worf928

Interesting. Ok. Easy enough. I'll try your test and report back tomorrow.

That's a non-event ABS-wise for a 5-speed.

I make no proposals. Other than what is evident in the shop manuals via text and wiring diagrams I based my question on my experience. i.e. the manuals indicate no different procedure is required, the wiring diagrams are not sufficiently illuminating, and my experience does not indicate the need. In the absence of disassembling the hydraulic unit or in the absence of information from someone who has done so, I am skeptical of a requirement to somehow operate the ABS hydraulic unit to achieve proper flushing.

But, you propose a test to support your claim. I'm all for it. I may learn something new.

I remain skeptical that pressure on the brake pedal has anything to do with operation of the hydraulic unit. It may serve as the observable for your test, but it isn't an input to the unit or the ABS computer, unless, as I wrote above, there is a pressure transducer in the hydraulic unit.

worf928

I tried this on my '91 and an '89 5-speed. In both cases there was no buzz, vibration, or sound from the pedal or the ABS hydraulic unit.

no doubt

I just tried it myself on my 1988; couldn't get the pedal to buzz until after I started driving the car in Reverse after the initial start. Went about 12 feet and it buzzed once (unmistakable).

WallyP

From the Porsche Document WKD 492 121, "928 with Anti-locking Brake System ABS"

"Hydraulic Unit
The hydraulic unit consists chiefly of three fast operating electric solenoid valves and a return delivery pump.
One each of the electric solenoid valves is allocated to the left and right front wheel brakes as well as the mutually regulated rear wheel brakes.

The hydraulic unit can change the hydraulic pressure to the wheel brake cylinders independently of pressure in the brake master cylinder. However, a higher pressure than that of the brake
master cylinder is not possible. Depending on the amperage, with which the electric solenoid valves are activated, the hydraulic
pressure in wheel brake cylinders can be
increased - pressure building up phase,
maintained - pressure holding phase, or
reduced - pressure dropping phase."

"Pressure Dropping Phase
If the braked wheel still tends to lock in spite of constant hydraulic pressure, e. g. because of road surface changing from dry to slippery, pressure in wheel brake cylinder must be reduced.
Current higher than approx. 5 amperes on the electric solenoid valve causes a greater piston stroke, which opens the outlet.

Hydraulic fluid flows to pump reservoir and return delivery pump. Return delivery pump returns the brake fluid into the brake master cylinder against existing pressure. A damper is provided in
each brake circuit to suppress the delivery noise. The pump always runs durmg regulation, even if return dellvery IS not required."

"Test Cycle
An integrated test program runs off in the control unit when switching on ignition.
A further test program runs off at a road speed of 6 km/h.
Testing at 6 km/h can be felt, since pump in hydraulic unit will run briefly.
Test cycle is repeated each time car moves off, if ignition had been turned off previously."

"Multiple Pin Plug on Electronic Control Unit
Never disconnect or connect multiple pin plug on electronic control unit while ignition is switched on."

"* Hydraulic unit and electronic control unit may not be repaired or disassembled."

AFAIK, there is NO WAY to cause the hydraulic pump in the ABS system to run while bleeding the brakes.

The pump will run briefly when all four wheel sensors first show a speed of 6 KpH after starting.

While it might be possible to manually control the pump by hooking into the pump relay socket, I don't think that is a good idea...

I doubt that having the ABS relay pulled, or having the ignition switch on has any effect at all during brake bleeding.

no doubt

Nice post, Wally. Because the ABS reservoir must be filled/pressurized (ditto for ABS pump), it may be that a full 4 corner brake bleed could require driving the 928 a few feet (enough to reach the 6 kilometer per hour threshold for the ABS pump test) prior to bleeding (each corner or just once?!) "to be sure" of getting a firm brake pedal with no air...and to be certain of having removed all old fluid during a brake fluid change (ignition remaining On during the entire 4 corner bleed to prevent loss/change in pressure).

Just a thought.



*of course, if just pressing the brake pedal enough times will activate the ABS pump, then no 6 km/h speed would be required. The 928 could remain stationary during the bleed.

worf928

I agree.

Yeah. I think we've learned something here. If after the first pressure bleed you aren't satisfied with the pedal then drive the car and re-bleed. That should probably be SOP if you've done something to the hydraulic system that might have introduced air to the hydraulic unit (e.g. MC, bias valve, etc.) And also, if you suspect that the fluid is really, really old and you want to get 99.99% of the crufty stuff out.

no doubt

I don't think that there would be a downside to simply suggesting that for your first brake bleed (or for every brake bleed for that matter), that you turn on your 928 Ignition, drive for a few feet above the 6 km/h speed threshold that turns on the ABS pump, and then proceed to do your bleed without ever turning the ignition off (thus insuring that the pressure is maintained in the entire system).

WallyP

"proceed to do your bleed without ever turning the ignition off (thus insuring that the pressure is maintained in the entire system). "

The ABS pump does not build pressure in the system. It returns the fluid that is bled off to reduce braking pressure to the master cylinder.

No advantage in leaving the switch on.

no doubt

Returning fluid requires pressure. No pressure means no flow.

The advantage to leaving the Ignition On after the 6 km/h ABS self-test activates the pump is that the ABS reservoir will be properly filled and pressure will be maintained systemwide.

Thus, an ideal brake bleed is more easily obtained.

Turning the Ignition Off is what offers no advantage...and could result in some old brake fluid remaining inside the ABS reservoir and/or ABS pump after a brake bleed.

Ditto for air.

So why advise people to turn Off their ignition?! That offers only disadvantage; a path to leave air in the system or to leave old brake fluid there.


Utterly no harm in leaving the Ignition On, in contrast, with the added benefit that a perfect brake bleed might be accomplished that way.

Thus, it makes more sense to recommend leaving the Ignition On (after driving the 928 at least up to 4 miles per hour) for a brake bleed.

worf928

The 'downside' is that twice as much time is required to do the flush and if one changes the fluid on or ahead of the two year recommended schedule the amount of 'old' fluid left should be vanishingly small.

For a 5-speed, you also need to flush the clutch circuit. So, the quickest way is to place the car on stands, remove the wheels, remove the undertray, flush everything, put the wheels and undertray back on and remove the car from stands. Doing this procedure twice has little marginal utility for that gram or two of old fluid that might be left in the ABS hydraulic unit.

Now, if the fluid is really old, or if you've introduced air into the system, then the double-flush is a good idea IMO.

no doubt

No need to do the job twice.

Step one: start your 928.
Step two: drive your 928 at least 4 miles per hour so that the ABS pump kicks on.
Step three: leave your 928 running.
Step 4: bleed all 4 corners (plus clutch if you have a 5 speed)

All done.

Rob Edwards

Can I 'drive' my '90 at least 4 miles an hour while it's up on liftbars? Or does only the rear wheels turning while the fronts are stationary freak out the ABS and not activate the pump?

no doubt

All 4 wheel sensors have to see 4 mph.



*the question that is so far unanswered, however, is if you can trigger the ABS pump on 928s simply by pumping the brake pedal (ignition On) enough times in a row.

Nicole

Since my soft pedal problem started (after replacing brake flex lines and calipers), we have bled the system about 10 times, and the car was driven in between.

The pedal is still soft, and the pedal travel way too long.

Yes, it brakes well, but you lose a lot of time pushing the pedal so far down, and you don't have the same feel as before.

Mrmerlin

Nicole, this could be a messy job but , why not try cracking each of the ABS units lines open while some one is pushing the pedal, put a rag under the line so it will catch the fluid, see if any air comes out?