Roger and all - I think figuring out the function and finding a better way to do it with a simpler relay is the way to go here. Replicating such an item does not seem to be a a worthwhile venture to me (this sems to fall into the over-complicated solutions camp) . Can anyone with a '79 with working relay figure out when the clutch is disabled? That is the only function of hat relay here....

I'm suspecting it is just when RPM's are below idle level.

If thats all it does the later method of feeding everthing via X-Bus is a possible solution path - just disable the relay during starting to reduce starting load....

Alan

 

Alan,
I have another 79 with a working relay so we will check out the function and get back to you.
Thanks,
Roger

 

This is exactly right; the relay disables the compressor while cranking.

 

I was able to find this relay, which is for a volvo k-jetronic fuel pump. Whole story can be found here (actually nice summary of how K-Jetronic works): http://www.volvoclub.org.uk/tech/fue...lInjection.pdf

I see the 928 part number is a four-prong/pin type and this a five pin type, but do the other four pins function the same? Alan? Also, does the relay bank have a dummy provision for a fifth pin? Thanks.

 

Well its a fuel pump relay - seems to be like many of the Porsche ones... it has a similar function enabled only after the engine is detected to be rotating - however the fuel pump triggers at cranking speed - quite likely at a lower RPM than the AC relay takes to determine the engine is actually running (at least thats what I'd do). It also has a short priming pulse triggered after the ignition (15) is turned on. So it has a similar function.

However I'd use a 53b relay in its place with the coil fed by Ignition (15) and the coil grounded by (50) the starter solenoid feed. This has the effect of enabling the AC only when the ignition is on and the starter is not activated. Its not the exact same functionality but is close enough and very robust.

Alan

 

I would try to repair it first. Usually it would be a semicondutor part that went bad...probably one of the ICs. I would guess they are something simple and easily available. Can you get a shot that clearly shows all the marking on the top of the ICs? Might also be a cap or the magnetics..
If it were me, and I were trying to replicate it, I would just figure out what it does and reproduce the same function in a $1 microcontroller, and figure out how to tie it in to a standard relay.

 

The original was rated at 16A "ind" which suggests that the relay also contains a zener or similar to suppress the back pulse from the highly inductive clutch solenoid. I would think that a standard relay might have a short life. Also a risk of that back emf pulse damaging something else in the car's systems...

 

By all means try to repair it - but if that fails - reconstructing a version of the original is overkill... ( besides what are those RPM specs - so how would you do it...?)

I happen to think the original design was also overkill... Porsche apparently agreed because in the 80's+ cars they dropped it in favor of basically what I proposed earlier.

Replicating similar functionality is not hard with a readily available & cheap relay and terminals - not much more...

John - remember that common SPST bosch relays are 30-40A rated - compared to the 8A (optimistically) rated head relay on the later vehicles (with a supressor - yes but still). The back EMF potentially damages primarily just the contacts and might cause clicks over audio - but I doubt it would do much more (ignition 15 load being amps - would quench this easily)... Adding an ~16v Zener from OP terminal -> Gnd would be a reasonable option to reduce noise & contact damage...

Alan

 

Alan--

The pulses for the FP relay (and this one...) are likely pulled to ground (almost) with engine speed if the trigger is similar to my now-passed-on CIS Saab Turbo of the same vintage. Don't know on the 928 whether this is 2/rotation (as it is on the 4-cyl cars IIRC) or 4/rotation. A freq counter will find this quickly. I don't know if there's a pull-up in the AC relay, might be worth a little o'scope sleuthing. (Dave A?) That's a tiny detail... I'd figure a way to duplicate this with a simple 556 circuit and a couple little transistors. One side of the 556 to detect the engine speed and provide basic engine-turning pulse detection similar to the FP relay. The second timer would be a basic on-delay chained off the first timer, so the compressor doesn't come on until the engine has been running for a few seconds.

Installed in the 928, the relay has power only when the AC button is depressed and the freeze switch is true. So using the second timer for on-delay in a 556 would cause that same start-up delay from the time the AC button is depressed. I don't think that would be an issue even noticed by most owners-- press the button and the compressor actually starts a few seconds later.

The early cars have a fixed-opening idle air bypass solenoid for AC operation, wired parallel to the AC clutch. That would need to be considered when sizing a suppressor diode set.