75 Brake Fluid Reservoir

[Colin Edwards]

Fitted a new reservoir recently to cure the final ARC warning light alert.  The fluid level sensor had failed.  Unfortunately the new reservoir's sensor is also RS - only discovered this AFTER I had installed it and filled it with fluid!!!

Are these sensors notoriously unreliable or am I just plain unlucky?  The new reservoir is identical to the original and was purchased from a very reputable OS supplier.  I'd rather not just try another reservoir as the rubber grommets in the master cylinder have a finite life and as I understand can't be purchased separately.  Has anyone performed open heart surgery on the fluid sender?  I've just performed successful open heart surgery on the 75's electric mirror control joystick so I'm feeling dangerously overconfident at the minute!

[shiny_car]

Seems unlucky, and thus unlikely.

The level sensor is a simple switch AFAIK. With the fluid topped-up, the level sensor (switch) should be 'open'; when low on fluid, it is 'closed'.

[caveat: the following is based upon the above presumption:] Use a multimeter in continuity mode. Remove the plug from the sensor, and probe the two terminals on the sensor; if open, the multimeter should not beep.

Also, what happens with the warning light with the plug removed from the sensor? It should be 'out'. If not, the issue is with the wiring or the ARC.

Note: the workshop manual suggests using a test light/globe to check the closed circuit; the wiring diagram indeed shows one lead is +12V, so don't 'short out' the terminals on the plug with just wire. 

[Colin Edwards]

Hi Shiny Car,

Tend to agree - unlikely the new reservoir sender is RS.  Also agree the sender is probably nothing more than a simple reed switch - similar to the widget on the washer bottle and coolant reservoir.  These are both operated by magnets.   

I had assumed the brake fluid level sensing would be fail-safe - if the system goes open-circuit the ARC alarm light flashes.  Now appears I assumed incorrectly!! 

I have the wiring diagram, however the test procedure is not included in the doco I have.  The inputs to the ARC TTL IC's seem to be resistor buffered to some degree so as to limit the sink current.  If the Brake Reservoir sensor input is capable of driving a test lamp (10W globe?) hopefully it should handle a dead short bridge.  I hope so................mine was bridged by the previous owner in some sort of attempt to address the winking ARC brake light!

I'll take your advice and sus out the wiring and the ARC.  After changing the voltage regulator on the ARC to a switching power supply it has been behaving itself well.

Colin

[shiny_car]

Good luck  .

This is all the workshop manual says about testing:

[Colin Edwards]

Thanks for the doco.  Looks like the 75 electrics are in for more serious attention.

Let you know what I find.

[Colin Edwards]

Perusal of the 75 Electrical Schematics I have indicate cars with and without brake pad wear monitoring.  Looks like simple earthing of a wire embedded in the pad material triggers the "brake" alarm function in the ARC.  My 75 does not feature pad sensors.

Given my 75 is a November 1987 build and does not feature pad wear sensors but features brake fluid level sensing, would the respective wiring looms all be identical?  Would a generic wiring loom provide general chassis support for comprehensive brake monitoring and if pad wear sensing required, the base loom is extended locally? 

The brake fluid level switch is active low - grounds to trigger ARC "brake" alarm.  This suggests all brake pad wear sensors also being active low are wired in parallel?

[Colin Edwards]

Well the plot thickens!  After a bit of poking around I can confirm on my particular car the brake fluid float switch makes when fluid level is LOW.  When  the contacts close Relay I14 (position 15 - 3rd relay in from the front near battery) energises.  According to the workshop manual this is the "Brake liquid level switch relay" - makes sense!  However, the closing of the brake fluid float switch does not directly control the respective input to the ARC.  Perusal of the electrical schematic suggests contacts on Relay I14 "arm" the brake pad wear circuitry.  Depending on what version schematic of the 75 and 90 electricals (the 75 workshop manual references the 90 document - Group 43 - electronic devices!) you look at, the brake pad sensors seem to operate in three entirely different modes.  Earth to alarm, open to alarm or close to alarm.  Given all sensors seem to be wired in series, two of the modes cannot work!
Next task is to essentially ignore the schematics and look at what happens the the ARC brake inputs when the external circuitry is "manipulated".  Regardless of the outcome, I'm pretty confident the reed switch in the brake fluid reservoir does not directly control the "Brake" alarm on the ARC.

[Colin Edwards]

All Fixed!

Managed to source Section 43 of the Alfa 90 Workshop Manual.  This has a detailed description of the ARC.  The 75 Manual references this 90 related doco.  This publication also has the same error with regard to the way relay I14 is drawn.  In reality, I14 features a single normally closed contact however the doco illustrates it as normally open. 

The ARC inputs 8 & 9 are connected to a series circuit consisting of the contact on relay I14 and two brake pad wear sensors.  Originally pad wear sensors were fitted to the lhs front and rhs rear brake pads.  The relay I14 normally closed contact and the wear indicators complete a circuit bridging inputs 8 & 9 of the ARC.  If the brake fluid level drops enough, the contact in the float switch makes and energises relay I14.  Opening of the input 8 & 9 bridge sets the "brake" light on the ARC flashing.  Another mode of monitoring (described in the Alfa 90 doco) is earthing / grounding of the bridging circuit.  When a pad wears, the sensor embedded in the pad is exposed and contacts the brake rotor - earthing the circuit.  This earthing is the initial wear warning and causes the "Brake" light to illuminate.  If the lhs front or rhs rear pad were to wear further and open the sensor circuit the ARC "brake" light flashes continuously.  Given my ARC "brake" light flashes and the pads are not fitted with wear sensors, an open somewhere is likely.

After a bit of dash disassembly and tracing of cables, sure enough ARC inputs 8 & 9 are floating free.  The ARC is only doing what its inputs tell it to do - good!

Given the confusion with the drawing of relay I14 I figured I may not be the first to be mislead by Italian electricals!  Sure enough, I14 was fitted with a normally open contact.  Given the different pin-out between normally open fitted and normally closed fitted relays, this circuit could never be intact.  Replaced relay I14 with the correct device.  Fault 1 fixed!

Closer examination of the wiring loom about relay I14 confirmed an orange and a grey/black wire formed the remainder of the brake integrity circuit.  The orange wire went into ARC input 8 however it was not intact.  As no pads featured wear sensors the unconnected ends of the wiring loom had to be hidden - somewhere!!  After a fair bit of digging around I located the rats nest where the open ends of the brake monitoring circuit were supposed to be bridged due to the lack of wear sensor on the lhs front brake pad.  Unfortunately this very dodgy joint had failed.  Fault 2 fixed!

Figuring similarly poor electrical work had been carried out on the rhs rear wiring loom I set out to find the grey/black wires described in the electrical schematic.  Pretty soon I discovered a couple of electrical connectors attached to grey / black cables floating about but tucked out of the way under the fuel return line!  Traced these back to a point under the rear seat where they exit the inside of the car.  These wires are now permanently linked inside the car under the rear seat as should have been done originally.  Fault 3 fixed!

Now with the correct relay I14 installed, the brake fluid level sensing system works reliably.  With brake fluid low the float reed switch makes and energises relay I14.  Normally closed contact on relay I14 opens and link between ARC inputs 8 & 9 removed.  ARC "brake" light flashes annoyingly - like it has since Iv'e had the car.  Return the fluid float switch to its correct position and the flashing ARC "brake" light is gone.  Heaven!

[shiny_car]

What did you figure out?