral."

You are correct, sir.
 
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Bob_Spidell

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Still wrapping my head around this (and looking at the schematic). I think, yes, shifting into neutral, first or second cuts power to the solenoid--it's a single wire to and SPST switch--BUT ... the throttle switch keeps maintains current to the O/D relay until the dash switch is thrown AND the gas pedal pressed enough to open the open the points in the throttle switch. Soooooo ... if you shift from 3rd or 4th to neutral, without opening the throttle the required amount (~5/16" at the lever) then back to 3rd or 4th the O/D reengages. Note shifting from 3rd to 4th, or vice versa, keeps the gearbox switch closed.

IOW, the gearbox switch overrides both the dash switch and the throttle switch, but you aren't completely out of O/D until the dash switch is thrown AND accelerator pedal pressed the requisite distance. This is to prevent damage to the gearbox if you throw the switch with closed throttle, resulting in reverse torque to the drivetrain. It's kind of a 'Rube Goldberg' setup but, apparently DMH didn't trust his customers as, I'm told, TR6s have essentially the same O/D with none of the 'safeguards.' Ditto the somewhat crippled O/D accumulator.

Amiright?
 

CraigC

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It would go back into overdrive, but consider that you likely would have slowed to the point where throttle application, once back in gear and trying to maintain speed, would cut out the relay. It's kind of an odd scenario that I can't say I have ever found myself in.

The throttle switch is there primarily to prevent harsh downshifts and possible excess wear/damage.

The inhibitor switch is certainly the easiest way to prevent OD engagement in reverse. Consider that they could have mounted the switch to only be triggered by movement of the reverse shift rod, which would have allowed OD in 1st and 2nd as well as 3rd and 4th. Maybe this switch location was easier or maybe they didn't see a need for it in 1st and 2nd. Who knows? I once tried running my car for awhile with the inhibitor switch bypassed as an experiment. Did not see any benefit. Other vehicles that use the Laycock OD have some far more complex electrical control circuits than the Healey.
 

John Turney

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Still wrapping my head around this (and looking at the schematic). I think, yes, shifting into neutral, first or second cuts power to the solenoid--it's a single wire to and SPST switch--BUT ... the throttle switch keeps maintains current to the O/D relay until the dash switch is thrown AND the gas pedal pressed enough to open the open the points in the throttle switch. Soooooo ... if you shift from 3rd or 4th to neutral, without opening the throttle the required amount (~5/16" at the lever) then back to 3rd or 4th the O/D reengages. Note shifting from 3rd to 4th, or vice versa, keeps the gearbox switch closed.

IOW, the gearbox switch overrides both the dash switch and the throttle switch, but you aren't completely out of O/D until the dash switch is thrown AND accelerator pedal pressed the requisite distance. This is to prevent damage to the gearbox if you throw the switch with closed throttle, resulting in reverse torque to the drivetrain. It's kind of a 'Rube Goldberg' setup but, apparently DMH didn't trust his customers as, I'm told, TR6s have essentially the same O/D with none of the 'safeguards.' Ditto the somewhat crippled O/D accumulator.

Amiright?
As for the last paragraph, once you open the gearbox switch, the throttle switch is immaterial. If you open the dash switch and mover the gear lever out of 3/4 to 1/2 or R, power is cut to the overdrive relay and it opens.
 

Bob_Spidell

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I wouldn't say 'immaterial,' but 'overridden' works for me. Consider: Coasting--foot off gas pedal--in 4th/OD. Throw dash switch to 'Normal' (off) foot still off gas pedal, O/D remains engaged. Put gearbox in neutral (solenoid off), O/D will disengage then put gearbox back into 4th without pressing gas pedal; O/D will reengage--throttle switch still 'in play.' Circuit to gearbox switch is 'hot' until a) dash switch set to 'normal/off' AND throttle opened (regardless of gearbox switch state). Or, 'kickdown:' coming up on slower traffic coasting in 3rd/OD or 4th/OD; throw dash switch to normal/off, nothing happens. Get a chance to pass, stomp on the gas pedal and O/D is disengaged--effectively a downshift--then throw dash switch back to 'Overdrive' at max RPM to complete pass. Having that 'extra gear' in a tight spot has saved my bacon a few times.

"If effected with closed throttle, a change from overdrive to direct drive could result in a shock to the transmission[sic]. An interlocking circuit is therefor incorporated to override the toggle switch under closed throttle conditions."

You can switch from 3rd/OD or 4th/OD to neutral and back, but the O/D will reengage. Took me a while to grok this schematic:
 

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