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Discussion Starter #1
I still can't figure out what the problem is with my APC system. It just doesn't want to allow anything over basic boost. I recently bought a complete APC system that came off of a working 89' Saab 900 Turbo, just like mine. I have fitted everything I got from the other car and it still didn't want to work. I currently have a MBC connected and I don't like it (boost is all over the place) but I can't figure out what the problem is with the APC. The wastegate works great and holds pressure when the vacuum line was disconnected. The only visible problems I have noticed is that the seat belt light blinks because the fuse was pulled (passive seat belts don't work) and someone removed the housing for the oil filter and replaced it with the N/A housing so I can't connect the oil cooler to the engine. I can't think of anything else to do or what it could be! The APC solenoid will click when turning on the car as well as when I apply the brakes. I get a series of clicks when tapping on the knock sensor. I have two of everything related to the APC system and have replaced everything and still nothing. What else could cause it to run in base boost????? What color are the wires that go to the boost sensor under the dash?
 

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I had this problem on my car as well. Turned out the problem was the charcoal canister was a GIGANTIC vacuum leak.
 

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yep, my car ran great other than that it had no boost. the leak only showed up under boost, and caused my APC not to fucntion
 

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Discussion Starter #5
under load with no MBC I get about 6psi. but it bleeds off to about 3psi. in the higher RPMS, it that normal? (I have aftermarket exhaust too) I doubt its a vacuum leak though but maybe so. I've heard a lot of people replacing charcoal canisters and getting their boost back.

I also tested my cruise control and everything worked perfectly. When the brake pedal was pushed it cut off, same with the clutch. The only other thing I can think of is that the wires going to the boost sensor might be incorrect as the previous owner did some whacky wire work. I'm still wondering if the APC system reads info. from the oil cooler adapter and since it's not there might be throwing it into base boost.
 

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I've just retrofitted an apc to my lpt. Initially I had problems that couldn't be tracked down (by back-probing the contacts on the apc lead connector) to the
a) pressure sensor (gave correct ohms at 0 and 0.6Bar),
b) brake light (no 12V when brake not applied),
c) knock sensor (continuity thru' leads and thru' shielding),
d) 12V power supply or ground
e) rpm signal in (got a voltage at idle)
f) apv valve - clicked when applied voltage and allowed me to blow down the W connector and get air coming out the R side of the valve.
It turned out it was the connector to the APC valve giving an intermittant connection. Have you checked that you get +12V out of the APC wire going to the apc valve at idle?
Best of luck...
 

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Discussion Starter #8
I haven't checked power to the connector but it's a direct connection because I pulled the wires out of the connector thinking it was worn out and might be getting a bad connection. I still have yet to go through everything with a voltmeter and make sure everything is functioning properly. I guess I'll do some electronics tests later today and see if I can't figure out what the problem is.
 

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Discussion Starter #9
I found out today that the APC solenoid is functioning correctly. When the car is on and idling I blow through the Intake line or any line connected to the solenoid and all 3 ports are open which should allow the turbo to spool up because there isn't enough boost pressure to open the wastegate but I still get base boost. How is this possible?
 

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So your APC system is functioning correctly and you’re not getting full boost but you are getting proper base boost? It’s not your wastgate. Does your dumpvalve hold vacuum when you suck on it? I guess just because your APC valve is opening correctly at idle doesn’t mean it’s opening under load (but before knock comes into play). I guess I’d rig a test light from the APC soloniod control (i.e. APC pin 12) into the ****pit and see what happens when you drive. Under normal conditions you should have 12V, under knock conditions you should have zero volts, or pulsing at least. < edit ****pit? I'm not allowed to say co*kpit? What kind of screening software is this?!! />
 

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Discussion Starter #11 (Edited)
I'm getting proper base boost (about 6 psi. on my boost gauge) under load but once the engine revs. up it backs down to about 3 psi. How do I determine where pin 12 is on the APC connector? I'd like to rig up a test light. So if the solenoid is open at idle it's working correctly, so the only thing this could mean is that there is enigne knock (or thinks there is) once the engine RPM is increased? I have unbolted the knock sensor from the block before and set it on top of a rag and went for a test drive and still got base boost. I have tryed everything! Everything SHOULD work perfectly. What else could cause the solenoid to close under load?
 

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If you disconnect the APC connector from the unit, undo the screw in the top of the connector, remove the gasket and pull the connector apart, you can see the connector numbers. When probing for signals using a voltmeter probe from the back ("backprobe") in order to prevent damage to the metal contacts.
Best of luck,
gus
 

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Discussion Starter #13
I still can't figure out why the boost won't build if the APC solenoid is fully open at idle. Right now I can push the boost all the way up to 16 psi. quickly and it hauls butt with my manual boost controller, plugs look great and I didn't get any knocking. Could it be the TPS sensor, when I give the car gas the sensor sends a message to the APC making it stay in base boost if the sensor is failing? Other than that, everything is functioning correctly.

I have swapped out individual APC parts with a working APC system.
1. APC unit
2. Knock Sensor
3. Boost Pressure Transducer
4. Pedal Switches
5. APC Solenoid
6. Cruise Control Vacuum Switch

After replacing all this I still could not get out of base boost. :cry:
Sounds like my problem is the only one people haven't been able to figure out since everything SHOULD be working. :lol:
 

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You cant look after the solenoid if its fully open at idle. It is on boost you need to know! Lets say the solenoid is open on off boost and when there is comming boost, the solenoid have to close. The systen works like when there have to come more boost, the solenoid have to close, if there could be knock or something else, the solenoid have to open in a short time!

The more pressure that goes in the wastegate actuator = Less boost
If you take off the hose to the wastegate, the turbo will boost all what it can! = No pressure in the wastegate actuator!
 
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