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Base boost causes?

1K views 5 replies 5 participants last post by  SAAB Snob 
#1 ·
OK - I know this one seems to attact a lot of threads (16 pages according to the search), but wonder about some parts of it.

My newly acquired (4 days ago) 92 2.0 looks like its at base boost (goes half way into the yellow and no further). Now i'm currently chasing the first problem (dash gauge never goes into the white) which sounds like it might cause the APC to drop into base boost (vac leak). Have now replaced manifold to Y piece, and Y piece to bulkhead hose and the gauge now dips slightly into the white at idle. Will do the pipe down to the dump valve tomorrow (ran out of light tonight) and see if that improves matters any more.

Other issues which i may have - plugs are unknown (service due this weekend - BCPR6ES is right?), which may have an effect (or is this only Trionic?), plus read about brake light bulbs...now this ones got me stumped. On starting, the ABS goes out after the requisite 20 seconds, but 2 red bulb icons stay lit for a little longer - are these brake light bulbs not quite right, or tail light logos? Seems a most odd thing to disable max boost if the bulbs are at fault - or is this just a Saab quirk?

The idle valve and APC will get a clean out this weekend also, just in case. Any other really silly things that might upset it?
 
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#4 ·
I discovered that the "APC" Fuse in my car was blown as well, which would explain my lack of boost as well as my Cruise Control no longer working. Since the same fuse for the APC is also tied to the radiator, this may explain why the radiator fan is acting up, but is not too bad at the moment.

So, in an effort to troubleshoot this problem, it took only a few 10amp "APC" fuses in the glovebox being blown at startup to conclude that the APC was on the fritz. So, as a result of reading these INVALUABLE Forums, I not only tested the pins on the APC after taking off the electrical harness and concluded that pins 1 & 2 were semi OK with an OHM registering b/n 1-2 Ohms on my DIMM Meter (Not the ideal 2-3 Ohms, mind you). When testing pins 2-3, there was NO reading which suggests that the circut is blown or shorted. Further, I tested the BPC by taking speaker wire and attaching battery clips to one end of each of 2 wires, one for positive and one for negative and touched the positive attached to the car battery to the middle pin and the negative attached to the negative on the battery to each of the outer pins. This confirmed what my DIMM told me as I heard the APC "Click" when touching pins -1 & +2 and heard nothing when touching pins +2 & -3. Moreover, after taking off the 3 hoses of the APC/BPC and affixing another new hose to the "W" (Middle) part of the valve to "Blow" through, air came out both the top "C" and bottom "R" holes in the valve...Not good!!! So a new APC/BPC is needed obviously but not before making sure that the 3 Mosfets on the ECU are in working order first, otherwise I realize (after reading these invaluable forums) the "New replacement" APC/BPC could easily Short Circut if any of the 3 Mosfets of the ECU are not in good working order (by testing the ECU Mosfet with a DIMM by taking off the Mosfet clasp and measuring Ohms by touching each of the Mosfet pins with the DIMM and the back of the ECU plate).

So until I get a replacement APC/BPC, I'll just have to enjoy the rather substantial Base Boost as it is for now!!

SAAB Snob
 
#5 ·
you shouldnt ever test an APC solenoid (BPC) by hooking it up to straight +12 VDC. The coils inside the valve can melt as a result, leaving you with a certainly dead solenoid. the ECU controls the valve with a modified PWM signal (it "vibrates" the voltage), so really, the way to test one is either checking that the coils are still a correct resistanec, and that the disc inside isnt stuck (blowing through the ports to check airflow.)
 
#6 · (Edited)
Tested ECU MOSFET w/DIM

OK, I tested the ECU MOSFET's with a DIM and noticed that the inner and outer (Of 3) gave a reading. Each has three "tree Pins" soldered into the ECU board. I noticed that one of the three tree pins (actually located on the outer sides, each towards the Heatsink on each side) registered noticeably higher than the other 2 tree pins on the same MOSFET (ie-with DIM set on 2000 Ohms, it registered 36 or so!! Whilst the other two pins registered 1 or 2). The Middle MOSFET did not register at all, even if I measured for kilo-ohms by turning the DIM from 2000 (Mega-Ohm) to 200 (Kilo-Ohms), so I de-Soldered it and replaced it with another MOSFET. I know the spec of the MOSFET is right, BTW! Now that one only registers a 1-2 on just one of the out of 3 pins. I am reluctant to replace my BPC Valve solenoid (As I have a new one ready to go) without first making sure that the ECU MOSFET's Work properly because I understand from the SAAB 9000 website (procedures/electrical/repair Trionic ECU) that all the Tree Pins are supposed to register approximately the same and, if found to be faulty, then the NEW BPC could short once the new fuse in #5 spot is put into the Glovebox and the ignition is turned on. I don't want to make that $120 mistake!

Anyone out there have any advise as to how to test the circuit with the NEW BPC without the risk of Shorting it if the MOSFET's in the ECU are still messed up??? There may be a problem with the ECU Board itself as I cannot rule that out. But I was pretty confident that once I got the BPC and the MOSFET's to replace any damaged ones in the ECU, that I'd be ready to make my way out of the BASE BOOST BLUES!!!!, but since I'm not sure that I "Repaired" one of 3 MOSFET's properly, as it doesn't register nearly as strong as the first one in the line closest to the ECU Clamp, I don't know what to think????
 
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