News

TH400 Governor adjustment

Discussion in '365 GT4 2+2/400/412' started by fcf, Apr 13, 2020.

  1. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
    Hello,
    my th400 is shifting 1-2 and 2-3 to early. Presumably, the springs in the governor have become somewhat weaker during the last 35 years. With the B+M-Kit (different springs and weights) I want to readjust the shifting points.

    Does anyone have experience with it? Can the governor be removed at all (with the gearbox installed)? Or is there not enough space?

    Frank
     
  2. It's Ross

    It's Ross Formula 3

    Jul 30, 2007
    2,000
    Barrington, Ill. USA
    Full Name:
    Ross
    My understanding is the governor controls full throttle shift points, the vacuum modulator the part throttle operation. Not sure which you are finding objectionable.
    NO way you'll remove the governor with the trans in place. The modulator looks like it could be done no problem. Adjustable modulators are available for the TH400.
     
    raemin likes this.
  3. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    1,221
    Romulus, NY (Finger Lakes)
    Full Name:
    Ken Battle
  4. SouthJersey400i

    SouthJersey400i Formula 3

    Mar 14, 2007
    1,221
    Romulus, NY (Finger Lakes)
    Full Name:
    Ken Battle
    The "Vacuum Modulaor" can be changed easily but I do not think this is what you bought. There is plenty of room to change the vacuum modulator; I did it years ago.
    See You Tube:
    Video shows adjustment nut.
    Ken
     
  5. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    As Ross said, spring and weights are for high RPM (kick down). No point in messing with it as it's quite a good match for our engine (very low weight). On my second rebuild (I've got a heavily customized tranny), we've ended up removing the optional governor and used the stock one instead.

    Vacuum modulator can be adjusted with a very short flat screw driver. That's definitively the adjustment to be performed in order to suit your driving style.

    Is your ATF up to spec? Before making any adjustment I would first flush the gearbox twice and drive the car for a few hundreds kilometres so as to stabilize the ATF viscosity.
     
    au400i likes this.
  6. Al Campbell

    Al Campbell Formula Junior
    Silver Subscribed Owner

    Nov 22, 2013
    421
    Australia
    Have you checked the operation of the micro switch on the accelerator linkage? My car was changing too early when I purchased it & fitting a new micro switch transformed it. Under full throttle it now changes 1-2 at 100KPH and 2-3 at 160KPH.
     
  7. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
  8. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
    Actually, an (aftermarket) adjustable vacuum modulator is already installed. And I tried already to do the adjustment by drilling the small screw. But the result was only a slight difference in shifting. So, this was not successful.

    Going further, recently I found the following TH400 instruction:

    1) There is no "adjustment bolt on the side of the trans".
    2) The only way to make major changes to the shift speeds is to modify or replace either the governor or the valve body.
    3) You can make minor changes to the shift speeds by installing an adjustable aftermarket vacuum modulator.


    A few comments about how the trans works: decisions about what road speed to shift at, and how harsh the shift is, are made by a simple hydraulic computer. The harshness of the shift is determined solely by engine vacuum; the vacuum modulator is used to sense the vacuum and to change the vacuum signal into a pressure signal inside the tranny. The higher this pressure, the more harshly the trans shifts. This modulator pressure is also used to decide at what road speed to shift.

    The road speed is measured by a governor which is at the tail of the tranny. It is gear driven from the tailshaft and therefore it responds to driveshaft RPM which is proportional to road speed (unless the tires are spinning). The governor generates a pressure signal internal to the trans; this pressure increases with road speed but not in a linear fashion. Since the governor uses weights which are spun out by its rotation, the force (and therefore the pressure) generated is proportional to the square of the RPM.
    In an attempt to somewhat linearize this the designers have used two sets of weights, one light and one heavy. The two are separated by small springs. The heavy weights operate at lower RPM (road speed). When a sufficiently high RPM is reached then the weights hit stops and do not contribute any more to the pressure signal. At this RPM the light set of weights begins to provide the increases in the pressure signal. Higher RPM always means a higher pressure, but not necessarily in proportion to the road speed.

    The transmission decides which gear to use by comparing the pressures provided by the modulator and by the governor. If the difference between these two pressures exceeds an amount determined by a spring in a shift valve assembly then that shift valve operates and selects the next higher gear. There are two shift valve assemblies, one for the 1-2 shift and the other for the 2-3 shift. The system works as follows: assume that you are idling and then begin to open the throttle. The engine vacuum at idle causes a very low modulator pressure (nearly 0). The governor pressure is 0 because there is no tailshaft RPM (no road speed). As you open the throttle and begin to accelerate the engine vacuum drops. This causes the modulator pressure to rise. Assuming that the engine vacuum remains constant due to a steady throttle position, the modulator pressure will remain at a steady value. As the vehicle picks up road speed the governor pressure rises. When the governor pressure force exceeds the modulator pressure force by a small amount then the 1-2 shift valve operates and engages the clutch which shifts the tranny into 2nd gear. The rate of clutch engagement is a function of the modulator pressure; low modulator pressure causes slow engagement and a smooth shift.

    High modulator pressure due to large throttle opening causes harsh shifts. As the vehicle continues to gain road speed the governor pressure continues to increase. Eventually the governor pressure force exceeds the modulator pressure force by enough to operate the 2-3 shift valve. This activates another clutch pack which shifts the transmission into third gear.

    Note that I spoke of pressure forces above. This is due to the fact that the pressures operate on the ends of small pistons (the shift valves). The pressure applied to the area of the piston develops a force. Since the shift valves have two ends, and since pressure is applied to both ends (one end from the modulator and one end from the governor), the difference between the two forces causes the valve to move to one end or the other of its travel. Because
    the areas of the two ends of the valve do not have to be equal, the valve will operate in accordance to the difference between some constant times the governor pressure versus the modulator pressure. This fact allows the designer quite a bit of latitude in determining at what road speeds the transmisison will shift. There are also small springs which apply force to one end of the shift valves and these also affect the road speed at which the shifts occur.

    There are other complications in the tranny. For example, it has a means of deciding when the throttle is wide open (the detent cable) and WOT causes the modulator pressure to be ignored and instead a constant pressure from a detent regulator is used on the shift valves. This causes shifts to occur at the maximum possible road speed for each gear. There is a manual valve which is connected to the gear selector lever and which selects reverse, neutral, and can lock out either 3rd gear or lock out 2nd and 3rd gear. There is a main pressure regulator which is controlled by the modulator pressure; higher modulator pressures increase the pressure of the main oil which supplies everything else. Most of these complications can be ignored for the present time because Jon's interest is in the road speeds at which the tranny shifts.

    Now, to get to the question of adjusting the shift speeds. There is no "adjustment bolt on the side of the trans". The only way to make major changes to the shift speeds is to modify or replace either the governor or the valve body. You can make minor changes to the shift speeds by installing an adjustable aftermarket vacuum modulator. The total adjusting range of these modulators is about 4 to 5 MPH at WOT. At cruising speeds you will get about 2 MPH total adjustment range. This probably isn't enough to suit.

    Working on the governor is probably the best approach to adjusting the shift speeds. The factory actually offers a high shift speed governor; it was used in a few HP applications such as Camaros. You can just swap one of these in and you will see a dramatic increase in shift speeds. In fact, it may shift at too high a
    road speed after the change. E-mail me if you are interested in the part number; I will have to look it up and it may take a couple of days.

    Another approach to modifying the governor is to get a B&M governor kit and to install it in a NEW factory governor. Governors are very sensitive and delicate beasts; they "take a set" when they have been run for a while and they _don't_ like to be messed with. A new governor will probably save you the grief of a sticking governor after modification. The B&M kit comes with an assortment of small weights and several springs. You have to experiment to find the right combination. It took me 10 or 11 tries to get the WOT shift speeds that I wanted. The B&M instructions are downright poor; you need to change the weights to get the 1-2 shift speed that you want, and you need to change the springs to get the 2-3 shift speed that you want. Unfortunately, these changes interact and you will need to iterate to get both of them where you want them.

    The governor is fairly easy to change because it is accessible by removing a small cover on the side of the tranny at the rear. Caution: the trans will be hot when you are down there messing with the governor. It is easy to burn yourself. I suggest gloves. And you will probably want to lay in a small stock of gaskets for the governor cover.

    As I mentioned above, governors are finicky. Absolute cleanliness is a must. If ANY amount of dirt, grit, grinding compound, metal shavings, or any other foreign material gets into the governor then you will have all sorts of shifting problems which will likely be intermittent and erratic and near impossible to troubleshoot. The contamination can get washed out of the governor and end up in the valve body where it will cause bizarre problems.

    One final thought: your original question suggested that the shift speeds are too low at all throttle openings. One possible cause of this is a leaky governor. The leakage could be due to scoring of the valve journals, or to a loose fit between the valve journals and the governor shaft. Or it may be due to wear
    on the journals which the governor engages in the case. If looseness is the cause of your problem then just replacing the modulator should fix it. If case journal wear is the problem then you can get a kit from a tranny specialist which will allow you to put new bushings in the journal area. This fix requires removing the tranny from the vehicle so that you can properly clean out the metal chips after you have finished.
     
  9. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    #9 raemin, Apr 14, 2020
    Last edited: Apr 14, 2020
    1/ Bear in mind that our cars do go from 0 to 240km/h, so the 8km/h (5mp/h) adjustment that can be expected on a regular (0-170kmh) car is likely to become 12 km/h on your car which is not bad.

    2/ As said the stock governor is really skinny, I doubt you do need to be more aggressive than that.

    3/ Valve body kits (i.e a few additional holes in the "plumbing" of the gearbox) do modify the transition between gears. That's how a TH400 can be super smooth on a RollsRoyce: there is a long overlap between the 1st and 2nd gear that makes the shift point unnoticeable (so to say) on these cars. Smooth shifts are bad for the friction bands (? sorry for the poor wording) inside the gearbox and this gets even worse at high rpm. So upgrading for a more aggressive valve body kit can be a good idea if you like high rev and play with the gear stick. BUT... firm shifts are bad for the rear axle and the convertor. A firm shift above 5000rpm is a lot to ask for the stock convertor, as our engine do still have a lot of torque even at that level of rpm. (I've destroyed 2 converters ...)

    4/ As suggested above, you can play with the accelerator linkage and adjust the trigger point of the kick-down switch.
     
  10. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
  11. Al Campbell

    Al Campbell Formula Junior
    Silver Subscribed Owner

    Nov 22, 2013
    421
    Australia
    This graph looks like mine was with the WOT micro switch not working.
    The 1/4, 1/2 & 3/4 look about right but it should take a big jump up when the WOT is switched in.
     
  12. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    Could you elaborate on this graph? Is this actual measurement or your own calculations?
     
  13. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
    Interesting! I will validate it...
     
  14. fcf

    fcf Rookie

    Jan 30, 2015
    35
    Germany
    Full Name:
    Frank
    This was the actual measurement. I was driving some laps and recorded speed, rpm and shifting on video...
     
  15. Al Campbell

    Al Campbell Formula Junior
    Silver Subscribed Owner

    Nov 22, 2013
    421
    Australia
    rovexienus likes this.
  16. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    You could also investigate the convertor: at WOT, you've got very little slippage (approx 10%) on the red graph. I presume you are still on the original (first generation) bulky convertor. I would expect the convertor to free a bit more the engine.
     
  17. It's Ross

    It's Ross Formula 3

    Jul 30, 2007
    2,000
    Barrington, Ill. USA
    Full Name:
    Ross
    #17 It's Ross, Apr 16, 2020
    Last edited: Apr 16, 2020
    So, a little practical experience of my own with this transmission, albeit in other applications.
    Shift points are "modulated" according to manifold vacuum as seen at the modulator, some are adjustable, not sure what came with our cars. I'm now curious so will investigate this as my car is currently on the lift for other work.
    The governor "governs" hydraulic pressure applied to the clutch packs according to speed. Faster speed, more pressure applied. This is not adjustable by any means other than modification to the unit.
    If FIRMER shifts are desired there are a number of modifications to the valve body, mostly involving changes to the orifices in the separator plate which regulates fluid flow. A very simple modification is to entirely remove a specific check ball. All these modifications are documented elsewhere.
    "Shift Improver" kits are readily available from B&M among others. These typically consist of new separator plate, check balls and instuctions.
    The switch is for a forced downshift only. If the switch remains closed(or is it open?)the transmission will see this as full throttle so shift very late always and apply more pressure but at low RPM this will be counterproductive. You can fool the trans into this mode a bit sooner than WOT by adjusting the linkage to close the switch sooner.
    Also, the TH400 torque converter is quite robust and serves FAR more powerful engines than ours. As for the diff., automatics don't apply too much shock load so unless really dialing up the pressures I'd not worry about the diff either.
    Oh, something I accidentally discovered. The governor will, given enough RPMs(which will vary by model), command a 1-2 shift with full pressure applied, even with 1 manually selected. I've not taken my engine past 7k and this has not occurred to that point, not sure when it would with our version and I'm not interested in finding out. It caught me by surprise many years ago when winding out a big domestic V-8 in first.
     
  18. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    Raising the switch point to high is not such a good idea as the (heavy) drums have to be stopped and started during each shift:

    1/ while in first gear, the forward drum and direct drum are counter rotating (forward drum revs at engine rpm speed, direct drum at 84%)
    2/ during 1/2 shift point the 14 pound direct drum must be stopped while the 8 pound reaction carrier has to start rotating at 50% of rpm speed
    3/ during 2/3 shift both drums have to rotate in the same direction at rpm speed.

    The energy adsorbed by the friction bands in order to stop and start these drums is related to the square root of the rotation speed, so a few more rpm can significantly shorten the life of the gearbox. Unless you invest in ultralite drums ($4000), its much more safe to get an additional 1000rpm through slippage on the torque convertor rather than higher shift-points.

    A good convertor is $400 and not as labour intensive as other options.
     
  19. It's Ross

    It's Ross Formula 3

    Jul 30, 2007
    2,000
    Barrington, Ill. USA
    Full Name:
    Ross
    When you consider all of what raemin has just told us it's no wonder these devices sap so much power.
     
  20. Newman

    Newman F1 World Champ
    Professional Ferrari Technician Owner Consultant

    Dec 26, 2001
    13,446
    Canada
    Full Name:
    Newman
    Its a motorhome transmission. The only reason I can see that Ferrari chose that particular auto transmission is the sturdy case is a good candidate for the torque tube arrangement. The Turbo 350 weighs less and uses less power but probably isn't stout enough to be a supporting structure in the drive train.
     
  21. raemin

    raemin Formula Junior

    Jan 16, 2007
    377
    Lyon (FR)
    Full Name:
    R. Emin
    Back then there were not so many transmissions available. Turbo 350 does not offer additional versatility in terms of gear ratios and as you said it was not as beefy as the Turbo 400. Some "case saver" kits were sold in order to make the 350 a bit more durable, which speaks volume on its durability.

    700R4 and 4L80 were not available at the beginning of the 400era and when the 700R4 was made available in 1982 Ferrari elected to stay with the TH400 with a new rear-axle (shorter ratio) and a better convertor. I can see why Ferrari did not upgrade the transmission: apart from the numerous failures of the first 700R4, it does not fit in the tunnel (4L80 is even worse) unless you perform major surgery in the fibreglass (and upholstery). Also it's got an odd gear ratios, coupled with the fourth overdrive that more or less requires to borrow the rear axle of a manual 400. So assuming you've got the proper rear axle, you end up with a not so pleasant transition between short first gear and way to long second gear. In 2014 there was on ebay a twin turbo 400 updated with a 400R4, you could see the botched upholstery, and the honest seller quoted the car as sluggish at lower speed.

    Nowadays, more modern transmissions could be contemplated, but they rely on sophisticated electronics which is an additional level of complexity. No nonsense approach is to upgrade the TH400 and either hit the bullet with lite drums and/or borrow parts from the 4L80. The 4L80 is basically a more modern TH400 with an overdrive. Many parts were updated (eg: sprag), but are still compatible with the older TH400.
     
  22. Newman

    Newman F1 World Champ
    Professional Ferrari Technician Owner Consultant

    Dec 26, 2001
    13,446
    Canada
    Full Name:
    Newman
    Those transmissions weigh a ton but the extra gear and with a better converter would be worth it. The tail stock bolt pattern is very similar on both if not the same - would be best to not have to fabricate the ferrari specific piece if possible. Not sure about the output shaft though but not a big deal to fabricate a coupler. As for axle ratios, I have them made in custom ratios for Boxers and Testarossa's so I wouldn't be limited to whatever Ferrari offered or the Ferrari tax associated with their parts.
     
  23. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    72,219
    Houston, Texas
    Full Name:
    Bubba
    The TH400 was designed for Pontiac high power applications, the big blocks.

    Motorhomes....well maybe!!
     
  24. BigTex

    BigTex Seven Time F1 World Champ
    Rossa Subscribed Owner

    Dec 6, 2002
    72,219
    Houston, Texas
    Full Name:
    Bubba
    That science it tight, to really alter the shifts very far you modify the springs and ports of the valve body.

    In the day you needed a "stall converter" to launch it off the line burning tires, and a B&M kit to do the valve body.

    As noted above, sludged up fluid and filter can really cause problems so a clean and flush might make a while new world...
     
  25. Newman

    Newman F1 World Champ
    Professional Ferrari Technician Owner Consultant

    Dec 26, 2001
    13,446
    Canada
    Full Name:
    Newman
    No it wasn't. It was designed initially for Cadillac's and Buicks then used in Olds and heavy trucks and yes they are in motorhomes. Its not a good transmission for drag racing because it eats a lot of power.
     

Share This Page