Baffling door pin flasher
 

So I had a little garage time xmas eve, and decided to address the problem of door pins not flashing when alarm set, by installing a new pulse unit I'd picked up on eBay years ago, since it was likely the original had failed (I knew the pin flasher worked, since I could power up the new flasher unit on my desk, and sit the pin in the coil.

First mystery. According to PET my MY91 should have 'pulse sender' 928 637 137 00. That's the p/n on the one I'd bought (on Porsche box too), but the one I find under the door card in the car is 928 637 237 00, which is the one listed in the 92-95 PET. I'm pretty sure the one in door is original - it's dated 1990 on a sticker.

So I remove the original, and sure enough the multimeter shows the coil is broken somewhere, which probably explains the absence of flash.

Now here's where it all gets very puzzling. The pin flashes on the bench in the new 928 637 137 00 pulse sender, but only if the coil is oriented upside down to how it's normally mounted on the door! If I turn it up so it's oriented to the pin as it is in the door, I can't get the pin LED to flash no matter how it sits relative to the coil. The photos show the LED caught during a flash, but with the coil the wrong way round. The other photo is the LED failing to flash, but coil oriented as it would be installed in door. If I want it to work when it's installed in the car, I'm going to have to try desoldering the coil wires and reversing the connections. Makes sense as if the LED is powered by inductive pick-up, it's only going to work when current flows in one direction.

I'm not finished yet! The pulse sender is connected to the door wiring by a two pin plug, with red wire (presume +12V) and green/black (switched ground by alarm?). It looks as if this is wrong way round to operate the new pulse sender, which is something that's mentioned in another thread on this, but the pin flashed when the connector was forced the other way round onto the sender (that thread also mentioned the different 137 and 237 part numbers).

So yet again a quick job turns into a deep conundrum. Can anyone shed any light on what I'm experiencing? Is it possible the coil in the sender was just mounted wrong way up originally (the green and black wires to the coil are same orientation in the original and new sender units).https://cimg9.ibsrv.net/gimg/rennlis...de3b67d0b5.jpg

Coil positioned as in door
https://cimg0.ibsrv.net/gimg/rennlis...49e6f3829b.jpg

Coil inverted from door install position
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https://cimg8.ibsrv.net/gimg/rennlis...38f6fb1a2e.jpg
Well I just checked the wiring diagram (Sheet 11), and it does suggest the keyed connector is wrong way round. In diagram Green/Black switched ground to alarm unit should go to pin 2 of pulse sender, red to pin 1. That's the reverse of how the plug would fit with the new pulse sender, but it's pretty easy to force the connector in the other way round.

​​​​​​​I've also reversed the coil wiring, and now it flashes when oriented as in the door.

The more I look into this, the more curious it gets!

I'll call the pulse sender I found installed in my 91 S4 the 237 part. Turns out it has no electronics whatsoever - it's simply an induction coil connected directly to the alarm unit. The alarm unit can pulse the coil (and so the door pin LED) by grounding one side of the coil. In this way the alarm unit can create different flashing patterns - normal flash when the alarm is set and all doors etc are closed, or a double flash to show one alarm sensor (e.g. hatch) is still open.

The 137 part is the one which should be in a MY91. It has electronics onboard so it will pulse the LED when voltage is applied to it. It can only create one flashing pattern. Though it works on the bench to create a steady flash, it will also pulse the LED once immediately when power is applied, so should work in the car (not installed it yet, as will mean reassembling most of the door card so want to check will be ok). But it can't provide a double flash function, as you have to wait 2-3 secs after applying and removing power till it will flash again on applying power. This also explains why the connector to the 137 part has to be inserted the other way round to the 237 part.

So it's pretty weird that the 137 part contains electronics for flashing which are redundant if the alarm unit works by momentarily grounding the pulse sender. Maybe the 137 'electronic' version is used in other Porsche or automotive applications where the alarm controller flashes the LED by permanently grounding it?

​​​​​​​So no question from me, but leaving this here in case someone is similarly puzzled in future! Oh and I note the 237 part (listed in PET for 92-95) is quite a bit cheaper than the 137 electronic version, so if the unit on my passenger side also has open coil winding I will buy one for that side and have non-matching numbers!

All installed and working now on the RHS door. For anyone reading in future, the part installed is 928.637.137.00, and I've had to reverse the wires to the induction coil (I suspect that was manufacturing error) that the pin sits in, and jam in the connector from the alarm control unit the other way round. The LED has the fast pulse before the alarm sets, then switched to the one flash per second alarm armed status.

Don't know how it does the fast flash, as I couldn't flash it that fast pin the bunch by momentarily applying power. Maybe testing with 9V makes a difference compared with 12V from car.

Anyway will install a new pulse unit on LHS when the new part arrives. I've ordered the 928.637.237.00 for that side as it's cheaper, and though not taken the door card off I suspect that's what's in there going by RHS. Also it's the 'non-electronic' version, so shouldn't require reversal of connector.

I also had to switch the connections on the one I bought back in 2005.

Just checked my receipt and it was a 928.637.237.00.

It looks like both parts have connection problems.

No big deal if you know what to do.

Thanks for posting this. I'm sure it will be useful for others in the future.

Thanks Bertrand - I found your old posts on this component when I was scratching my head over it!

The 237 is the part where the alarm control plugs straight through to coil, so if you had to reverse connector, the coil in your pulse sender must have been mounted the wrong way round too. With the 'electronic' 137 part, the coil connections need to be reversed if they are wrong way round, as you can't change current direction in coil just by reversing the connector.

I think the pulse sender sub-system must have been delegated to the same team who designed the alternator cooling hose on back of splash shield, and the hatch release mechanism!

+1. :p