ABS Operation

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signs
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Just wondering if anyone can give information on operation of ABS. My 1994 300gd has these fitted, but recently had a few bum clenching incidents where brakes locked up causing a skid (weight+speed+wet road= oh sh#t) Is theABS set up to operating on all wheels, as I thought load sensing valve wouldn't be required if  so. My ABS lights come on with ignition, go off when started and go on / off with diff locks on/off and there is no evidence of anyone crossing wires to allow lights to operate with charging circuit (a known dodge to pass test).

Is there any way to test ABS system to ensure it's working correctly or should I get a warning light if it's faulty?

 

Arnie
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Re: ABS Operation

The ABS system is a 3-channel system. The two front wheels are independent and the rear axle is the 3rd channel. It uses s a 'select-low' algorithm to activate based on the wheel with the least grip (highest rotational speed). The ABS light should extinguish after a second or two, when the key is in position II, and this indicates that the system has passed its initial tests.

When moving off, if you keep your foot lightly on the brake pedal (this works best when going down-hill), you will feel a pulse as the speed reaches approx 4 km/h, and the system is primed. This is an indication that the pump-block and wheel sensors are working as they should. If not, you will get an ABS warning light at this point.

Indeed there is a brake-pressure limiter for the rear brakes and you should check that its adjuster lever is properly connected to a point on the top of the rear diff housing. If it is all connected, there is a test procedure for calibrating its adjustment according to test loads on the rear body. The length of the tie-rod can be adjusted to adjust the operating point. If you have changed or altered your rear springs or shock absorbers at any time, the brake pressure limiter may need re-calibrating.

The wheel sensors are tested for circuit continuity when the ignition is first turned-on and thereafter, checked that they produce pulses when the wheels are rotating and compared with each other for plausibility. Therefore, unlikely to be a wheel sensor fault or it would be flagged-up by the ABS light.

Check, however, that all 4 tyres are the same size and have approximately the same amount of tread. Also, check that your rear brakes are in good order with equal brake force left and right, as drum-brakes are often neglected. Preferably all 4 tyres should be of the same brand and model, but if not, at least those on the front axle and those on the rear axle should be the same.

Unfortunately, this ABS is somewhat primitive with regards to diagnostics and there are no fault-codes to be read, other than the ABS Light.

 

 

 

 

 

signs
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Re: ABS Operation

Thanks Arnie, this is the exact information I was looking for. The back brakes need to be adjusted up a small touch, and the valve & linkage seem to operate OK. This linkage seized up a few years ago, can't remember exact details now, which resulted in most of the braking going to rear, and this caused some 'interesting' moments on wet road. The tyres are relatively new BFG all-terrain. We did replace springs & shocks last year so maybe valve linkage needs adjusting.

JASONGDS
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Re: ABS Operation

Since fitting white springs on the back of my 463 brakes not as good as with weak springs, any info on adjustment of the rear load sensing valve?

Arnie
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Re: ABS Operation

The adjustment procedure is in the service manual. Basically, if you lengthen the link, you will increase the braking force.

 

Pistonhead
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Re: ABS Operation

Apart from the limiter valve rod perhaps needing checking and adjustment as necessary; if you have a case where the brake shoe remains stuck on after the hand brake is released, it could be a case a slight hub oil seal leak having caused oil contamination of the rear drums and the brake shoe can becomes twitchy and snap on when brakes are applied due to oil and brake dust combination (particularly sensitive under wet road conditions). Clean out the excessive brake dust build up and sand paper the drum surface and brake shoe lining thoroughly and re-fit.

If the oil leak is excessive then of course you will need to put that right. 

mgrays
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Re: ABS Operation

As a 3 channel system it will be scary on split mu surfaces; In less technical terms .. if you meet a car on a narrow road and so pull one half of the car onto verge/grass/gravel at the edge of the road while keeping the other side on the tarmac .. it will detect the wheels on the verge locking up (and the rear axle particularly).. so at that point it limits braking to the grip of the verge. You are left feeling like the brakes failed/reduced as you know on a non ABS car you could have stopped fine. A more modern 4 channel system will realise that the brakes on the tarmac have braking capacity and so the car is not about to spin.. and so it will brake normally while modulating the wheels on the verge. So the trick is do you braking on the hard stuff and only go to the verge at the last moment!

For stability you never want the rear wheels to lock as then the car loses stability and spins.. hence we have a rear load limiter. The crude method of checking the load limiter I seem to remember is to reverse at 10 mph on a hard surface and see if you can lock the rear wheels.. the more technical manner is to ensure that in any braking scenario that the rears never lock.. but do brake as much as possible.. so adjust until it locks .. then back it off a  bit.. then do lots of test in laden/unladen .. guess unladen and then 6 bags of cement! Alternatively keep everything factory and follow their rules.

My first experience of ABS was on a 3 channel car and I found this stability issue out on the first test drive on a brake evaluation test car (as a brake developer) .. so after that I stayed away from ABS cars for a while for personal use .. since then they have progressed to the point that they are the best thing on sheet ice and in the wet so I have no problem (except in deep snow where they are still a menace!) The ideal is selectable ABS but the EU outlawed it so they had to pull the button on the G.

Arnie
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Re: ABS Operation

interesting points there. The funny thing is that the ABS pump (at least on early Gs) is a 4-channel unit - and it looks at the response from all 4 wheel sensors, but the two rear hydraulic channels are commoned. There was a lot of talk in the early G brochures about the ABS being specially developed/ modified for the G and its permanent 4-wheel drive. I guess they could have used a 'select-high' algorithm for the rear and allowed the wheel with least grip to slip, while braking the wheels with the most grip as much as possible. But with 4-wheel drive, if you lock-up 3 or the 4 wheels, the 4th will spin away.

mgrays
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Re: ABS Operation

4 channel would imply 4 hydraulic channels/valves? All of ABS systems have 4 wheel sensors (excepting the mechanical ABS fitted to Fords (SCS) which were pre electronics and only ran off the front drive shafts). Presume the 463 G is still "I-I" layout so the rear axle is tied together left/right on one circuit and the fronts on another. To run with the rear load sensor valve they would have had to have two load sensor valves and hoses.. and hence they stayed with 3 channel. Now it would have been custom coded a bit in the brain/ECU for G but that is the same on any car.. so not that "special" but it was an early system.

On ice at slow speed I lock all axles so they all slip at the same speed.. but then I have no ABS on my G.