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Broken boost control solenoid - 88'


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#1 kepico

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Posted 07 May 2020 - 04:28 PM

1988' Quest 5spd

So replacing my vacuum lines I ended up breaking one of the ports on my boost control solenoid (at least I think that is what it is called) it is the solenoid attached to the top of aircan.

Instead of replacing it (if I can even find a replacement) I thought I read that it doesn't really need to be used if I follow the instructions on the 3 port wastegate trick - leave open inner port (closest to turbo) and cap one of the two outer ports (the other port will use vacuum to OVCP as normal) - if I do this then instead of 7.5 low to 10 psi, it will instead go straight to the 10psi?

Is this correct? And is it ok\safe to do (it won't over boost?)

If that is ok I had a few questions;

1. No post I read mentions what to do with the solenoid itself? Does it need to stay an be capped and remain plugged into the electrical? Or can it be removed all together? Obviously I cannot cap the port that I broke off!

And

2, What is the correct size vacuum line? I thought it was 1/8 after reading some posts but that size is smaller than the current OEM vac lines and it seems like it'll be a hard go to get it on some of these vac ports?

Thanks for any help!!





#2 mikec

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Posted 09 May 2020 - 06:47 PM

That solenoid controls which side of the wastegate actuator, on 88-later StarQuests, sees boost pressure.  One side opens the wastegate at 7.5 psi, the other at 10psi.  The solenoid itself exists for two functions:
1: on stick-shift StarQuests, the ignitor commands the solenoid to pass boost to the 7.5psi side to limit max boost to protect the transmission a bit.
2: on all StarQuests, the ignitor module checks the engine for knock.  When knock is present, the ignitor retards timing a bunch to reduce knock.  At the same time, on 88-later StarQuests, the ignitor opens the solenoid to reduce boost pressure which should also reduce/eliminate knock.

So those are the issues to be aware of before removing the solenoid.  You can run a vac hose directly from the over-the-valve-cover pipe "sensing" nipple to the 10psi port on the wastegate actuator.  Cap the other 10psi port.  And then completely remove the solenoid.  Keep knock in mind if you use cheap or no-name gas that may not be as high-octane as it claims to be... and use high octane gas whenever you plan to use full boost pressure.

Note that it is the ignitor that monitors for knock - not the engine ECU.  The ECU does send a "knock signal" to the ignitor; this signal basically tells the ignitor when to monitor for engine knock.  That signal is ON whenever the factory boost gauge has the needle in the filled-in part of the display arc.  When boost is not present, the ignitor doesn't monitor for knock; that way ignition timing won't be needlessly retarded if a rookie stick-shift driver lugs the engine.  There is no feedback from the ignitor, to the ECU, when knock is detected.  The StarQuest ECU and ignitor do not "learn" anything knock related.

I forget the exact vacuum size.   3.5mm if I remember correctly which is a tad larger than 1/8th inch.

mpc

#3 tux

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Posted 09 May 2020 - 08:44 PM

Mike, do you happen to know what voltage is output from the knock sensor when knock is detected?  Or is it's a sliding scale and what those values are?

#4 mikec

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Posted 09 May 2020 - 10:52 PM

The knock sensor is basically a piezoelectric microphone element epoxied into a metal casing.  The output is low voltage and with very low current capacity.  Whatever is connected to the sensor must have very high input impedance, otherwise it'd load down the sensor.  A typical multimeter is enough to "short out" the knock sensor, loading down the output voltage to almost zero.  A high-impedance meter, like a quality digital meter, won't load down the sensor but it still won't read anything as the knock sensor output waveform is a series of short-duration spikes/shocks.  Ever see an earthquake seismograph trace: small wiggles...  spikes...wiggles... more spikes... wiggles?  That's what a knock sensor output looks like.   The spikes are at most a few tenths of a volt unless the sensor is really whacked where you might get a little over a volt generated.  I put a junkyard knock sensor on an oscilloscope, with a 10x probe (divides the input by 10 but also increases the input impedance of the oscilloscope by a factor of 10) years ago (so this is from memory) and smacked the sensor and smacked the sensor with metallic objects.  My oscilloscope, with a basic (not 10x) probe has a 1megohm input impedance and that noticeably loaded down the sensor.  The 10x probe (so 10 megaohms impedance) reduced the loading-down effect 90%.  To get even a few tenths of a volt took some significant smacks with a screwdriver shank... that's probably larger smack inputs than a knocking engine is likely to produce.

The StarQuest ignitor includes a high impedance amplifier connected to the knock sensor; the output of this amp then feeds a comparison circuit: once the output voltage exceeds a certain value, knock is present.  The comparison circuit is turned on/of by the ECU based on RPMs as I noted earlier.  Once the comparison circuit detects large enough spikes (knock), the ignitor retards ignition timing; it also starts a short timer that keeps the timing retarded even after the knock goes away.

mike c.

#5 tux

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Posted 09 May 2020 - 11:11 PM

Thanks Mike.  I'm trying to figure out how to scale it in an aftermarket ECU.  I have the ability to do mV, but all I could find out from the manual is that it's tuned to ignore engine vibration and over-torquing or hitting it with a metallic object can damage it permanently.



#6 kepico

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Posted 11 May 2020 - 01:35 PM

That's some really good info there, thanks a bunch Mike!




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