North American Turbocoupe Organization



BCS frequency?
Rob.. Offline
Member
#1
Does anybody know at what frequency the factory BCS functions? I have an 88 TC that the boost controller is on the fritz. If I'm not carefull I ring the overboost buzzer quite easily. I have a manual boost controller that I can replace it with but I'd rather keep the factory high/low selectability. I was thinking of useing a MAC boost solenoid that operates at 30Hz. If the POS Ford BCS is much different than that there is a good chance it will ruin the boost circuit on the EEC. If I could find out what frequency the factory uses I can match it with just a few $$$ in parts from Radio Shack. I'm hoping someone has the answer before I end up needing a headgasket. Thanks.
I want to live in a world where monkeys don't dig up the dead and parade about in their shoes.
88 7F, SS big valve head, port matched in/ex, gutted and powdercoated intake, A-237, stealth T3/T4 Garrett hybrid, Abaco 85mm mass-air meter conversion, tripminder, Walbro 255 high pressure, SVOJohn tuned.
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Jeff K Offline
Administrator
#2
Early TCs dithered the BCS at 40 Hz (Pulse width modulation). Later TCs (87-88) just used the BCS as either on or off.

To keep the high / low boost selectivity with a boost control valve (Gillis, etc) you can do what I did years ago: Put in the boost control valve, and put a vac T at each end of the valve. Connect a solenoid to the Ts so when open, it bybasses the boost valve, giving the stock WG boost limit. When the solenoid is closed, the boost valve controls max boost. You can either wire the solenoid to a seperate switch, or piggyback a relay on the reg/prem relay to control the solenoid (what I did). That way, with switch in reg, I have 9.5 psi max boost, and in the prem position, I have whatever the boost valve is set at. I used a Ford canister purge solenoid from a mid 80s 5.0 (free at the JY). It will hold 40 psi when closed and open / close at up to 30 psi. I have had this system installed for years and it has worked flawlessly. I suggest not trying to use a stock BCS solenoid in this application as the stock BCS solenoid is a cheap poorly made piece of crap.

BTW, Welcome to NATO!
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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Rob.. Offline
Member
#3
Thanks. I was just thinking of using the MAC solenoid in place of the stock BCS. I've had two TC's and two SVO's and on everyone of them the OEM BCS was crap. I've done the deal with the manual controller, and you're absolutly right, it does work flawlessly. I was just trying to do it electronically with a little panache. However, I don't want to burn up the boost circuit doing it. So you're saying the 88 BCS is just a switch? No frequency, just 12V on/off? How does it limit boost to ~15psi without modulation and dutycycle?
I want to live in a world where monkeys don't dig up the dead and parade about in their shoes.
88 7F, SS big valve head, port matched in/ex, gutted and powdercoated intake, A-237, stealth T3/T4 Garrett hybrid, Abaco 85mm mass-air meter conversion, tripminder, Walbro 255 high pressure, SVOJohn tuned.
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Jeff K Offline
Administrator
#4
All years BCS are electrically and mechanically the same. In 86, at least, the solenoid was opened to max over a second or 2 period via a PWM signal from the PCM (actually PCM grounds it to energize it). In 87-88, the PCM just turns it on and off..... no PWM modulation. Why?? No idea... maybe Ford needed that PWM signal for something else??

The PCM doesnt directly measure boost pressure, it roughly infers it from air flow, RPM, and probably a few other PIDs. Max boost is determined by the calibrated orifices in the "T" on the compressor outlet that act as a pressure divider so the WG only sees a given fraction of the actual boost pressure (stock, about 2/3 of the actual boost pressure). i.e., BCS closed, WG sees full boost pressure, BCS open, dumping some pressure from the calibrated orifices in the T back to the turbo inlet. Kind of like the old "fish tank valve" pressure bleed boost controllers no one uses any more.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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Rob.. Offline
Member
#5
So the MAC solenoid should be a direct replacement for the factory BCS. I'll get one and see how it works out. Thanks.
I want to live in a world where monkeys don't dig up the dead and parade about in their shoes.
88 7F, SS big valve head, port matched in/ex, gutted and powdercoated intake, A-237, stealth T3/T4 Garrett hybrid, Abaco 85mm mass-air meter conversion, tripminder, Walbro 255 high pressure, SVOJohn tuned.
Reply

Jeff K Offline
Administrator
#6
What is the coil resistance of the MAC unit? If I remember right, stock BCS is around 100 ohms. Be sure the MAC unit isnt real low, like 25 or 30 ohms, which might pull too much current from the driver in the PCM and fry it.

Keep us informed if it works. I have toyed with building a PWM controller to provide variable max boost dependent on RPM and throttle position. I know you can buy such controllers, but I would rather build one myself and learn something in the process.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
11 Crown Vic Interceptor
14 Toyota Camry (wifes car)
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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Rob.. Offline
Member
#7
I've checked the two that I have and they are 74 & 76 Ohms. They both suck so I'm not even sure they are within spec. But I can add a suitable resistor so the MAC solenoid won't draw too much. No biggie.
I want to live in a world where monkeys don't dig up the dead and parade about in their shoes.
88 7F, SS big valve head, port matched in/ex, gutted and powdercoated intake, A-237, stealth T3/T4 Garrett hybrid, Abaco 85mm mass-air meter conversion, tripminder, Walbro 255 high pressure, SVOJohn tuned.
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