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Base Boost - BPC internals detail. (long - sorry)
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Posted by Dexter J (more from Dexter J) on Wed, 11 May 2005 21:50:38 Share Post by Email
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Salutations:

Was within moments of saving myself $275.00 - however - it was not to be.

Basically, after fixing a bad TCS safety valve and cruise peddle switch, the Trionic BPC still had our 1993 Aero locked down to base boost. With the assistance of Bill Homer (thanks again) we were able to confirm that we were not seeing 3 ohms on right hand side (pin 2-3) on the BPC.

I stumbled across a post from Henry regarding opening up the BPC to see if one of the wires had come away. I was able to open the BPC up easily by finding the seam under the pin flange and giving it a careful pry with a good chisel instead of a hammer as suggested.

There was considerable brass dust in the unit - but all three wires were on the posts. I then carefully lifted out the internal selenoid using a pair of pliers. There is just enough slack in the unit to draw the coil drums (good quality chrome cases with a brass centre ring) so you can lay it across the case when it is out.

Upon inspection - I discovered the source of the brass dust as well as the lost connection on pin 3.

Basically - the internal control disk (not brass) that meters airflow between the two magnets had worn through the pin facing vent (there is one vent on each side) in the brass ring so that it had chafed a 2mm section out of lead to #3 pin.

It is machined so that the wire is actually pressed between the rear facing vent in the brass ring when assembled and thus, as the very hard carbon steel control disk is free floating and seems to favour the wire blocked port at idle, it makes a certain amount of sense that it would eventually gouge enough out of the brass to start to work on the exposed wire at this vent port.

While I very much appreciate that the Trionic BPC led APC system is an extremely advanced and effective turbo control system - I feel strongly that this is a very basic design flaw - in that the machined channel for the wire is deliberately cut above the vent instead of to one side.

Anyway - I was able to regain 3ohms across both pins by coating the remaining wire (there are two from each coil in a shared casing) with liquid tape and inserting a bare thin gauge copper wire shunt into the original casing from the top to regain connection on the second wire, again covering with liquid tape.

However, the damage (hot gas from the vent, chafing from the disk and the resulting shorts over time) was too far gone - so that when I tried to return the unit to the plastic case (having fabricated a small copper plate to keep the disk off the wire) - the remaining wires fractured.

I pulled the drum again and tried to cut back the wires to replace - but they were cooked and corroded right back to the drums in both upper and lower wire cases given that they share a common ground on #2 pin.

I took the opportunity to poke around a bit and confirmed that the internal face of the brass vent ring was worn through the rear side of the assemble as described. This I think explains why BPC's start to buzz over time. They effectively cut a groove - then ride in it and I suspect tilt a fair bit under various loads, which would also make boost irregularly apply as they age.

Finally - having confirmed that there was no hope - I put the drums and control disk back together making sure that they approximated original vent gaps. Then I removed all the wires and dropped the unit it upside down so as to favour over boost from the unit for the moment and will be looking for a MBC+A device on the local market.

Posted with all the usual disclaimers and limitations, with the footnote that it I ever do buy a replacement SAAB BPC APC Solenoid - I will be disassembling it before installation to machine a correct wire channel and avoid this seemingly basic bad design choice. I mean 15mm one way or the other and this doesn't happen and the control disk rides true for pity's sake.

Bill Homer - if you are ever again in touch with the firm in Europe who makes these things - please pass along my comments for their team. I'd be more than willing to pay $275 (Canadian of course) for a new one if I saw this simple, simple step added to the machining.

Dexter J

posted by 24.222....

http://www.dexterdyne.org

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