Early 93 USA Vanden Plas 671330 brake boost not switching on
I can jumper the ABS relay and the pump turns on.
The pressure switch shows continuity at low/no pressure between pins LS2-1 WN and LS2-4 YP.
Reservoir full
Brake hazard light is on
ABS light is on

Does this suggest Combined Pressure Switch failure or ABS module failure?

How does the ABS board come out of the housing seeing as all 6 torx screws are removed but it doesn't want to budge?

 

I haven't dug into the ABS on my own car but have you checked that the sphere is still pressurized?

Common failure point at this age.

 

Pressurizes fine when I jumper the relay, it's something in the control side. I've been having endless electrical failures in this chassis, resoldering boards, replacing components, every day a new failure. I'd love an aftermarket/dirt track racing solution that bypasses the factory BS and just gets this effing brake system braking w/o ABS. I finally get the chassis sorted and the electrical turns it into a no show.

 

Hi 87octane91,

It's been awhile since I had to deal with the brake hydraulic system in our '93 VDP, but at one point the combined pressure switch failed in the circuit that powers the pump, and after searching unsuccessfully for an affordable replacement switch, I ran a wire from the pump relay to the center console and zip-tied a small push-button switch to the hand brake handle so when the BRAKE warning light illuminated, I could just press the button and run the pump until the light extinguished. We drove it that way for a couple of years, until I found a good replacement combined pressure switch for less than nuclear submarine part prices.

I am attaching some relevant documents that may be helpful in diagnosing your issue. I also have the Brake section of the Workshop Manual but it is too large to attach to a post. If you don't have it, send me your email address via Private Message and I'll send it to you. The file size is around 48 MB.

Based on your symptoms, it sounds like the combined pressure switch is working in the circuit that powers the pump when pressure is low, and in the circuit that illuminates the low brake pressure warning lamp. The third circuit in the combined pressure switch communicates with the brake fluid level switch in the reservoir. Have you tested the fluid level switch for proper operation? With the fluid level full, there should be continuity between the White/Green wire and the Purple/Green wire.

P.S. If it turns out that you do need a combined pressure switch, I may still have a good working one.

Cheers,

Don

 

Don, you truly are the lost library of Alexandria Egypt, please never ever die.
As of this moment, I have continuity from the reservoir switch across pin 1 LS19-1 WG to pin 2 LS19-2 PG (from the combined pressure switch)
Pin LS2-5 WU to LS50-51 (in the ABS module) reads open at low/no pressure. At full pressure with 1 minute of pump run time, there is continuity from LS2-3 to LS2-5 WU to LS50-51 in the ABS module, suggesting the combined pressure switch is still good.
The ignition switch ground 11 has other circuits working just fine, thus presumably eliminated.
I still can't figure out how to rip the ABS board out of the housing to inspect. I assume it's glued together like so many other components in this rig.
What have I missed?

 

Thanks for your very kind words octane!

On most of the ATE ABS modules the cover is glued or ultrasonically welded to the case, so to open it you have to saw a thin shallow kerf all the way around the cover, taking great care not to saw into the circuit board. Once the cover is off, the solder side of the circuit board is accessible, so you can inspect for cold, starved and broken solder joints.

Try to find photos online specifically for the late XJ40 module. I don't think I've ever had to open one, but I have opened lots and lots of the ones in the X300s and X100s/X308s, which are notorious for bad solder joints.

 

Octane,

I am posting these thoughts from our PM exchange in case they prompt any ideas from other members that may help you get to the bottom of your brake malfunction:

I note this presumption from your Post #5. I would suggest testing the resistance between the White/Brown wire at the combined switch connector to a good ground.

Also, looking at the schematic, there are two diodes in the Yellow/Purple-White-Blue-Brown/Black wire series that connects LS2-4 at the combined switch with terminal 86 of the pump relay. If one of those diodes failed, battery power from 21 might be grounding through the switch instead of through the relay coil, preventing the relay contacts from being pulled closed. If your meter has a diode test function, you can test this theory. My assumption is that the diodes will be wrapped in the harness hear the splice shown on the schematic. The diode that failed in our '93's fuel pump harness was just crimped to the harness wires and covered in heavy heat-shrink tubing.

I can't remember if you reported testing the conductivity through the switch contacts when the system is pressurized, but that should tell you if all the contacts inside ths switch seem to be responding correctly.

Cheers,

Don