Stumbles Intermittently

[Jhalsey]

Yes, glad to see this, mine also looks factory except for the added connector w the red insulator.

I have studied further Sheet 5 of the 1989 Wiring Diagram Model 89. It shows the OB switch as “charge air pressure indicator switch”. It shows two WT wires leaving the switch, one travels to the “speed relay” and enters on the 7000 1/rpm side. I assume that is the rpm limiter which cuts the fuel pumps when 7000 rpm is reached.

The other WT wire runs to pin 7pin of a connector that has pins numbered up to 14 at T69. Don’t know what the T69 reference means, probably a physical location n the car. Guessing its the 14 pin connector that plugs into to back of the fuse/relay panel in the back left of the engine compartment where the YELLOW relay lives. From there the WT wire travels to pin 86 of the “Relay Air Meter Charge Air Pressure Delayed” relay. When this relay closes it connects pin 87 to pin 85 of the “Relay Fuel Pump II” thereby grounding it and causing it to close along with “Relay Fuel Pump I” which is slaved to II.

This all seems to jibe w what has been said by others (908/930 in particular). However, my OB switch has only one WT wire leaving the switch and looks to be the original wire, connectors and harness sheathing. I have looked under the drivers seat and can see no obvious mods to the wiring. I will have to remove the seat to look more closely. I think my next step is to see if the WT wire I have is the one that goes to the speed relay or to the 14 pin connector in the engine compartment.

I will test for continuity between WT wire i have and pin 7 of the 14 pin connector. If I have continuity there, there must be a break between there and the air meter relay. Not sure where that relay physically lives.

It appears my engine compartment main harness has been mucked with at some point, its is wrapped in electrical tape. It’s about 1” in diameter in the area behind the distributor going towards the relay/fuse panel. So its anyone’s guess what changes have been made. I’d like to get the whole mess back to stock but not sure its worth it just to get the OB switch to work as designed.

I’m in the middle of fitting the new vacuum canister and running into a problem with interference between the canister (which is maybe a half inch taller than the OEM canister) and the engine tin underneath. It prevents the canister from rotating fully clockwise to the end of the adjustement slot.

I may end up cutting the tin to provide clearance but would hate to do that. The oil feed for the turbo is not centered in the tin and the hole is offset to the right (see pic).




Not sure the best way to correct this.

As always, thank you for the assistance.

[908/930]

I found a copy of the wiring diagram you are looking at, same but different to my 1987, I'm thinking they had an engineering student redraw it to keep him busy, change name of charging pressure control switch to charge air pressure indicator switch, etc. I'm not sure the 2nd white wire going to connector T65 actually exists, the way it is shown it would bypass the speed relay.
What they now call "relay air meter charge air pressure delayed" should be the yellow relay.

I think it is worth the time to get the OB switch back to working, can save you from a $30k rebuild

[Jhalsey]

well, that was the goal, but to do that I have to follow the white wire and I'm now preoccupied with installation of the new vacuum canister. I should be done with that soon once I set the idle speed and timing. I will then see if there is continuity between the white wire at the OB switch and pin 7 of the 14. Update, there is no continuity between the white wire and pin 7 of the 14 pin connector at the left rear in the engine compartment. And the white wire is not grounded. So it may be that there is no second white wire.

I will resume investigating further tomorrow.

[Jhalsey]

Well, time for an update. Have not made any progress on solving the inoperative OB circuit. The car is running well, have driven it approximately 30 miles with the new microprocessor controlled vacuum canister (see picture).



There is however one small problem. When power is applied, the new canister goes through its start up routine, which is to advance 7 degrees and then retard 3 degrees to establish its reference location. In my distributor the outer plate rotates 7 degrees but when it reverses to rotate 3 degrees in the other direction, the outer plate and the star wheel appear to be locked together.

So it would appear when I disassembled the distributor earlier in pursuit of freeing up the advance/retard mechanism, I incorrectly reassembled it somehow. I’m pretty sure I installed the little oblong steel key correctly otherwise the star wheel wouldn’t turn with the shaft. I will have to remove the distributor again and inspect. There is plenty of axial play, so that’s not the problem.

Managed to install the distributor one tooth away from its prior location. Unfortunately, one tooth appears to be a big angular difference so the distributor is rotated all the way to the other end of the slot with only a tiny bit of slot left to turn clockwise. But the timing is right on the TDC mark on the fan shroud after the thermo time switch activates and the idle is steady at 750 rpm. Not sure why Porsche couldn’t come up with a design that allowed 10-15 degrees of adjustability in either direction, perhaps I’m still doing something wrong.

That’s it for now, will update when I have the distributor removed and inspected. May have to send it to the vendor who has kindly offered to go through it to ensure all is well.

[Alan L]

Thats not too far from where my dizzy adjustment (and most others) sits. Just imagine - it could be worse - the lock nut could be behind the can :-(.
As for the star wheel etc, been so long since I pulled one apart. No wait - a year ago I had to rebuild mine - the internal coil failed. But my memory is so bad, thats no help today.
Alan

[Jhalsey]

Well, time to close out this thread, The inoperative OB switch circuit has still not been solved. I have traced the non-original ground wire and found it soldered to a small gauge red/br wire behind the front fuse panel which is no doubt the ground circuit to the fuel pumps. It is a well done modification, just not sure why it was necessary. Probably because it was easier than trouble shooting the OB circuit. I will keep after it and hopefully one day the mystery will be solved.

I have finished installation of the new vacuum canister and it is working well. The problem with the advance plate and star wheel moving together was not repeatable. The ignition timing is right at the TDC notch on the fan hosing at idle. I ended up moving the distributor one tooth and now it sits at the other end of the adjustment slot which I slightly elongated, There is now enough adjustability to retard or advance. I believe lack of a working advance mechanism was the cause of my original problem with hesitation upon opening the throttle. Not a cheap solution but it is well made and supported and there is no alternative if the original canister is unrepairable.

I am now in the midst of dividing the fuel pump circuits to provide the relays/pumps each its own dedicated 12 v supply. I hope to make it as original looking as possible. Also ordered two new relays from the Canadian company that is making round relays equivalent to the red relays in our cars, will transfer the red plastic caps to complete the look.

Will post pictures when finished. Thanks for all the help received here, not possible otherwise.