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Discussion Starter · #1 ·
Should I be worried?

The sensor itself gives a voltage on the 'scope when hooked up and tapped with a spanner. But if I tap it while bolted on the engine block and connected to the APC with the ignition on, the solenoid doesn't chatter as expected. Plus the solenoid doesn't click when the ignition is first turned on.

Other than that, it boosts well, and is limited to 1-2mm into the red.

Is the "click" and "chatter" a definite test of things working, or can it work, but not display those two things?
 

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Is the ignition "on" or is it turned to "run" Pretty sure it needs to be at run to chitter-chatter.
 

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Is the ignition "on" or is it turned to "run" Pretty sure it needs to be at run to chitter-chatter.
No it doesn't. It should chatter with just the ignition on. Other than that it's not easy to answer your question. Perhaps you're not tapping it hard enough. Other that that the only idea I can think of is that the APC is not working and all your boost is because of an over adjusted base boost. Just a thought. Disconnect the APC solenoid plug and see if the boost falls back to mid orange. If it does it would appear the APC is working.
 

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I think on later models, the APC won't work at low RPMs. Try tapping the sensor at 2000 rpm.
 

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I think on later models, the APC won't work at low RPMs. Try tapping the sensor at 2000 rpm.
That's not my experience Jim. It should chatter with just ignition on, especially on late models. No rpm signal does not disable the system. Try it at 2k as Jim suggests and see what happens.
 

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Discussion Starter · #6 ·
Disconnect the APC solenoid plug and see if the boost falls back to mid orange. If it does it would appear the APC is working.
The APC is working for sure. It wasn't originally after I transplanted the system from a donor car but it turned out to be the box. It was boosting halfway in the yellow, as expected for base boost, and then just past the red when a working (160bhp) box was installed.

I just managed to get another known-to-be-working box (175bhp), and will try that out.

In the meantime, I've been fueling with 98 Octane just in case....
 

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The APC is working for sure. It wasn't originally after I transplanted the system from a donor car but it turned out to be the box. It was boosting halfway in the yellow, as expected for base boost, and then just past the red when a working (160bhp) box was installed.

I just managed to get another known-to-be-working box (175bhp), and will try that out.

In the meantime, I've been fueling with 98 Octane just in case....
I think if knock sensor is faulty it will revert to base boost anyway, so no risk of engine damage with component failure I don't think.
 

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It tends to need a fairly smart smack with a long heavy screwdriver. When you tried the voltage on the scope was it connected to the APC as the loading will make a difference.
Have you rigged up a knock led? Might be better way to check if the APC unit is recieving a signal.
 

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Discussion Starter · #9 ·
When you tried the voltage on the scope was it connected to the APC as the loading will make a difference.
Have you rigged up a knock led? Might be better way to check if the APC unit is recieving a signal.
Nope, it was not connected to anything as I had to bring it to a friendly TV repair guy who had a 'scope.

Yes, I should wire in a knock LED to see if it flashes.
 

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If you are convinced the APC is working properly James, why are you bothering to get the solenoid chattering? If you connect a knock light to pin 19 it only duplicates what the solenoid is doing. BTW, the sensor does need a good smack to get a chatter and the engine does not need to be running, just ignition on.
 

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Discussion Starter · #11 ·
If you are convinced the APC is working properly James, why are you bothering to get the solenoid chattering? If you connect a knock light to pin 19 it only duplicates what the solenoid is doing. BTW, the sensor does need a good smack to get a chatter and the engine does not need to be running, just ignition on.
The APC might be working properly, but the knock sensor amplifier and trigger circuit may not be - that's my focus right now. That would be troubling as the knock light will not light and I won't have knock protection.
 

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The APC might be working properly, but the knock sensor amplifier and trigger circuit may not be - that's my focus right now. That would be troubling as the knock light will not light and I won't have knock protection.
The APC will not operate unless the knock sensor and associated circuitry is functioning, and you say the APC is working. Without that you will have uncontrolled boost. Anyway, I am following with interest.
 

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Discussion Starter · #13 ·
The APC will not operate unless the knock sensor and associated circuitry is functioning, and you say the APC is working. Without that you will have uncontrolled boost. Anyway, I am following with interest.
I don't understand the APC box fully so don't know if such a fail-safe feature is built in. My APC box may be correctly controlling boost & the tapering of the boost may be due to the pressure tranducer/rpm limits and not due to the knock sensor - I don't know. Hence the question.
 

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I don't understand the APC box fully so don't know if such a fail-safe feature is built in. My APC box may be correctly controlling boost & the tapering of the boost may be due to the pressure tranducer/rpm limits and not due to the knock sensor - I don't know. Hence the question.
I think Peva mentioned a simple test, here is how you test the APC system and get your answers. The safety features you are not sure of are built into my 84.

Disconnect the plug to the APC solenoid, if your boost doesn't go as far when disconnected but goes over to red when connected your APC system is delivering proper boost.

IF the knock sensor is faulty the APC reverts to base boost. You will get the same effect (base boost) if you disconnect the lead from the knock sensor IIRC.

SO after saying that, If when you disconnect the plug from the APC solenoid your boost doesn't change from being over in the red (or near the red), it means your APC is NOT functioning and you have the waste gate rod wound clockwise way too much and your car despite boosting into the red is on base boost (no APC intervention and no knock protection)

Hope this helps.
 

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Discussion Starter · #15 ·
Thanks. Yes, it does help.

It all functions as described. My base boost is safely in the mid-yellow, and 2 APC boxes of differing rating indeed give different (expected) boost characteristics, just past the red. It also reverts to base boost when I unplug the fuse to the APC.

So the presumption is that if the APC works insofar as increasing boost from base boost in concerned, then the knock protection is also working. That is a presumption I question, as the system isn't really all that smart. Given that the signals from the sensor are so minute, the margin for false or no readings is great.

But I might be worrying for nothing....
 

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Unplugging the fuse to the APC disables everything, to test the separate components I listed the individual components you should unplug etc.

It's not really that sensitive I mean you whack the thing with a screwdriver pretty hard for it to trip the APC.

What do you mean it isn't smart? Generally speaking If the sensor fails - APC detects and reverts to base boost, if the sensor is unplugged the same situation.

The system is intelligent - they pioneered automatic boost control, making turbocharging in the 70s possible. With your system if the hose to the waste gate breaks the fuel cut out (separate device) prevents engine damage from overboost.

The system is designed in way that, should a component fail you will not destroy your engine. I don't think it's possible for the knock sensor to stop functioning in a way that it doesn't revert back to base boost.

IF the car is not on base boost, the knock sensor is functional. As long as you have the correct torque on the nut and it is loctited the readings won't be inaccurate and there isn't any more risk of destroying your engine than anyone else with a 25+ year old car.
 

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As S900t8v says, the APC system is intelligent and has fail-safes. If any part fails it will go to base boost. He describes a reasonable test. Another test to apply is to mark your dizzy position so you can return to it. Now advance the dizzy on the collar by about 1mm and lock it. Go for a drive and observe the boost level you get. There should be significantly less boost. Retarding the dizzy by a similar amount will give you increased levels of boost. If you can observe these happening your APC and knock sensor are working correctly. Return to your correct dizzy setting.
 

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Turbos all came off the truck with the APC solenoid valve unplugged, to lock it in base boost.
During the Pre-Delivery Inspection test drive, we were instructed to watch the boost gauge to see the initial drop when the APC kicked in, to ensure it was working. It's just a little twitch of the needle, but you can see it plainly if you know what you're looking for. With the big 12" diameter gauge (special tool) the needle twitch was obvious to anyone.
I never saw a need for a more 'accurate' test.
 

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Good point Jim. When the APC is working you can definitely see the needle twitch or dip when the APC starts controlling boost. But you have to know what you're looking for, but once observed you can't miss it, even on the stock gauge.
 
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