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MG MGF Technical - Emergency Braking w/ ABS

Happy New Yr.

Question on how the MGF's ABS brakes handle emergency braking situations (dry conditions). I had two instances over the past two months where I have had to engage hard braking and I was a bit surprised by how the brakes/ tires handled. I have a ABS installed 98 VVC with the factory installed Goodyears (yes I did checked under the hood to see that the ABS unit is indeed installed!). In one case (fortunately, on an empty stretch of road in a local warehouse district), I nearly ran a red light (doing about 80-100kpm) and had stomped on the brakes about 50m from the intersection. (driver stupidity - fiddling with the radio and didn't notice the upcomeing red traffic light). The tires gave a long and loud screech for the full length to stop without the telltale signs of ABS engagement (brake pedal vibrating). No turning was involved, but the car did jiggle a bit side to side as it came to a stop. Is this normal (tire screeching and slight side waggle) for the MGF w/ ABS? Now my only other experience with ABS has been with a 95 Saab 900 coupe where the only time I've ever got tire screeching was from accelarating hard through corners. In hard braking or slippage, the ABS clearly engaged and the car was as steady as a rock through the braking.

Similiar screeching happened to me last nite when a truck suddenly swung into my lane forcing me to brake hard.

While braking on a straight path, I thought screeching tires indicate that wheels are locking. Isn't that what the ABS is suppose to prevents? Should I get my ABS unit checked?

Earlier in the summer, I did test out the ABS on a gravel road and the ABS did engage then. Haven't had to brake hard in wet conditions yet.

Also, does anyone know off-hand what the braking distances are for the MGF with ABS for various speeds?

Mike
Mike

Mike,

On the 99 VVC ABS (should be the same as the 98) there are 5 channels, 1 for each wheel and a backup (I don't know how the back channel works)

The ABS detects brake lockup and lifts the brake pad up off the disc (the tyre will be screeching at this point) and then once the lockup has gone it allows the brake pad back down onto the disc. Since this up and down motion happens a number of times per second the screeching will sound like one long screech and not a hundred small screeches. The side to side movement has to do with ABS working independently on each wheel, The front left may be locked up but the front right may not... so the car will pull to the left (Stick one side of the car on the road and the other on the dirt at the side of the road and test it out)

Since it takes about 2-3 sec of hard braking to lock the brakes, ABS is only useful on long hard braking....

In the wet & cold the brakes take longer to lock so ABS takes even longer to kick in ( I learnt this the hard way and took an inch of the front of the "F")

Matt





Matthew Minion

Matt,

Thanks. That's what I understood how ABS works and explains the screeching tires. But what I was troubled about is that I did not feel the usual rumbling of the brake pedal signaling that the ABS have been engaged. Maybe I should find a safe place to test out the ABS brakes this weekend.

Michael
Mike

I get a distinct rumbling from my ABS as you'd expect.

I'm pretty sure that it engages way before 2-3 seconds though. If you think about it 2-3 seconds in an emergency situation is often too late.
Julian

whenever it is icy, testing your ABS is dead easy. Drive 10mph (yes, TEN mph is enough) and push the brakes very hard. ABS will engage instantly!
(plus during darkness, you'll notice some current fluctuation in your headlights due to power hungry ABS system plus a loud rumbling noise)

I always do it for fun at my appartment during icy evenings :-)
Dirk Vael

Yep Dirk, I've tried that. Good fun it is aswell. A perfect straight line stop on sheet ice (there was no one around and I was on a private wide road when I tried it!) The ABS cut in within a microsecond of me hitting the brakes.
John

Mike,

I think you have something wrong with your ABS.I have tested my ABS on an empty road and the judder from the ABS comes through the pedal straight away and without any tyre screeching. I have used them in "anger" in France when pulling away from lights on wet cobblestones - agian instant pedel feedback and no tyre noise. Finally I have been taken around a race track in an F to be shown how well the car handles under full braking from over 100mph - again no tyre noise and total control.

If your car is laying rubber for 50 feet it ain't got ABS working!

Hoe this helps

Cheers

Patrick
Patrick

"... I have used them in "anger" in France when pulling away from lights on wet cobblestones "

Surely your F is fitted with traction control then?!

;o)
Julian


Hi all, has anyone experience from Racelogics (or any other) traction control ? Is it just a launch control or has it also good effect on slides and rear end outbreaks ??

Carl.
Carl

Mike, your car is behaving exactly as my non-ABS equipped car does under heavy braking. This is as sure a sign that something is wrong as any!

Best get the local dealer to have a look at it.

Carl, the racelogic system is more than simply a launch control- it actually utilises the standard car's ABS wheel sensors to modulate the engine power (I don't know the specifics- perhaps an e-mail to their customer services dept?)

Rob
Robert Bell

Thanks guys for the input. That's how I thought a PROPER working ABS should act. Wasn't sure if the F, being so light, would show a little wheel slippage (thus screeching) between ABS engagement as Matt mentioned. I was planning to have the car taken in for other minor problems, petrol fumes (back w/ a vengeance - despite a new tank with a new lining), so I will definitely have them check the ABS. Out of curiousity, if there is some fault with ABS would it show up on the diagnosis (just had the car in before x-mas for the first 6mos service) or wouldn't the ABS light on the dash (actually is there one? haven't checked) stay on if the ABS unit is faulty? Unfortunately (fortunately?), it is still relatively warm in Tokyo (relative to Northern Europe...), so I won't be able to find a ice patch to easily test out the ABS...

By the way, what kind of slip-differential/ traction control does the F have if any?

Mike

Mike

Mike, the F has no traction control/ LSD of any kind- although LSDs are available for the F (recent Brown and Gammons catalogue, or Rover 220Turbo Torsen diff). Traction control is available from Racelogic (web address on the traders section of http://go.to/mgfgallery).

The ABS does have a warning light on the dash. I do not know the cirscumstances under which it is triggered to go off. I guess these triggers are listed in the manual.

Rob

Robert Bell

Rob

The warning light will only come on if the ABS fails
in any way.......

matt
M66 MAT
matt

The Bosch ABS system works irrespective of vehicle weight. Each wheel speed is measured via the toothed sensor attached as part of the rotating hub assembly. Very simply the ABS functions as a three stage device.

1, is the noraml transmission of fluid pressure from the brake pedal direct to the wheel cylinders.

2, is when the system 'reads' the wheel speeds to have a variation of more than 20%. Usually this is when one wheel is turning slower than the others and indicates that this wheel is on the final stages prior to locking. When this is detected the ABS valving isolates the fluid line to that affected wheel. Pressure in the line is maintained, but additional pedal presssure results in no more pressure being applied to that wheel.

3, if the wheel speed doesn't stabilise then the system will then 'bleed' some of the fluid pressure from that line. This will reduce the braking effect on that wheel with the intention that it's rotational speed will stabilise with the others. If the wheel speed doesn't increase, for example when on Dirk's ice, more fluid is 'bled' from that line until it does stabilise.

This all happens in a very very short period of time and once the wheel speeds stabilise and if the brake pedal is still being applied then fluid pressure will be allowed to pass once again to the wheel cylinders. If the wheel speeds then venture outside that 20% threshold then the above is repeated again.

The speed of each cycle is so fast that a number of cycles will have occurred before you can reduce pedal pressure. This will mean that during each cycle an amount of fluid will have been 'bled' from the line between the ABS valve block (The under bonnet unit) and wheel(s) affected.

If nothing is done then you will suddenly find your foot on the floor with no pedal, so to ensure that you retain a pedal and control the ABS pump comes into operation and forces the returned fluid back into the system between the master cylinder and the ABS unit. This requires very high pressures and is why the pump draws so much electrical power. The pulsing (buzzing) is the operation of the pump and is felt in through the pedal.

The operation of the system will occur on any surface from ice to the grippiest surface available. The only proviso being the variation of individual wheel speeds.

Clearly in Mike's case the ABS is NOT functioning and the dashboard lamp should illuminate to indicate the loss of the system. The ABS system always defaults to normal braking if any fault is detected.

The most common fault by far is a failure of one of the whel sensors. Which one can be located quite easily with a sensitive voltmeter. Find the loom connections where the wheel sensor lead plugs into the main loom and unplug each sensor. Connect the two probes to the sensor terminal.

Jack that wheel off the ground and spin it over by hand. You should get a small pulsating current generation corresponding to each toothed segment of the wheel sensor. If the pulse is present move to the next wheel. You will find one that doesn't give any reading and that will be the one that has failed.

Problem is that replacement senors are not cheap, are delicate and easily damaged, and as such are often difficult to remove from a crashed car if you want a second hand one. I have come across many ABS equipped cars where sensor replacement has been a low priority of the owner and a bulb removal in the dash is the preferred option. Check your bulb hasn't been removed if the car was bought second hand!!

Carl,

I have seen the Racelogic system work to great effect on a 2WD Cosworth with a silly power output. It really does make the best use of all available traction AND is fully adjustable so if you want to have enough power to just break traction then this is possible. The system I saw was a couple of years ago so current versions almost certainly will have even greater adjustability.

I am off the the Autosports Show this Thursday (Trade day) and if Racelogic have a stand (I expect they will) then this is one planned stop off.

Rog
Roger Parker

There is one ohter thing to bear in mind when breaking hard approaching stop lights (particulary where heavy traffic frequent) is that as cars and trucks slow they can displace the road surface, sometimes causing noticable groves/ridges in the road surface.

This can have a dramatic effect on the way the car pulls up.. ie "Tramlining effect". I experience this alot around here.

So, the fact that the car is not pulling up straight may not be a problem with the ABS.. It could be an issue with the advers road surface. Sometime it is difficult to see this effect in the road surface when driving normally, or when you are looking at the red light approaching quickly !

Scott
Scott Martin

Roger - thanks for the excellent technical advice. I don't have voltmeter handy, so I'll leave it the dealer to check since my F's still under warranty. Meanwhile, I still haven't had a chance over the weekends to test the ABS.

Scott- yes, I've noticed that especially on roads which get heavy lorry traffic. The day I picked up my F, I came across such grooves coming to a stoplight and was caught by surprise by how much the F wanted swirve.



Mike

Scott, a very good point and more especially applicable to cars with the 16" wheels and 40% profile tyres.

Mike, if the dealer is to do the job then there is a diagnostic facility that they can plug into. This will be able to verify continuity of the wheel sensors and note any other error messages.

Rog
Roger Parker

This thread was discussed between 05/01/2000 and 16/01/2000

MG MGF Technical index

This thread is from the archive. The Live MG MGF Technical BBS is active now.