Alfa Romeo VSD design

[stevewaclo]

Hello all,

I have travelled here many miles across the circuits of the interweb from the Alfa Romeo BB, optimistic the accumulated wisdom of Automotive Electrical & Sensor Forum members will assist me in my quest.

Series 3 Alfa Spiders, from the early to late 80's used two different Bosch computers for their 2L, DOHV, L-Jetronic injected engines. One computer handles fuel, injection duties and the second, ignition timing. The latter computer has the following inputs: TPS (1/0), flywheel position sensor, flywheel RPM sensor (2 sensors on the flywheel?, don't ask and finally, the reason I'm here, a Vacuum Sensor Device (VSD).

The VSD receives a vacuum signal from the intake manifold, and by means of a copper bellows (hey, I'm talkin' the 80's here ) moves a core within a coil to feed variable inductance to an oscillator in the Ignition Timing Computer. The computer massages the inputs and sends an appropriately timed (no pun intended) voltage to the ignition coil. Subsequently, the distributor functions simply as a four way, high voltage switch and does not factor into ignition timing in any way.

You're way ahead of me if you identified the weak link in this system to be the copper bellows, and tragically, all over this end of the galaxy, Alfa Spiders with failed VSD's have reverted to their base timing of 10degrees BDC, never again to provide improved fuel economy and performance :-(.

A number of us on the Alfa Forum have considered, without success, a replacement A/D/A circuit for the VSD to no avail and are optimistic engineers here may have some ideas.

Here are the electrical specifications we have measured, using my VSD: Coil inductance at Max Vacuum (about 20#, limit of core travel), 3.6 mH; at zero vacuum (WOT) 2.5 mH. Resistance of the coil is 50 ohms. As an aside, have also discovered removing the electrical connection between the VSD and Ignition Computer results in ignition timing at idle going, to use the technical term, "nuts", with the timing mark jumping all over the harmonic balancer and manifold vacuum a shaky 5#.

On behalf of the hundreds of unhappy Alfa owners, I beseech you all to give our problem some thought and rise to the challenge!

Thanks and best wishes.

Steve Waclo, EE, ret (knew T. Edison personally )

 

[MikeHalloran]

I'd be inclined to hack the sensor rather than replacing it.

E.g. substitute a rolling elastomer bellows and stainless spring for the copper bellows.

I might start with a Ford product from the 80's, their Variable Ventruri Carburetor.
It looked rather like a Holley or Motorcraft 2 barrel, but where you'd expect to see a choke plate over a pair of round venturis, it had a pair of rectangular venturis, one side of which was hinged, and actuated by a fairly delicate rolling rubber bellows opposed by an internal hairspring.

The rubber bellows was its Achilles' Heel. Those were the days when gasoline was in transition, and if you got a tank of the sour swill that's standard now, it would cause the bellows to swell and bind, or develop a leak. A replacement only cost a couple of bucks, and with practice, you could replace it in a few minutes yourself. You'd go into your Friendly Local Dealer, and say you needed a bellows for your VVC, and the parts guy would say "Oh, those never fail.", and reach into a big box below the counter and pull one out; he didn't even have to take a step or look up the part number.

Very late in the production run, when they were giving up on carburetors entirely (and when it worked, it was a very good carburetor), the diaphragm material was changed to silicone, which was much more resistant to fuel potions.

So I'd hacksaw an entire bellows and spring assembly out of a VVC casting, and add or change a spring so that it would stroke the coil like the copper bellows, and make it into an assembly.


The other way is to use a vacuum signal to modulate a virtual inductor. I'm sure it's possible, coupla op amps and a handful of resistors, etc., but by the time you get the electronics protected from the automotive environment, a mechanical solution is probably cheaper.



Mike Halloran
Pembroke Pines, FL, USA

 

[stevewaclo]

Mike,

Thanks for your reply.

Parts from later version Variable Venturi Carbs?! Excellent concept.

You have definitely been around the block a few times to remember that old chestnut. Another example of automotive technology introduced before the bugs were out, then re-engineered better than ever, but too late to salvage the original concept (final versions of the much-maligned Corvair come to mind).

Here is a link to additional 411 on the VSD, including pictures! Check out all the posts on the page.

I'm with you on a mechanical solution. As much as I'd like to go all electronic, I fear electrical gremlins patrolling the wiring may make reliable design difficult.

http://www.alfabb.com/bb/forums/spider-1966-up/35224-reposting-missing-pictures.html

BTW, a response from one of your homies here on the Forum appears to have vanished, as well as my reply ??.

May be able to locate the heads-up via Yahoo! in my Trash and see what happened.

Best wishes and let me know what you think.

 

[stevewaclo]

Ooops! My bad.

I need to get a score card or avoid maintaining conversations on multiple sites.

Suspect you young'uns multi-task regularly, but too much for me :-(.

Here's what I was looking for:

http://arduino.cc/forum/index.php?topic=146575.new;topicseen

Best wishes.

 

[MikeHalloran]

A message that is solely hobby oriented and generates no useful engineering discussion may be Red Flagged by any member, which often results in its removal by forum management.

The problem you are trying to solve seems like a nice engineering challenge to me, but I am not the final arbiter of its fate.



Mike Halloran
Pembroke Pines, FL, USA

 

[stevewaclo]

Mike,

Following up on your suggestion to use part of the Ford VV Carb as a replacement actuator for my VSD, I found this site and suspect the potentially useful part is the unit shown in the second drawing here:

http://www.geocities.ws/julianvz/Sierra/Carbs/ford_vv.html

Unfortunately, I'm stuck at that point and unable to source the part or even it's name.

Help?

Best wishes.

 

[MikeHalloran]

The VVC on my 5.0l V8 had two barrels and was different in detail, but the part of interest to us would be the "Carburettor Control Diaphragm" in the illustration you supplied. ... which doesn't make perfectly clear the linkage between the diaphragm and the air valve, but you need essentially none of that to move a core in a coil.

You do need some kind of housing to clamp the diaphragm's periphery, a straight rod from the diaphragm to the core, probably some kind of perforated diaphragm to support the core radially, and a spring inside the diaphragm to scale the vacuum signal to core displacement.

It will probably be a custom spring, of the kind you can make yourself by winding some music wire over a dowel with your bare hands.






Mike Halloran
Pembroke Pines, FL, USA

 

[stevewaclo]

Mike,

Thanks again for your response and I'll keep trying to dodge the referenced "Red Flag".

"...but the part of interest to us..."

Does that mean you have become trapped in my web of intrigue?

The "carburetor control diaphram" you refer to in the drawing I sent appears to be the name of the internal componant of whatever the assembly is on the right of the illustration, or is that the proper nomenclature for that entire part of the carb?

I'm optimistic you're familiarity with VV carbs (or is that an unpleasant part of your life you would prefer to forget ), will help me track down specifics on the mystery assembly. I'm drawing blanks with Google. Would very much like to get some "hands on". Perhaps a print out and off to NAPA?

Best wishes.

 

[MikeHalloran]

You don't need a working carburetor. I'd start at a junkyard.


Mike Halloran
Pembroke Pines, FL, USA

 

[kennewton]

Hello Steve and others.

Taking an electronic approach to this problem should not be that bad. For starters two primary parameters have to be determined;

- Characterize the signal driving the VSD, and the VSD output signal to the timing computer over the pressure range, this could be some type of phase or frequency signal. Some understanding of the computer circuits associated with the VSD maybe necessary.

- Select a common automotive manifold pressure (MAP) sensor, and determine its supply and output signal versus pressure. An analog signal maybe preferable.

Once these signals are determined, they can be interfaced to a small microprocessor. After all the VSD does interface with a microprocessor. The enclosure, electronic circuit, and software is fairly standard stuff, been there. The electronic interface would not have to be that complex, or pass environment tests. Designing for EMI and ESD is not that difficult if the basics are known.

Would be interested in discussing and possibly working on this project.


Regards,

Bob Schenk schenk.robert@newtonsystemsllc.com

 

[stevewaclo]

Hello followers and casual lurkers!

Cleaning out some old emails and thought it may be of interest to readers of this thread to know Bob Schenk's generous offer of assistance has been warmly received and our humble group's net IQ subsequently rose 15 points!

Bob hit the ground running and Alfa hardware for evaluation in his secret laboratory (unlisted phone number with unlisted phone company) should be in his eager and capable hands anon...anon and a half at the outside . Presuming Bob will survive the deluge of annoying, amateur chatter within the group, we anticipate great things of him.

Thanks again Bob!