multi-purpose relay advice?
 

I've had some problems with my '74 shutting off while driving. Traced the problem to a burned out main power relay. This is one of the black, 5-pin, multi-purpose relays. Diagnosis was complicated because the burned out relay would still work intermittently, and most of my old, spare relays were in the same shape.

Went out and bought new relays (URO brand), but I had one of those fail after only an hours of driving too.

So, what are the opinions out there about what the best brand or source is for these relays Prices vary from $11 to $40, from several different manufacturers. Any opinions out there about brands and reliability? Porsche OEM worth the extra money?

Thanks,

Andrew Blyholder

Hey, Andrew! SmileWavy

I think OEM is the way to go. URO does not have a good reputation for the quality of their parts. Most of what I've heard in that vein is about their tie-rods, but that makes me think the new failure you are experiencing is also due to the part.

If price is an issue, there are several local Porsche boneyards that will have used relays on hand. They were used in a lot of different Porsche models through the years.

You may want to double-check the current flowing through the relay. You didn't say if the problem was the switch part of the relay or the coil, but if you get creative with wires and bullet connectors you can make a test harness that would let you check how much current is actually going through the part that burns out. I'm frankly not sure how much should be, but if it is very high you may have an underlying problem that is causing the relays to die.

--DD

Yes, I'm still mystified why this has cropped up, when the car was previously reliable for many years. I sawed open all my failed relays and they are all going in the same way. There is a small length of braided copper wire connecting the #30 post to the moving arm of the electromagnet connection. That wire was melting in half. But even after it melted, the melted off ends could apparently still touch the metal frame of the electromagnet, allowing it to continue working intermittently. Why these are overloading without blowing the fuse is the mystery. Nothing on this circuit but the ECU. So does that mean I have some sort of ground short in the ECU that's pulling too much power - enough to overload the relay but not enough to blow the fuse? And what's the point of having a fuse if the components it's supposedly protecting burn out before the fuse goes?

I don't know what else could be causing that sort of draw. It could even be something that is shorted only when the relay is closed? But the braid is definitely the path from #30 to either 87 or 87a (depending on if the relay is open or closed). It's probably worth it to check how much current is going through them. Though again, I don't remember what they're rated for.

Maybe the braids are corroding and have some real resistance now? That would certainly generate heat.

Metal braids are available in various sizes. It should be possible to get new material and solder it in if you are good with an iron. How permanent the fix is depends on how good the joins are and how much current flows through, of course.

BTW, the cover is either pressed on or glued on to the base of the relay. I have one that the cover is loose on; I can just pull the cover off the relay to see the guts inside. No saw required. You might be able to convince yours to open up the same way instead of sawing them.

--DD

Yes, the cover of one of the new Uro's has already come in-glued. The old German ones are bonded much better. But they saw off quick with a coping saw.

I'll report on the current flow.

Not much interested in trying to repair these. I was hoping new units would get me back to reliable running again.