The load side of R34 is at pin 87.
You can test for a direct short at the relay R34 socket at pin 87 to find out why Fuse #16 is blowing.
You can use a test light or a digital vom.

- ABS control unit unplugged
- Pull relay R34 out of socket.
- With a test light, connect test light clamp to B+
- Probe the socket at pin 87. Test light should not light up.
- If it lights up, there is a short to ground from central electric to ABS connector.

Is the diode shorted or open?
If diode is shorted, power at pin 30 will get shorted to ground and will blow fuse #16.
You need a new relay if you cannot find a replacement diode.
Diodes are cheap.



http://forums.pelicanparts.com/uploa...1615735851.jpg

Zener Diode inside relay R34
 

Yes exactly! I suspect that is what is happening. I have not had time to test the diode with 12 volts, but my multimeter shows continuity regardless of polarity across the diode.

This is the weirdest diode I have even seen. I will post some pictures below.

The screwdriver is pointing to the diode.

http://forums.pelicanparts.com/uploa...1615835065.jpg

This is a view of the diode from the top. The item that looks like a dime is the diode.

http://forums.pelicanparts.com/uploa...1615835240.jpg

This is the insulation board that was rattling inside the relay.

http://forums.pelicanparts.com/uploa...1615835313.jpg

Ok, I confirmed that 12 VDC flows easily in either direction through the diode. I tested with a 12 volt light bulb and got equal brightness regardless of polarity. So the diode is shot and I need a new relay unless I can source this strange looking diode.

But the more important question: Why did the diode fail? Did something else cause it to blow? I don't mind spending $100 for the new relay but if it goes poof the moment I plug it in, I will be annoyed and sad. But if that is the only way forward, it is what it is.

A couple of questions for bazar01 or anyone:

In the diagram in post #22, pin 31 of relay R34 is shown as going to B8. I assume B8 is a reference to some other location, but where?

Also in same diagram, immediately to the left of R34 is the Rotary Switch For Lock Control and pin 5 is shown as going to M4. Where is M4?

B8 is R34, pin 31 ground point GPVI. Wire color is brown. Brown is used for ground connections. Pin 15 is the 12V control voltage from central electric.

M4 is the rotary switch ground point at GPXV.

If you have a test light, test to confirm for ground connection.
Fuse #16 is blowing because a shorted power diode, DO-5 will conduct power direct to ground.
If you connect the test light clamp to B+, test light will will be ON when you probe B8 and M4.

Thanks Bazar01,
Where did you get the ground points info that says B8 is GPVI and M4 is GPXV? Is there a table somewhere?

You have to learn how to read the Porsche wiring diagrams. LOL.

Do you have a pdf file with you so I can walk you through?

It's a steep learning curve for some of us... :)
Yes I have the PDF wiring diagrams. Mine is a 1992 C4 manual trans.

You lost me on how B8 connects to your most recent post of ground points.

Not really that hard if you have the pdf.

Those boxes with numbers are just reference grid locations on the page.
The page has a horizontal and vertical grids.
Horizontal in letters A,B,C....
Vertical in numbers 1,2,3....

Say B8 on the "ABS" wiring schematic page is located at grid G38 of this "ABS" current page.
Scroll up to a previous page where grid B8 is located, this will be the "Lights" wiring schematic page.
This grid on the "Lights" wiring schematic will reference it back to G38 of the "ABS" wiring schematic. This will point you to GPVI grounding point.

Example of above in picture,

http://forums.pelicanparts.com/uploa...1615905887.jpg

If it was mine, I'll make sure I don't have any more power shorting to ground at the relay R34 load side contact at pin 87.
Then just put a fused jumper wire across pins 30 and 87 and see if fuse #16 remains good.

Ok, I settled in with a strong cup of coffee and studied your instructions and the Porsche wiring diagrams and got it figured out now. Thanks!:)

You should title your instructions with "All you wanted to know about Porsche Wiring Diagrams, but were afraid to ask".

Great idea on the fused jumper. The relay is rated at 10 amps, so I guess I should use a 10 Amp fuse?

Do you have a dc clamp ammeter?

Maybe start with a 5 amp fuse and use a dc clamp ammeter.

I do have a clamp ammeter. I will do as you suggest! Fuses are cheap.

Success!!! I am so pleased.
 

I tested the car just now with a 5 amp fused jumper to bypass the bad relay R34.

When I did that, the ABS/PDAS controller came to life and I was able to talk to it with my UDT999 diagnostics tool.:D:D:D

So it looks like another major hurdle to getting the 964 back on the road has been overcome.

Thanks so much for everyone's help in particular bazar01. Your persistence in helping me out is greatly appreciated.

Now I have to order a new relay, test the battery charging system, consider waterproofing the controller or mounting it upside down. Also have to flush all old brake fluids. And lots of other minor repairs and maintenance, but it is so great to get past this hurdle.http://forums.pelicanparts.com/support/smileys/wat6.gifhttp://forums.pelicanparts.com/support/smileys/wat6.gifhttp://forums.pelicanparts.com/support/smileys/wat6.gif

Thank you for reporting back. It will help a lot of people with similar issues. Glad your persistence paid off.

UDT999 just for ABS/PDAS manual activation? Can it do anything else?

I rigged a box of toggle switches to cycle/activate/excercise the solenoids and the pump to cycle the ABS. Useful when bleeding.

I plan to get a Pico scope and a Durametric enthusiast option 2. The electricals on these cars are getting old and getting a little difficult to handle.

Just to close off the repair/troubleshooting of the relay, I want to add one more thing.

Anyone experiencing trouble with this relay should understand that if the diode has failed, the relay will bench test as being ok when it is not.

This is because on most relay bench tests, you apply power to the coil and then test for continuity on the switched terminals. This means you never have power on terminal 30 and 31 at the same time. But in the car, the relay will have power at 30 and 31 and if the diode is fried it will result in a short circuit which will blow fuse #16.

Most relays don't have a diode and I almost missed this and was ready to conclude that the relay was NOT the problem when in fact it was. A multimeter with a diode test feature found the problem in the diode.

In regards to the UDT999:

What I found was the UDT999 was as close to the Porsche Hammer as I could find. I am very happy with it. It will bleed all hydraulics including the locks, will read and reset all alarms including airbag alarms and can count engine knocks and lots more. I looked very closely at the Durametric but in the end went with UDT.

Web site is here. http://928gts.jenniskens.eu/

That is cool how you rigged up a device to bleed your system. That was one of the main reasons I was looking at Durametric and UDT but I am happy to be able to read and reset codes.