| Author | Message | |||
     Stefan Morley Frequent User Username: myupctoys Post Number: 395 Registered: 07-2009 |
Hi all, Does anyone have any experience with how the pressure transducers work? I call them transducers (not switches as the manual refers to them) because they vary their resistance in proportion to the pressure applied to them and the dash lights respond to this change in resistance (simple voltage divider); not acting as a switch. My brake warning lights are blinking, before people tell me not to drive I have tested at least the Rear Brakes and Height control system are running at 2275psi. Have yet to measure the front brake system, hopefully today... I replaced the diaphragms in the switches in response to the brake lights flashing and being confident the pressure was correct. New diaphragms are 1.3mm thinner than the originals, which has caused a secondary issue which I'll address separately. A third issue has also been discovered, the systems are not designed to be fail-safe.... There is no difference, as far as the dash lights are concerned, between a correct high pressure reading and the sensor wires being left disconnected.... My question is how accurate "should" the pressure "transducers" be? I only have a sample set of two, hardly conclusive. Many Thanks Stefan | |||
| Stefan Morley Frequent User Username: myupctoys Post Number: 396 Registered: 07-2009 |
Update, A possibly more valid question to ask; based on tests. How repeatable should the comparator thresholds (related to the dash lights) be between the two sensors. AND is this adjustable! The only clues in the manual about tolerances are the comment 1100>>1200psi the lights should come on and should be off between 1400>>1425psi. Obviously, my sensors have a problem; unrelated to pressure, but exactly what is the question. Do need to follow through on the front brake system pressure. Side note, the rear brakes/height system only interact with the right brake warning light. The front brake system affects both lights. Might help someone out. There is one other symptom the car has, that might be related... OR not. Whilst back, the voltmeter in the centre console pegged all the way to the right, indicating over voltage. I have measured the battery and battery when running and the voltage is fine. The gauge used to run somewhere in the middle. But it is "possible" the brake light warning issue occurred at a similar junction in time. Or simple co-incidence. Just wondering if the two might be related. Can vouch for the fact that the brake sensor excitation is battery voltage, and thought that a bit curious if any form of accuracy was needed. IE are the two connected with a regulator failure..... Not talking car alternator regulator, talking a low power simple three terminal electronic regulator. Be easy enough to test if someone could help out. Simply disconnect a pressure sensor and stick a multimeter on this wire and to ground. Battery voltage (less drops through the fuse panel and wiring) then both cars are the same, regulated at something below this, then I have a regulator failure. Cheers Stefan | |||
| Stefan Morley Frequent User Username: myupctoys Post Number: 397 Registered: 07-2009 |
Car year might help. But these sensors are used in a wide range of cars. Bentley Turbo R 1988 SCBZSOTA4JCH22888 | |||
| Stefan Morley Frequent User Username: myupctoys Post Number: 398 Registered: 07-2009 |
Answered some of the questions after a few hours reverse engineering the ECU for the brake warning lamps. They are crudely simply. Attached in case it's of use to anyone else down the track.
Excitation does come from the battery through a 620r then on to the sensors. Thresholds and hysteresis is purely set by the thresholds of a Schmitt trigger in a 40106 inverter and is supply dependant. Not adjustable. Only thing that is adjustable is a low speed oscillator for the flashing lamp rate. Will test the unit to determine if something is amiss with my 40106 thresholds, but static tests looks fine. Assuming 40106 is working to spec, then the root cause is the sensors themselves. | |||
| Stefan Morley Frequent User Username: myupctoys Post Number: 399 Registered: 07-2009 |
Hi Peoples, Edit my comments about the transducer/switch. The previous owner must have had the pressure switches rebuilt but got the important bits the wrong way around. My UD14074, UD17876 and diaphragm were assembled in the correct order but installed 180 degrees end for end. Dumb luck the resistance through the diaphragm was enough to "sort" of make the brake controller ECU function AND the units didn't leak. This is the correct order: https://www.introcar.com/cms-page-556978 Assembled correctly, they work as a switch, and would seal better. Both my pressures are good, root cause, just this incorrect assembly of the "switch". | |||
| Robert J. Sprauer Frequent User Username: wraithman Post Number: 840 Registered: 11-2017 |
When pressure drops, the internals make contact with ground closing the relay to turn on the warning lamp associated with that hydraulic circuit. It's a simple switch...either on or off. |
Brake Warning Lamp ECU