Fabio421

Thank you very much. Dr. Bob, can you explain all of the different diodes in the head unit so that future users may get some help from this thread? Thanks.

I appreciate all the people who have helped me in this thread.

the flyin' scotsman

.........and given the close proximity of the relay; is it smoked also?

Fabio421

Which relay would that be? Is it on the panel or the circuit board?

dr bob

I made a mistake on the diode spec. The 1N4001 has 50 PIV rating. It takes a 1N4002 to get the 100 PIV rating.

The 'logic' in the control head varies slighty by year, but generally it's consistent. I diagrammed the head in my '89 when I did the diagnosis that lead to the external relay solution for the AC clutch. I also did the same for a friend's '91, whic has a somewhat different internal circuitry but looks like it's functionally interchangebel with the one in my car.

The relay I mentioned is a little PC-board mounted device in the control head. The original is underspec'd for the service, and is a known failure point. Plenty of info here on the AC relay replacement, easy to find with the Search function.

Fabio421

So do I need the 100 PIV rating or was that a mistake? I guess what I am asking is do I need the 1N4001 or 1N4001?

Thanks

Alan

You should get the 100v version - its unlikely you will see that much voltage often but its possible. All these diodes serve the same purpose - spike protection. If you trace the connectivity they are all reverse biassed between ground and switched outputs from the head unit that drive inductive loads (relay coil, control solenoids). When these switch (off) they can create a signficant switching spike. These dodes are used to supress that noise (at least the negative spikes below ground level).

The exact diode specs are not critical as long as the have sufficient PIV for the positive spikes they won't supress and sufficient current capacity to dissipate the negative spikes. Zener diodes would actually make more sense to me in this application (to suppress in both direction) - but thats not what is shown in the diagrams for any of these.

Alan

dr bob

FWIW, these diodes are all pretty much the same price when you buy them. Buy the highest volatge rating you can and sleep well at night. The 1N4007 has a 1000 PIV rating, for instance. These are all general purpose silicon rectifier diodes, hardly a precision semiconductor, etc.

69gaugeman

Try the 1N4001

Fabio421

Whoops. I didn't notice I wrote that. I bought the 1N4003's . Actually I bought a few of the 1N4001, 1N4003 , 1N4005 & 1N4007's. The reason was that they were all out of the packs of the correct diode so I had to buy a 25 pack of assorted diodes. Still it was only $2.59.

Angel84-928

I opened mine up and the last digit is a 7 visible. All three are 1N4007.

Landseer

Interesting.

Mine had a smaller dia diode.

But it was marked with a what appeared to be a reference to 1988.. must have been a replacement.

docmirror

Have your relay replaced with a bigger one now, or suffer this summer. You can send it to me and I'll take care of it if you don't want to mess with it. Any of the 1N400x diodes will do in here. I suspect Porsche got a lot of 3s at a good price and used those, then got a better price on a lot of 7s and used those. Onything over 50V would be suitable.

Fabio421

Update: I installed the 1N4003 and it worked for one day. Now it starts to smoke if I push the A/C button that is below the head unit ( mounted on the central indicator). I thought that maybe my soldering job was supect so I resoldered it but it is doing the same thing ( smoking and not working). Should I replace it with the 1N4007 or is there something else going on here? The relay appears to be OK. The windings all look good and the clear housing is still clear and without a smokey tint.

One other symptom that it is now exhibiting is that the fan will not blow unless you press the A/C button and then it is only blowing very slowly. The selector **** has no effect on the fan speed.

Alan

Diodes are unidirectional items - did you install it the correct way around - the white circle at one end is the indication, you must match this.

You will need a new one if this is what happened...

Alan

Angel84-928

I think you have a short. If its smoking it means too much current running thru there. The spec sheet shows Average rectifier forward current should be 1.0 amp for the entire diode series (1N4001 thru 1N4007)
1N4001-1N4007.pdf