Info on BCDD - Boost Controlled Deceleration Device

[Captain Obvious]

First off, hopefully you already know this, but I want to make sure you understand that I wasn't chastising or anything with the wording. I was just trying to standardize on the language with what is in the FSM, etc. Sometimes with things (like the altitude corrector), the function is also the name and can help with troubleshooting. So no prejudice intended, and hopefully none taken.

So I'm no expert on the BCDD device, but I do know they made several different varieties over the years that all did the same thing (which is limit the intake manifold vacuum at a certain level and not let it decrease below that level). Think of it as your air compressor outlet pressure regulator. Same concept.

In 75 (and I think 76) they used that sideways lock screw to hold things in place, and in 77 and 78 they went to an external lock nut instead. Also in 78, they offered two different versions, one for CA and one for everyone else. The CA version was pretty much the same as they had been selling all along, but now starting in 78 the version to NON-CA cars was a simpler version that no longer included the altitude corrector. I guess they decided that other states didn't have enough altitude changes to warrant the compensator anymore.

Point is... There might be some small variations between one and another depending on the year of manufacture.

If both the units you have use the sideways lock screw, then they are both from 75 or 76 and should probably be pretty much identical inside (including the springs).

Can you take a pic with the device disassembled and the parts laid out roughly in the position of assembly?

[MattSaint]

No Sir, Non taken.  I really do appreciate your input.  So much so that I went and grabbed the FSM and re read the pages.

So, what you said struck a cord about differences for CA cars.  The original owner purchased the car in NJ, but it appears to have been a 'partial' CA car as it had for example the floor temperature light in the dash, but did not have the temperature sensor in the floor.  Maybe there was a variance in other parts of the car as well, i.e. the BCDD.

Both have the side lock screw and aside from the presence of the internal spring they appear identical.  I had good luck taking mine apart, accept for the screws which help the BCDD itself together.  i.e. the diaphragms are all intact.  I think I will try and put it back together with the spring and see what happens.

I'll take and post pictures tomorrow in the AM.

[Captain Obvious]

I'm not sure there were "partial" CA cars, but there are minor differences between CA cars and others. If you're looking to identify conclusively, I'm sure I can come up with a list of other items to look for other than the catalytic convertor stuff. Stuff like the altitude switch under the dash.

I've got two BCDD's here. One is on my original 77 throttle body, and the other is from a non-CA 78 throttle body that I bought as a spare some time ago.

I'm reluctant to take my original apart (for fear of damaging one of the diaphragms), but the one from the non-CA 78 is fair game. If I get a chance, I'll snap some shots of that one exploded. Only issue is that since it's non-CA 78, it doesn't have an altitude corrector on the bottom. Just a simple screw on the bottom and the whole thing is an inch shorter than previous years because they simplified the underside.

So a question about the altitude corrector... On your BCDD, when you turn the adjustment screw on the underside, does the entire altitude corrector turn with it? Or is the adjustment screw threaded into the underside of the altitude corrector? I'm guessing it's the latter, and that would explain the need for the dowel across the top of the altitude corrector that fits into the slot in the valve. They need that feature to keep the altitude corrector from turning when you adjust the screw underneath.

[MattSaint]

I'll have to take a closer look, but on first view the adjuster screw appears to be just an extension of the altitude adjuster.  In other words just a single unit and the entire thing turns when you turn the screw.

I had wanted to leave mine intact, but alas I took it apart and ended up damaging the altitude adjuster.  The diaphragms are all unharmed and in really good shape, the altitude adjust not so much.

[MattSaint]

I confirmed that the adjustment screw is just an extension of the altitude corrector.  Everything is soldered together on both of the ones I have.  They spin together.

[MattSaint]

Here is an exploded view of the BCDD.  Note that 1 of them had the spring and the other did not.

 

Here is a close up of the altitude corrector.  Note that both sides are soldered...  So the adjuster screw isn't really a screw.

[MattSaint]

In conclusion.

I cleaned everything up, after removing all the diaphragms, and reassembled the BCDD with my spring and got it back into the car.  A little fiddling later and the car now operates much better than it did.  Is it perfect, no, but now I don't keep the high idle when coming to a stop and the car is in neutral.  I think I can still do some minor tweaking as the idle is a tad high now.  I am going to pull the solenoid harness and see what the RPMs are at at idle.  I was doing everything by feel and hearing.  Need to put some eyes on the tach.

Thanks everyone for your input!

Edited October 18, 2017 by MattSaint
grammer

[siteunseen]

Glad you got it back together. I'm by myself and now have a timing light that shows rpms but before buying that I would use my phones recorder on the tach gauge while fooling with the motor. Then replay the video and see what the readings were. Almost bought a baby monitor in a momment of brain farts.

[Captain Obvious]

Cool, and thanks for the additional pics. Before conclusion... Yes, both of them should have that spring. Someone must have been inside your BCDD in the past, and left that spring out. You probably already know this, but without that spring, it won't work correctly. You didn't get into what kind of problems you were having, but I surmise that without that spring, it would be either unpredictable and erratic, or hang like crazy at high idle the first time you decelerate. Or both?

And also just to close up some of the analysis.

So if the whole altitude corrector spins with the adjustment screw, then the purpose of the cross dowel is to convey that rotation to the slotted piece that it mates with. And that makes sense since the shaft of the lower valve (the green valve) is threaded. The later model non-CA version essentially ends there with that slotted stub sticking out to be used for adjustment. The rest of the unit is pretty much identical from that point upwards.

So if you are interested,  basically how this thing works is like this:

• There is a vacuum chamber above the lowest diaphragm that senses the intake manifold vacuum.
• If the vacuum in that chamber gets low enough, it sucks the diaphragm upwards and opens the green valve.
• When the green valve is opened, it allows vacuum into the chamber above it which pulls the upper diaphragm down.
• Pulling the upper diaphragm down opens the big round nose plunger valve and allows air to bypass around the throttle butterfly plate.

Simple, huh?  

The whole altitude compensator thingie simply automatically adjusts the preload on the lower valve by growing or contracting in length depending on the ambient atmospheric pressure. Again, simple, huh? Haha!!

Way too many parts, and be thankful that todays cars do the same thing just by using the computer to open the idle air control valve a little when decelerating. We don't have a computer, and our IACV is a slotted head screw sticking out the top of the throttle body.

[MattSaint]

Hi Capt,

Mine had the spring, the one I purchased did not.  Mine was completely frozen because of all the coking surrounding the adjuster screw.  There is a hole which reveals some of the coils of the altitude adjuster that I sprayed PB Blaster into, but it never freed it up.

I was having the crazy RPM hang at high idle.  It made life difficult driving in a parking lot at slow speeds and coming to a stop.  I would keep the car in gear until the last possible minute and then go to neutral.  Making sure I was below 10 mph.

Thank you for the description of what it does.  I had a rudimentary understanding of its function, but that does clear up some of the mystery of this troublesome device.

As I mentioned it isn't one hundred percent fixed but it is so much more drive-able now and behaves way better.

[MattSaint]

14 hours ago, siteunseen said:

Glad you got it back together. I'm by myself and now have a timing light that shows rpms but before buying that I would use my phones recorder on the tach gauge while fooling with the motor. Then replay the video and see what the readings were. Almost bought a baby monitor in a moment of brain farts.

That is a great idea!  Thank you.

[Captain Obvious]

Cool. Glad you're out of the woods. Here's my new tag line. I was talking about carbs at first, but it applies to lots of other things as well!

"I've found that surgically clean and actually assembled correctly goes a long way to fixing a lot of issues! Go figure!!   "