I have read that fitting big brakes can upset the ABS and it kicks in too early and in effect makes your brakes not work very well at all.

 

Anyone know what this is all about?

 

Is it when you go over a certain diameter?

"same ECU on a bog standard C180 as they do on a AMG36"

 

Same ECU but different calibations. As John says, it's not simple, there are many variables. ABS is not simply reacting to wheel speed sensor signals, it has some predictive capability also. This is partly to enhance the ABS function, partly because the ABS system also controls the front / rear balance to prevent premature rear wheel lock up.

 

As for how the system will be affected by fitting different hardware, it is difficult to predict. There is a certain amount of lee way built in for variables such as friction variation, temperature etc. If the modification is a big one such as huge new brakes the effect on the ABS will be significant, if it's just an effective radius increase the effect will be small. Best way to tell is through others experience, or commisioning Teves or Bosch to perform an ABS re-calibration. As this will take 18 months, cost quite a few million quid and involve shipping your car to Sweden, Spain etc it's probably not practical.

"same ECU on a bog standard C180 as they do on a AMG36"

 

 

no, i mean really exactly the same! same part no to be ordered for both with same S/W installed!

ok, if it calculates on volume then it would need some way of measuring the volume, & i know there isnt! like i said, there would be too many variables - ie battery voltage differences would effect the volume of fluid displaced, fluid temperatures, position of the pistons in the calipers, etc etc.

 

also have just checked on a early C class merc & they use the same ECU on a bog standard C180 as they do on a AMG36 with much bigger calipers!

 

I'm not saying the volume is calculated just inferred and assumed when doing the calibration on the unit. Battery voltage would only affect speed of piston stroke if anything, fluid density over the sort of temp range we're talking can be assumed to be constant and the piston position will only affect where it starts/ends, not the amount it moves and so the the difference in pressure would be equal.

 

Possible merc use the same algorithms in the ECU then the ABS master unit with pistons etc is matched to the callipers?

"same part no to be ordered for both with same S/W installed!"

 

It's very possible both cars use the same algorithms but I can't imagine they'd use the same parameter sets. Probably the ABS module stores parameter sets for all Merc models built at a particular location (to reduce the number of parts to choose from) and the end of line diagnostic machine tells the ABS module what the car is (you're a 190 diesel, you're an AMG etc) at which point it loads the correct parameter set.

 

"Possible merc use the same algorithms in the ECU then the ABS master unit with pistons etc is matched to the callipers?"

 

The industry has moved away from separate ECU / valve bodies now to save cost and weight. The units are no longer servicable but they are so much smaller that replacement costs have reduced (to the manufacturers if not the end customer). The volume of fluid required to be removed to reduce the pressure in even a large caliper is very small, most valve blocks can cope with a wide variation of brake system. The blocks are so expensive to engineer that the brake system suppliers design them to suit the widest range of applications possible at the time and cope with the variation using software.

The article does imply that the unit uses its set parameters to release fluid by the correct amount to attain the desired wheel speed, and not that it actually calculates the amounts needed.

 

But the article on warped discs is a bit strange, I have put discs on a lathe and seen massive runout (warp) and yet she says they have never seen it.

 

Makes me wonder about them.

 

Playing safe, looks like I am going to use Skyline discs, which are not much bigger (44mm) and retain the original calipers, so the ABS should not be too upset, if at all, and if the article is right.

Possible merc use the same algorithms in the ECU then the ABS master unit with pistons etc is matched to the callipers?

 

no, i looked up an early non programable one!