PDWA Internal Arrangement

All,
My S3 PWDA has been tripped so the light is on. I have performed the reset procedure in the ROM several times with no luck so have just unplugged the light. Unfortunately, my OCD kicks in and I know it is not right so want to fix it.

There is a cap on each end of the PWDA. I tried removing one the other day and it is on real tight. Before I =gain better access to the switch to try to remove it, can anyone answer a couplle questions?

Well, something went wrong and I did not get the questions before it posted!

  1. If I get the caps off, does the spool have an end such that I can I manually move the spool to re-center it and turn off the switch?
  2. If I remove the caps, does the spool have seals on the ends that will maintain integrity of the hydraulic system or will I have to bleed the system again if the caps are opened?

Thanks for the help.
Doug

If you unscrew the end “caps”, then you’ll find there is a bolt running from end to end. It has a groove at either end which houses an O-ring to maintain hydraulic integrity. In the centre is a spring loaded switch which either conducts to earth or not, depending on whether the bolt has moved across or not…

A sensible and cost free approach is to simply omit the PDWA. If you are careful, then the brake pipes can be manipulated to join the booster connections directly. The front and rear pipes would be reversed in this case, but that won’t affect the braking, simply what the failure mode is if hydraulic pressure or vacuum is lost.

The booster connections on the s2 car started one way around and it switched sometime during the production run, so there is no great significance in which is the master and which is the slave.

Do make sure you don’t have a genuine hydraulic problem first.

kind regards
Marek

Thanks Marek.

If I am understanding the internal design you are describing, then the caps should be dry and if I remove the correct end, I should be able to slide the spool/bolt to the center without having to re-bleed the system. I know the switch does nothing but I have in the back of my mind it is not per the design and want it to work as planned!

Yes - if it works as it should.
Be also prepared to find that the O-rings have leaked, it is horribly corroded inside and the bolt is covered in 50 year old congealed muck - you just don’t know.
kind regards
Marek.

1 Like

Just unplug the dang thing, your most likely going to have to bleed the brakes when you open it up and it’s never going to work reliably. I rebuilt mine many years ago and it worked for about a month. One caveat though, I always check my brake fluid before I take the cars out.
Cheers,
LLynn

It is a bit of quirky safety feature.

“You’ve already lost brake pressure. Maybe.”

I’d quite like to know before that happens, not afterwards. I think the seat of the pants will have worked this out long before I notice a red light coming on.

kind regards
Marek

1 Like

Tip: If you click the little pencil at the bottom of your original post you can edit it rather than adding a second post. (But maybe you already knew that . . .)

Doug,
The PDWA (C37217; RTC2525;Girling 64048953) is a safety device, much like seat belts. Sorry to hear you’re having difficulty resetting the valve. Here are some pictures of an original valve and it’s components. Hope it helps. Corrosion inside the valve is usually the culprit. It’s the result of improper brake maintenance. The ROM is so-so in the reset operation. I"ve found it’s best to remove the valve, overhaul it, reinstall and then bleed the brake system. I know it’s a lot of trouble but the only way you’re going to know what’s going on IMHO.


There are two pistons within the valve body. Each is fitted with a square o-ring to hydraulically seal the unit. The right piston has a groove in which the small steel ball resides. The ball groove keeps the switch from making contact. If a leak develops the associated pistol will move causing the ball to ride up on the piston, actuating the switch which turns on the BRAKE light.

The spring in the center is fitted to the switch (C37468; RTC-826A;Girling 6467738).

Girling made a Service Kit - SP2615 but as of late, it’s not been available. The kit comprised two square seals, 2 copper washer and the steel ball. Maybe one of the major suppliers will see fit to re-introduce the kit! The switch was used on numerous British vintage car PDWA valves along with Ford Motor Cars.

Without the ground spring the system won’t turn on the BRAKE warning light if actuated. They’re easily fabricated.

PDWA body Girling part number.
With care the o-rings may be reused. They’re square and the material is resistant to brake fluid. Standard o-rings will not work!
If you have not idea when the last brake flush was, when the last time the PDWA was overhauled then it’s time to remove it, take it apart and see what’s going on. It’s 40+ years old and probably never been serviced.
Hope this info helps. Nice stable of cars. Also have several daughters living in the area.
image

Stay Well and Enjoy Happy Trails,

Dick

1 Like

Doug,
Two corrections to my previous post.

  1. C37217 PDWA Valve - Girling 64068955
  2. C37468 Switch - Girling 64677381

Dick

Doug,
Some additional information.
image
Photo showing typical corrosion found on a long overdue valve service. The valve would not reset per the ROM because of internal corrosion. Once overhauled and a new service kit installed it worked as new! Tip: Pipe cleaners with brake cleaner spray are good for cleaning the small fluid passages.
image
FORD OEM brake switch.
image
Inside the switch connector end the two terminals are shorted together via a cross wire.


OEM switch. The same switch was used on circa 1970’s MG, Triumph and other British Leyland products sold to the North America market. Ford used the switch, among other applications, 1986 Bronco.

Cheers,

Dick

Dick,
This is exactly what I was hoping to see! Thank you very much!

Doug,
Glad to be of assistance. Post some pic’s of your PDWA if you remove and overhaul it. Since the SP2615 service kit is NLA I’m looking into PTFE square o-ring seals. The small ball bearing shouldn’t be a problem finding either. The original end washers may be super heated with a torch to anneal them back to specs for reuse. Any questions don’t hesitate to ask the Forum or send me a PM.

Dick

For anyone reading this thread who does not know what PDWA is short for it is Pressure Differential Warning Actuator. (I googled it). The information in the thread is valuable and I would encourage the original poster to correct the order of the letters In the title for the benefit of anyone searching in the future.

Done …

I used to have a T shirt with “DUCK” written on it. Underneath it said “Dyslexics Connection UK”. I’m guessing our cousins across the pond won’t get it, but the Brits will nod knowingly…

1 Like

Five out of Three of us are so we may get it!!!

I am a bit of a dyslexic – but my doctor said if I just concentrate everything will be KO

1 Like

The French Connection UK T-shirts were very good advertising for the company. Everybody noticed them, big time.

Doug,
As a follow-up here’s a tool that will aid in keeping the PDWA valve centered while bleeding the brakes.
image
Found this little gem at National Parts Depot - part number 914-7. It arrived today and I gave it a test drive. Works great with the Girling PDWA valve even though it’s listed for American vehicles.
To use the tool remove the PDWA electrical switch and simply screw this into the port, finger tight only. The tool pin will keep the ball seated in the shuttle piston detent, holding the shuttle valve while bleeding the bakes.
After the brakes are properly bled remove the tool, install the switch and wiring connector. Check the dash Brake light insuring the PDWA valve has not operated/tripped.
NOTE: If the PDWA has operated/tripped this tool will not reset the shuttle valve. To reset, follow the ROM procedure (70.25.08; 70.25.17) or remove the valve assembly, overhaul and reinstall with the tool in place of the switch.
Another note: If the PDWA internal seals have hardened (age/heat) brake fluid may migrate up thru the switch. The switch does not contain fluid. Lastly, if rebuilding the PDWA use new end Copper Crush Washer (according to Girling).
.