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Discussion Starter #1
Can anyone help with information about the basic setup of the TH400 gearbox.
We are in the UK and have been supplied with a new UK built sailplane winch with a new chevy 8.2 litre engine driving through a "new" TH400 gearbox.

We believe we are having problems with the gearbox changing down at too high a speed when reducing speed and consequently shifting down too hard but the supplier insists there is nothing wrong with the gearbox because he has supplied about 50 similar systems, although he admits he does not know how the gearbox works.

There is no tachometer and no milometer installed, there is no identification plate on the gearbox.
I understand these gearboxes were last made in about 1990.
It has an adjustable vacuum modulator adjusted fully out, we dare not yet touch the governor, it is still under warranty
My questions are:-

1 Is it possible that this gearbox has been kept in store in the USA since manufacture, or is it more likely to be a reconditioned unit?

2 When the gearboxes were originally built were they modified inside to suit a particular car/truck or were they all identical apart from the makers improvements?

3 If there were modifications to suit the car, would they have significantly affected the gear change speeds and softness?

Thanks in advance for your help.

colin milner
 

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Colin,the unit is more than likely remanufactured,as opposed to being new,then stored.

Transmissions are calibrated for specific vehicles,of course,but was this unit calibrated for your particular needs,I don't know. Since the supplier has 50 unit sales under his belt,he should be able to answer this question.

There are many modifications that can be done to this unit,as with most.

Again knowing what may have been done would be helpful.

I would think this company should have tech support for their units.

Knowing what the vacuum is doing during your concerns,may help as well,as in could there be a leak.

I really don't know what a sailplane winch is,or how it is used,relative to the trans. If you like, a further explanation on it's function may help with diagnosis.

Look forward to your future post.
 

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Discussion Starter #3
Briscoe,
Many thanks for your answers. The winch is just a drum with about 1000 yards of steel cable on it driven by the engine and automatic gearbox so that the cable pulls in fast enough to launch the sailplane.

So we accellerate the sailplane in about 5 seconds to 60 mph and then as the sailplane gets higher the drum is slowed down to keep the sailplane speed constant.
The winch is stationary and the driver just has a throttle and a lever for selecting neutral or drive, all other gear options are inaccessible.
We do not know of any modifications carried out in the gearbox , and I have a feeling that none have been done.

The gearbox starts in 1st and gets into 3rd before the sailplane is off the ground, all this is ok, but as we reduce throttle to stop the glider speeding up in the top half of the launch, the gearbox changes down into 2nd and jerks the cable and sometimes breaks the cable.
What we would like to know is have we got a gearbox which has been designed to change down gear at too high a speed, we would like it to stay in 3rd once it is there and not risk changing down and breaking the cable, we think it is doing about 2000 rpm under half throttle when it changes down , is this what you would expect.
Once again, many thanks for your interest.
colin milner
 

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So as the plane gets higher of the ground, you throttle back the engine and the trans downshifts from 3rd to 2nd???

It sounds as though the transmission thinks that you arebringing the engine back down to idle and therefore it brings the trans down into 2nd and eventually into 1st gear.

The400 trans was last used by GM in the trucks in 1990, it was replaced by the 4 speed 4l80e trans.

If there was a handle that controlled the upshifts and downshifts, you could install a manual shift kit into the trans and it wouldnt downshift into 2nd on you.

To me, the problem sounds like a simple adjustment in the trans control.

Question, You throttle down to keep the plane speed constant, then the trans downshifts. So being in 2nd gear, this would allow slack to form between the winch and the plane correct? Do you break the cable increasing the throttle to take up the slack?
is there any way to launch teh plane by not throttleing back the engine and by just releasing teh cable from teh plane while under throttle?

here are the gear ratios 1st gear 2.48:1, 2nd 1.48:1 3rd 1:1 or direct drive.

I bet that since its powered by a 502, that you have spend a decent amount of money on this power winch. I would be all over the person who sold you the winch, esp. since it is STILL under warranty. I am sure his other 50 installed winches dont break the cables.

It sounds to me that you need to adjust that vacuum governor. While you are under throttle accelerating the plane, the trans goes through the gears normally, then as you back off the throttle(thus creating more engine vacuum) the trans downshifts because it thinks the engine is returning to idle.

I would get that vacuum modulator adjusted, being adjusted completly out doesnt sound correct to me.
I always thought a thm 400 had an electric kickdown for downshifts.

goodluck
peace
Hog
 

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Colin,2000 rpm on coast should not cause this trans to downshift.

I wonder, is the governor causing this,and I'll get back to the gov in a minute.

If there is no tech support with this trans,and this is on your shoulders,can you do a line pressure test?

This would tell us the condition of the pump,detent circut, and help to narrow down the possibilities.

But simple stuff first. Do you have a vacuum gauge? If so what is the vac doing when this trans downshifts?

Is there electricity to the trans for passing gear,I doubt it,you wouldn't need it for this application,but you never know. But if so,the detent connector[passing gear] is on, what would be the drivers side of the trans.

Like I said,doubt it's there but if it is,disconnect it and see if it makes a difference.

Now back to the governor. On the passenger side,there is a four bolt cover,towards the rear of trans. This is where the gov is housed.

I would remove that cover,pull the gov out and measure the shinny lands.

The lands should be smooth and measure .800 Not much wiggle room here.

A governor that measures .799 would be acceptable,but no less.

Please keep me informed. Dana
 

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Discussion Starter #7
CHEVY 502 & TH400 IN SAILPLANE WINCH

Johnsongrass1,
I will post a photograph when I get a good one.
cm
 

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Discussion Starter #8
CHEVY502 &TH400 IN SAILPLANE WINCH

Hogg,
Yes as we back off the throttle the cable whips when it changes down from 3rd to second. We do not have chance to take up slack, the power is there all the time because the guy in the sailplane is still climbing, but because of the cable angle the drum has to slow down to maintain the same speed for the sailplane.The sailplane still has half of the climb to complete at this stage,so we have to keep some power on.
I am sure the electric kick down is disconnected if there is one.
colin milner
 

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Discussion Starter #9
CHEVY 502 & TH400 IN SAILPLANE WINCH

Briscoe,
Yes I have ordered a vacuum gauge and will get some figures asap.
No electrics to the box. Connection, drivers side half way along with no wire on.
Will check the governor, thanks for the figures on the diameter.
colin milner
 

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why not just run a full manual shift tranny? Shift the gears as you wish?

:cool:
 

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Crosley,I was wondering the same thing.But I went back and checked, Colin states in post #3 that the driver has a lever for selecting neutral or drive,all other gear options are inaccessible.

Perhaps not the unit itself,but maybe the lever will not physically travel past drive???

This may explain why the manufacture declined a manual shift unit.

Of course this is not to say this couldn't be modified.
 

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briscoe said:
Crosley,I was wondering the same thing.But I went back and checked, Colin states in post #3 that the driver has a lever for selecting neutral or drive,all other gear options are inaccessible.

Perhaps not the unit itself,but maybe the lever will not physically travel past drive???

This may explain why the manufacture declined a manual shift unit.

Of course this is not to say this couldn't be modified.

Any thing can be modified , it's just steel and aluminum.

:D

If vacuum drops low enough at the modulator the t-400 will down shift out of 3rd gear.

An adjustable mod may help , I dont know.

Never dealt with this setup. I've Fooled with boats , air & water (prop) driven .

:D
 

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That launch really got my attention. I googled v8 sailplane winch and found this:

Winch Launches
If you can, try to get a flight at a gliding club which uses winch-launching. Yes, gliders are actually winched into the sky on the end of a long cable. The winch is powered by a powerful motor, often a V8, and it pulls the glider up like a giant kite. Sometimes the cable breaks, but this is quite routine and simply results in a short flight and a bit of inconvenience for the ground crew! Come to think of it, this situation could be considered a thrill ride for an unsuspecting passenger! Firstly, the jolt when the wire snaps. Secondly, the fairly sudden nose-down movement of the glider as the pilot makes sure the airspeed stays up. Sudden but smooth.

Now, why are even normal winch launches thrilling? Because you go up like a fighter jet, that's why. The nose of the glider points skywards at an angle of 45 degrees or so, and you reach release altitude in less than 30 seconds. Even an experienced glider pilot tends to enjoy it time after time! I certainly did.

An even more thrilling form of launch is rarely done now-a-days. It was called a reflex launch. The tow rope, yes a rope not a wire or cable, had a bit of stretch in it. It was attached to the glider's release point at one end, and the other end was attached to a powerful car. A loop was laid out to set a certain amount of slack in the rope, before the car went charging off down the airstrip. TWANG. I remember seeing a Blanik L13 with 2 people on board lunge off the ground in maybe 3 meters (10 feet) and commence climbing like a home-sick angel. Ah those were the days.



Oh yeah!
 

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About 20 years ago I spent a weekend gliding with both air and ground tows at Euroa Vic (1,1/2 hrs north of Melbourne). The ground tow sat on a 3 ton truck and comprised a 455 Olds,t400 and M/benz 1418 diff with a drum on each wheel and independent brakes. I remember clearly being told that the trans only used 2nd gear and even in the plane a kilometre away you could hear the 455 load up against the converter - perhaps this could work for Colin with a full manual valve body.
BTW, to Colin and myself a th400 has the kickdown wiring on the PASSENGER side, so I respectfully ask that people consider rhd applications and use a simple left or right description (as viewed from the drivers seat), thanks. Ian.
 

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My apologies to our friends down under,and to our other friends,in other countries that drive on the wrong side of the road :D

I sometimes forget that WWW stands for world wide webb.

I will be more considerate in the future.

Anyway,when I first was trying to picture this sailboat winch thing in my mind I envisioned someone sitting right on top of the trans,throttle in one hand,shift lever in the other,letting the cable out. Kinda like road warriors :D

Now if I had transferred that thought,to this thread,I could have said....Colin look beside your right foot,see that cover with four bolts,thats where the governor is!! :D

Unless he was sitting on top of the transmission....backwards. In which case I would have suggested he look beside his left KNEE.

Not sure how anybody else feels about it,but sometimes I just crack myself up.

Now,if that euphemism gets 'lost' in the application,it only means,sometimes,I'm easily amused. :D :D :D
 

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Discussion Starter #16
Briscoe
Yes it, is by my right foot thanks.
At this stage we are still trying to establish some facts so that we can discuss things with the supplier. Do you know if there are established figures for change down rpm at set manifold pressures?

cm
 

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Colin,as you may know, vacuum will decrease under a load.

Conversely,it will increase when you reduce throttle.

I would answer no to your question.

Since the governor in short, provides the fluid pressure to the shift valves to stroke them[for a shift to occur],problems with the governor circut will cause this trans to fall out of 3rd.

The gov lands,the gov bore in the case. If there is a thimble filter that is stopped up,these things will cause your problem.

Trapped vacuum,will cause a no line rise condition,which could hurt the clutchs.

Knowing what the vacuum is doing when this problem occurs will help us.
 

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Discussion Starter #19
Eliot,
Thanks for the suggestion, in stronger winds when it tends to change down earlier we may try that.
cm
 

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Could also be the wrong spring on the 2/3 shift valve. Hang on I just had a thought (first for '07). As the drum pulls the plane up it also winds wire onto the drum thereby making the drum diameter 'grow' just as a fuellers tyres do, would this not increase the load causing a downshift? Plot the vacuum readings over a full pull and if they drop dramatically as the shift occurs then you'll have to file the gearshift to lock 2nd, bash the bloke you bought the 'tug' from and buy me a beer. If I'm wrong all bets are off. Lock 'er in 2nd.
 
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