3-Way Boost Control Solenoid Switch for XR4Ti Have you ever wondered if you can do anything useful with the factory Boost Control Solenoid (BCS) besides bypassing it and installing a bleeder valve? Well you can put it to use in your modified or stock XR, and with great results! The modification will cost you very little -- some wire, a 3-way switch, and some time to wire it all up. I performed this mod back in December, 1997 and it has been working great, no problems, no downside and no serious headaches implementing it once I figured out the logic of the wiring. What it is: The BCS adds about 5 lbs of boost to the maximum allowed by the turbo's wastegate actuator. A 3-position switch is mounted inside the car, within easy reach of the driver. Position O (switch off) forces the BCS to stay off. Position 1 allows stock EEC-IV computer (ECU) control of BCS. Position 2 forces the BCS to stay on by isolating it from the computer and grounding its control lead. What it does: Three instantaneously selectable levels of performance. With the switch in position 0 (off, same as unplugging the BCS), you get rock solid low boost (about 10 lbs) at wide-open throttle (WOT). This is useful as a primitive traction control on slippery roads. Also good for going easy on the engine during climbs up extremely long mountain passes (Alaska's got lots of 'em) or for improved mileage when you don't need higher performance. With the switch in position 1 (BCS control wired to computer), you get stock computer control of boost. By observation, some computers (PF3 in my '89) are able to modulate the BCS and give boost levels between low and higher settings as the computer's boost algorithyms see fit (not uncommon to see 11-13 lbs at full throttle, with RPM's below 4000 on a mostly stock '89 XR). The behavior of the computer in this regard may vary by model year. In my '85, assuming no detonation was detected, its computer would just give low boost up until 4500 RPM and then high boost to redline. Either way, the bottom line is smoother power delivery than position 2 -- less frightening for your passengers and more controllable on slippery roads -- whether rainy, gravelly, or icy. Position 1 also keeps the computer's ability to cut back the boost when it senses knock at higher RPM's. With switch in position 2 (BCS control wired to ground), you get rock solid high boost (about 5 lbs more than or perhaps half again the value of low boost, roughly 15 lbs on a stock XR) at WOT. Position 2 is the most fun, because you get maximum torque from about 3000 RPM on up. :) How it's done: This can be done by just about anyone who has some experience with running wires such as in a stereo installation. Experience with a soldering iron may be helpful. But solderless connectors can probably be used with good results. Likely the worst that can happen in a situation of poor contact is the BCS staying switched off and the boost staying at the minimum level until the connection is fixed. Disconnect the battery while working with the ECU wiring harness. This may not be absolutely necessary but it's best to be cautious when working around the ECU. The three-position switch is wired as follows: About 5 inches from the ECU, cut the solid purple colored wire that runs between the BCS and ECU (pin 32) and connect switch leads: - Center (common) switch pin to purple wire segment heading to BCS. - Pin for switch position 1 to purple wire segment heading to pin 32 of ECU (for computer control setting). - Pin for switch position 2 to any good GROUND (for high boost setting). The reason for using ground is that the ECU continuously supplies a steady 12V to all of its actuators, and controls them by grounding the circuit. So, when controlling the BCS, the computer grounds the purple wire. I installed the switch, along with two others (one is for my subwoofer, the other controls my Halmeter air/fuel guage), in the blank panel on my '89 that covers the hole where the stereo fader joystick goes on the pre- 87's. Wire used was of similar or heavier gauge than that used in the ECU harness. I found a three-conductor wire that works very nicely at my local electric-supply store. Notes: This setup works even better than I expected it to. There really are three distinct levels of performance, all selectable with the flick of a switch, at any time, even while under full throttle. This may not be as good as an aftermarket variable boost controller, but for about 5 bucks in parts and supplies, I ain't complainin'. For non-intercooled XR's this eliminates the need for a bleeder valve and BCS bypass. XR's without intercoolers really shouldn't exceed 14-15 lbs of boost, IMHO. If you do this mod and you're already using a bleeder valve and previously had the BCS bypassed, make sure you either remove the bleeder valve or close it down before taking the car for a test drive since the BCS adds an additional 5 or so lbs of boost when it's activated. The primitive traction control really helps in the winter on icy Alaskan highways. Before this mod I had to be extremely cautious with the throttle, even in 5th gear at highway speeds with the ECU in control of the BCS. Stabbing the throttle, as soon as the boost would come on the studded rear tires would spin out from under me. This was a bad thing on curves and required very careful, skillful application of the throttle. With the switch in position 0 the boost is toned back just enough that wheelspin won't happen anywhere near as easily. Position 0 is also useful for when you want better mileage or when you hand over the keys to another driver and don't want them to have the full power of the engine at their disposal, for instance when letting your timid spouse, your little brother or your teenager drive the XR. This assumes the purpose of that little switch next to your stereo isn't easy to identify. ;) It's probably wise to use caution when driving the car with the switch in position 2, and be sure to use good high-octane fuel to limit the chances of knocking. While the ECU retains its ability to retard the timing when it senses knock, it has lost one of its controls when you bypass pin 32 and it can no longer cut back the boost. People who have already eliminated their BCS with a bleeder valve are probably already aware of this and don't consider it a serious risk. For me, the ability to have full boost below 4500 RPM is worth the risk. It's my opinion that the primary purpose of the BCS was for smoothing out the power delivery, not to protect the engine. This idea has some support by the fact that a primary design concern of the XR was limiting the Nosie-Vibration-Harshness and power-on-off behavior of the turbocharged four, retaining as well as possible the feel of the Cologne V6 in the European Sierra XR4i. However, for the most part I leave the computer in control of the BCS. I do enjoy having the option of overriding the ECU when I want more or less power. Idea: With the addition of a bleeder valve, one could set the switch to position 2, set max boost with the bleeder valve, and from then on use the switch to select between, say, 13 (roughly) and 18 lbs of boost. With the switch in position 1 you'd still get the stock computer modulation of the boost for smoother power delivery and the failsafe backoff to lower boost if knocking is detected. You'd still be able to tone the power down in the winter time and have full boost available for play time and stoplight races. Note that this shouldn't be done on an XR without an intercooler. I use this method now that I have the Modern Performance intercooler installed in my engine bay. Standard disclaimers apply. If you blow up your engine or crash your car I'll feel really bad but can take no responsibility. That's it. I like feedback. Please let me know if any details need clarification or if you have any problems that I can help resolve. If you try this modification, please drop me a line and let me know what you think of the results. This is, I think, a cheap modification that can make anyone's XR more fun to drive, and instantly adaptable to various driving conditions. Steve Munk fxstm@uaf.edu