boost control solenoid

jclars

Registered User
I have read all the boost related posts I could find, and did not see a picture of the solenoid that was used on the 89 SC. Can someone post one up? I have a 3.8L SC in a F100 and I cant seem to locate this component.

The reason I ask, is it appears I have been running without the SC kicking in. I unplugged the by-pass actuator vacuum line and then it closed the by-pass. Voila - boost! It opens and closes with engine start/stop, But nothing in between. The valve isn't sticking and the actuator does not leak.

so I am wondering if this solenoid is needed and missed when I reassembled the engine into it's new home.

BTW, I currently have the actuator vacuum line attached to the common vacuum tree. Is this the culprit? Does it need to pull vacuum from somewhere else? Would this eliminate any need for the solenoid?

Puzzled,
John
 
Except for the '89 SC, these engines don't use a boost solenoid...just the bypass as you've described. Open/pulled under vacuum and closed without it/under boost.

Do you have a working vacuum/boost gauge? That's the signal you want for the bypass.
 
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I thought I read in many of the previous posts that the 1989 models had the solenoid. They dropped it in subsequent years. That the later years had a dedicated vacuum port between the intake and SC where the 89 did not.

Maybe I was reading more into the posts than warranted, as I have absolutely no recollection of such a solenoid. That said, I didn't have a recollection of the BAP until the loose plastic terminal fried itself onto my exhaust manifold and made me realize that the sensor was missing.

John
 
Thanks for the link - for some reason, I did not see that one out of about 20 on the subject. It is the most complete discussion, but left one question unanswered: If I delete the bypass solenoid, where is the best place to tap into for vacuum. Where is the port located on the newer SC's?

Pictures?

Thanks,
John
 
I have an 89 with the boost solenoid. mine is still working so i have it hooked up now. However, I upgraded to the MPX blower and the matching inlet plenum which had an extra hole in the front side of it for hooking up the vacuum line for the bypass actuator. I installed a plug in this hole because I am still using the solenoid.

That being said, the ideal spot for hooking that vacuum line up is somewhere on the inlet plenum (between the throttle body and the back of the supercharger). This way as you accelerate hard and open up the throttle body the vacuum will drop to zero and cause the bypass actuator to close, which then helps you build boost. If you have an early style inlet plenum, there may not be a nipple to attach the vacuum line to. You will have to tie into another line or drill a hole and install a fitting to attach the vacuum line. something like this... http://www.glowshiftdirect.com/18np...c1458_a_7c6258_a_7cGS_d_FA_Non_a2s_Restricted
 
Where is the port located on the newer SC's?

There is a brass tee on the intake (pic 1) - it happens to be metered on one port. That tee feeds over to the driver's side of the firewall into a plastic multi-port manifold (pic 2) which splits for various needs, one of which is the bypass actuator. These photos are of my '90 SC.
 

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Super - now we are getting somewhere! But technically, that isnt the inlet plenum is it? Where you show the circled fitting?
 
Here is a picture of the MPX intake plenum location for the bypass actuator...
 

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Rear view...that tee is on the IC outlet path where it returns to the top of the engine, then turns down and attaches to the intake manifold rear...it sees both vacuum and boost there.
 

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I may be answering my own question, but with some of the input so far, it has dawned on me that maybe I have put the vac tube in the wrong location on the "tree".

On the attached picture, the solenoid tube is on the bottom. Just below what could be a one way valve? Not sure, but perhaps it should be above that, since the tree is fed from the same relative location as the circled one in KMT's post.

One way to find out...

The only dedicated vacuum port on the intake plenum is where I have the PCV valve hose. Perhaps a tee at that point would work.

John
 

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Again, are you running a single vacuum/boost gauge? If so, where does it tap in?

Which line in that photo feeds the bypass valve? If it's that tiny hose to the left in the photo, after the VAC valve, you might be seeing only vacuum that is held by the valve. Tee in upstream of that valve, instead of after.
 
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Okay - I moved the vacuum line from the bottom of the tree to the port on top that had the chrome acorn cap on it. I took it for a test drive and the boost is now active! It showed about 11 psi when revved up to about 5000 in first.

I must say it is the first time I have punched it from a dead stop with the boost available. Wow is all I can say. I would guess my truck is quite a bit lighter that the original car, but still just some spinning tires then it squatted down and launched! Very impressive!

My donor was a 1989 version, which started my questioning. I am still missing the solenoid, but the by pass problem is now solved! I really didnt want to run with the valve disabled. The air outlet got noticably hotter after a run that way, as well as being noisier.

Thanks for the assistance everyone. I appreciate the time taken for pictures which really helped me understand the problem. You all have helped make this project a success!

I have a feeling my gas mileage just took a hit...it will be hard to keep my foot out of this one.

John
 
That's good news! Just be sure the bypass valve is pulled open at idle, under vacuum. I think you're fine, tho.

Keep us posted when you have the time, John and good luck w/your truck :)

Ken
 
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Confirmed - valve opens upon engine start and remains that way at idle.

I found the attached component that I had marked "to vac boost valve", but as I remember back, some one said I could eliminate this, as it was only to prevent boost while in reverse and it was eliminated on later cars. So maybe we have connected the dots so to speak? If I had hooked the tube up correctly, I would never have learned all this!

Now on to another vacuum dependent device - cruise control...

John
 

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as it was only to prevent boost while in reverse and it was eliminated on later cars.

Sounds like that would make it an anti-boost valve :)

Dots connected √

How much does that truck weigh? Now we need a photo of the hunkered down back end.
 
Well, not hunkered down, as in wheel gripping action, but lower than stock to start with.

Truck weight is estimated at around 3400# vs the SC at about 3800#. But I will be beefing up the frame and a few other adds that will probably get them closer.

On a side note, armed with the vacuum knowledge gained in this post, I was able to trace the cruise control issue to a backward inline check valve. Reversed it and now have cruise control! Killed two birds (so to speak) with one stone!

Very productive day!

John
 

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as it was only to prevent boost while in reverse and it was eliminated on later cars.

Well this is my understanding what the "bypass solenoid" does.
the Solenoid itself is basically a Vacuum dump Valve

Now the ECU Calculates load, when the load it gets to a predefined Value, the ECU Energizes the Solenoid in turn dumping the Vacuum to the By-pass valve, making the supercharger produce "boost" sooner then it would of if the vacuum wasn't dumped.

I own a '89 XR7 AOD car and have noticed that in like 3rd and 4th gear, when i slowly roll into the throttle once and watch the Vac/boost gauge that when i go to a certain Throttle position that the Vac/boost gauge would suddenly drop to atmosphere if not go into 1-2PSI of boost (gear related) with a semi sudden surge of power when that happened

Now my '91 SC 5spd car, i could do the same thing with it in say 4th and 5th gear, I can smoothly moderate the way the engine reacted to the throttle input, with no sudden changes to the Vac/boost, and i could stay out of boost easier.


And that's my Two bits on that valve :)

and PS: Nice truck, Wish i could do a project like that with the drive train out of my '89 XR7.
 
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