substitute relay for a/c

Porsche (mine's a '96) uses a relay for the a/c condenser that looks like this part number 964-615-215-00: (sorry for borrowing and modifying the12volt.com's graphics)



It appears similar to the SPDT Relay discussed on the relay page except the center terminal is numbered 31 and is grounded. It is stamped with 12V 10A. This relay costs ~$50.

I'd like to substitute this relay part number 141-951-253-b:



This has no center terminal at all. It's marked 12V 40A. It only costs ~$5. So you can see why I'd like to use this as a substitute.

Any of you expert auto electricians have thoughts about making this swap?

Thanks, JC

You can get five pin relays for $5-10.  I would use a relay that has the same amount of pins as the stock relay.

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Kevin Pierson

Kevin,

Thanks for the reply. Any tips on how to back into a part number for a 5 pin relay? Or a website or other source that lists relays in such a way I can search one out?

JC

If the pin layout is like the one you posted above then that is a standard bosch style relay.  If the pin layout isn't like above you may need to go to a car parts store and see if they could cross reference it for you.

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Kevin Pierson

The pin layout is exactly like posted above. The challenge I've had is finding a relay that is designed with the center pin numbered 31. All the 5 pin relays I've seen number the center pin 87a.

Another thought. I wonder if this is actually a SPST Relay with the center pin (87a on a SPDT) numbered 30 and grounded.

I mis spoke on my last posting. The grounded pin is numbered 31. Here's a drawing of the schematic from the relay I'm trying to replace:

It looks like a reverse voltage breakdown diode.  That is an odd relay you got there!

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Kevin Pierson

KPierson wrote:

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It looks like a reverse voltage breakdown diode.  That is an odd relay you got there!

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Guess that's why it costs 50 bucks! So how about my suggested substitution?

JC

You would be best off to find the schematic for where that pin connects.  If it connects to ground you may be able to wire the diode up externally.

If it were me, I would personally bite the bullet and buy the direct replacement.

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Kevin Pierson

OK, just bought and installed the direct replacement. It doesn't work either! What a puzzle, the substitute works but not the new specified relay.

Anyone have any thoughts?

By direct replacement, you bought the $50 relay and it doesn't work?
What substitute did you use that works?

If you have schematics for the wiring showing the connections to the compressor and rest of the car you could tell for sure, but I'd guess it's set up to switch 12V coming in from terminal 30, with the clutch coil connected to term 87. The internal diode connection to term 31 would be for clamping the negative voltage spike when the relay opens up and protect the relay contacts from arcing.

If you open up your original relay you could probably see how it's connected and what diode rating you'd need, but you might have been able to modify the standard relay by bending away the NC contact and internally soldering a diode between the NC and NO contacts. $50 and it's done for you, but if you start with a good quality relay and a properly rated diode, you should be able to make up an equivalent easily enough.

Yep, I bit the bullet and bought the $50 relay, but the condenser still does not operate with it in place. But it does with the cheap substitute (without the diode to ground).

Man I'm confused. I guess now I need to find an A/C forum.

I'm no electrician and it's been (many) years since I studied electric circuits. So I've scanned the wiring diagram for the ac circuit. It's in 3 pieces so I could keep them large enough to upload and still make readable.







Again, sure appreciate all advise.

You have a condenser controlled by a relay?
How did you determine the relay was the problem in the first place, did you bench test it?

* With ac button "on" the compressor was not running.
* Began testing downstream from the compressor.
* The compressor power lead was plugged in tight but no 12v at plug
* Compressor fuse - good
* Compressor relay - swapped a #53 (141-951-253-B) for it and the compressor started functioning with ac button on - OK!
* Bought a new 964-615-215-00 compressor relay put it in - NO DICE!

If it works with the substitute relay it would indicate you don't have a problem with a cutoff kicking in for the relay coil drive, unless it's something intermittent that stopped working when you unplugged it and plugged in the new relay.
The special relay with the diode is just for shunting the spike in voltage when the relay opens, from what I can see in the third picture.
You won't be able to just use a standard SPDT relay because it would short out the 12V input on term 30 if you were to plug it in.
I'd first check that you have voltage across the coil terminals when you expect the AC compressor clutch to be engaged, if not, there may be some cutout like evap freeze up or low refrigerant pressure that's interrupting it, and you'll have to check back from there.

dualsport,

Sounds like you really know your stuff. What do I risk by permanently using the substitue relay (my second drawing on my first post), which "seems" to be working properly?

Without the diode, it'd be harder on the relay contacts and may eventually result in them burning earlier, but you could drop in another at that point.
I'd find out why your new relay isn't working though, since you already plunked down the bucks for it anyway. Make sure it's got the same configuration that you expect; compare the resistance readings across the coil terminals to see if they're in the same ballpark.
If you can bench test the relay by applying 12V across the coils, see that it clicks and you measure continuity between terminals 30 and 87.
Maybe you have a different relay than you expected or the new one is just bad out of the box, as unlikely as that might be.