Omega Owners Forum
Chat Area => General Car Chat => Topic started by: Webby the Bear on 28 March 2014, 14:02:11
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Hi guys,
Ok, doing level 3 transmission at the moment. It's enough to boggle the mind :o (well mine anyway)
So, we're talking about autoboxes and distinguishing torque convert faults and gearbox faults when the teacher starts talking about an auto stall test. I'd never heard of it but the premise is you chock the wheels, handbrake on, put it in 'D' and floor it for 30 seconds. In that 30 seconds it should stay on the red line. If it doesn't and only gets to say 5000rom then it could be a sign some brake bands are worn.
Now don't shoot the messenger (as mentioned I'm still only learning) but if this is true surely the car at that sort of revs would go over the chocks and the handbrake wouldn't do sh*t.
Anyone ever done this?
Your thoughts.
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He's doing it wrong! :o
The revs should get nowhere near the red line, and you should do it for nowhere near 30 seconds.
Think about what happens when you take off at full throttle in an automatic car. Engine revs rise immediately to perhaps around 2000-2500 RPM then, as the car gains speed, the revs increase until it eventually changes up at around the red line RPM.
Imagine that the car never accelerates but is anchored to the road. Engine revs will just sit at the initial 2000-2500 RPM.
The reason for this is that the torque converter has a "stall speed" which is the difference between the input and output speed for a given amount of torque applied from the engine. At maximum engine load and zero road speed, this is equal to the engine RPM, hence the method of measuring it.
The stall speed of the torque converter is selected to allow the engine to reach a speed where it's developing a reasonable amount of torque as the car pulls away, giving good acceleration from rest. It's analogous to the few seconds where you slip the clutch as you pull away in a manual car. If you don't do that, the car bogs down in the region between idle speed and about 2000 RPM where the engine is making very little torque.
With the torque converter stalled, the output torque of the engine is multiplied by a factor, actually giving more turning force into the gearbox than the engine is producing. This effect occurs when there is maximum load on the engine. As the engine load decreases and the vehicle gains speed, the torque converter unstalls, the torque multiplication reduces and it starts to behave more as a solid coupling like the clutch you have in a manual box. So, the torque converter behaves as a kind of continuously-variable gearbox. At a certain speed, most automatic transmissions pull in a clutch which disables the torque converter for added fuel economy (torque converter lockup). This happens at about 45 MPH in an Omega under light load, and considerably faster under full load.
If the stall speed is too low, the car is really sluggish off the line, as not enough torque is available from the engine until the vehicle has accelerated a little. If it's too high, fuel consumption is affected, as the torque converter operates inefficiently, unnecessarily multiplying the engine torque when it's not required.
The stall speed for a given application is fixed by the design of the torque converter. Testing it does give you some information about the condition of the engine and transmission, though. If the stall speed is not within the expected range, it can point to a few faults:
1) Engine performance is poor (low torque output gives low stall speed)
2) Brake bands / clutches in the gearbox are slipping
3) The torque converter is faulty (although there's not a great deal that can go wrong with them)
4) ATF is in poor condition or the level is low
5) Torque converter is not filling with ATF (filer blocked / pump problem)
The reason you only want to do it for a few seconds is that during that time, the torque converter is taking 100% of the engine output and turning it into heat, so you need to be careful that you don't overheat it.
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Thanks Kev, kinda confirmed what I thought.
I think I understand what you're saying. initially revs increasing but no acceleration is effectively like sitting in park and revving... i.e, just engine RPM vut no vehicle movement?
So what would you be looking at for the omega? put your foot down and it will sit at 2500rpm?
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Mr.Wood, I salute you! That is the most comprehensive and clear description I have ever read - and I almost understood it.
On an allied question, is there a danger of overheating if you hold the car on an incline in gear for more than a few seconds? I have to plead guilty to doing that on occasions due to laziness.
Ron.
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If the revs got anywhere near the red line during a stall test you'd have big problems :o
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Thanks Henry. So whats the actual correct procedure? :) And what are we looking for?
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Procedure from TIS (for a 3.2 auto):
Important:
The test should last for a maximum of 5 s , because the automatic transmission oil gets extremely hot. Check the oil temperature with the diagnostic tester before running the tests. If necessary, let the automatic transmission oil cool down first.
Parking brake applied
Brake pedal fully depressed
Engine running at idle speed, operating temperature
Automatic transmission oil temperature between 45 °C (113 °F) and 85 °C (185 °F)
Selector lever in position D
Accelerator pedal actuated to full load stop
Record stall speed in the form sheet for automatic transmission diagnostics.
Nominal Value
2200 rpm to 2500 rpm
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Pop to Sainsbury on your way to class next time, Webby.
Buy a large haddock.
Slap your instructor about the face with it when you get to college.
You can say it's from me.. ;D
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Seems like a fine way to wreck a gearbox! ::)
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Thanks guys and Aaron, yeah i'll be 'aving a word with him!!!!
Kev thanks very much for that. can you just explain what accelerator actuated to ull load stop means? am I looking to just get it to between 2-2500 revs? :-\
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Thanks guys and Aaron, yeah i'll be 'aving a word with him!!!!
Kev thanks very much for that. can you just explain what accelerator actuated to full load stop means? am I looking to just get it to between 2-2500 revs? :-\
"Floor it" :D
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Thanks guys and Aaron, yeah i'll be 'aving a word with him!!!!
Kev thanks very much for that. can you just explain what accelerator actuated to full load stop means? am I looking to just get it to between 2-2500 revs? :-\
"Floor it" :D
And a 3.2 Omega shouldn't exceed 2500rpm during the test. That is to say...
'With the accelerator pedal held to the floor, in drive and brakes on, anything outside the expected RPM will point to a potential problem.'
Or to put it another way... The engine shouldn't rev past the given stall speed, regardless of how hard the accelerator is pressed :y
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And anyone who regularly launch-start their Omega autos (who doesn't?) will know the sall speed of the TC is approx. 2.5k rpm, maybe a shade less.
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Seems like a fine way to wreck a gearbox! ::)
seems like a fine way to wreck a car
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Thanks guys. su just to get this right... in these conditions (handbrake on, wheels chocked, box in 'D') with my foot to the floor on the accelerator it will go to 2500rpm and shouldn't be below that or exceed it?
:)
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Precisely :y
Autodata should be able to tell you the expected stall speed for any given car/engine/gearbox :y
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Thanks guys. su just to get this right... in these conditions (handbrake on, wheels chocked, box in 'D') with my foot to the floor on the accelerator it will go to 2500rpm and shouldn't be below that or exceed it?
:)
Nominal Value
2200 rpm to 2500 rpm
The quote was for a 3.2 and indicates a stall speed in the range 2200 to 2500
Your 2.5 will probably have a very slightly higher range (I'm guessing 200rpm higher eg 2400 to 2700)
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Obviously, if you're going to do this test, the car park at beach head is probably not the best place. :-X
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Precisely :y
Autodata should be able to tell you the expected stall speed for any given car/engine/gearbox :y
You learn something new everyday! Cheers Taxi, Kev et al :y :y :y
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Obviously, if you're going to do this test, the car park at beach head is probably not the best place. :-X
;D I think I may leave this test until I've got some experienced OOFers watching my back :y
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Obviously, if you're going to do this test, the car park at beach head is probably not the best place. :-X
;D I think I may leave this test until I've got some experienced OOFers watching my back :y
They certainly won't be stood in front of you ....... :)
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Obviously, if you're going to do this test, the car park at beach head is probably not the best place. :-X
;D I think I may leave this test until I've got some experienced OOFers watching my back :y
They certainly won't be stood in front of you ....... :)
;D ;D ;D