Alfa Romeo Vacuum Sensor Device Replacement

Hello all,

You appear to be a group that welcomes a challenge!

Here's an email I recently posted elsewhere, with no response. I trust some here may rise to the challenge:

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 (and techies :-) 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 )

One other point, I have no way of knowing the characteristics of the vacuum/inductance curve of the VSD through it's range and presume linearity.

And one other, other point. Contrary to my usual practice, I did not search this Forum for previously posted 411 on this topic :-).

It sounds entirely possible to me to make a replacement sensor. You need a differential pressure sensor (these are readily available) and some electronics (probably including a microcontroller) to process the signal and simulate the variable inductance. Replacing an inductor by electronics to simulate one was a trick used in light aircraft autopilot adapters in the 1980s.

Before suitable electronics can be designed, there are a few other questions to be answered:

  1. What are the 2 ends of the coil connected to? In particular, is one end connected to ground, or to +12V, or +5V, or another fixed voltage?

  2. Does any DC flow through the coil, if so, how much?

  3. What is the approximate frequency range of the oscillator?


Thanks for making time to reply!

Your questions are very perceptive and important and, unfortunately, members of the Alfa BB Research and Development Committee have not had resources available to answer any of them :-(.

Truth be told, we have convinced ourselves the coil within the VSD is integral to an oscillator within the Ignition Computer (again, separate from the L-Jet computer, but connected by the TPS signal [1/0]), but have not absolutely verified the situation. As you observed via your three questions, simply hanging an actual (or electronically simulated) inductance may neglect other critical circuit components. I believe the relevant term is "unintended consequences". As you may imagine, incautious efforts to modify our IC may result in release of very expensive smoke from the unit, a situation no one wants.

As you have probably guessed, my efforts to reach out for a solution to our quandary included outreach to others and I have been in touch with members of another electronics Forum,

after lacking patience to await your response. Additional details are on the link and I will share your observations with them.

Additionally, I will research the IC as a tiny voice is telling me there may be other particulars available.

Once again, thanks for your input and I welcome thoughts from you and members of Arduino Forum. (BTW, you folks must have achieved some degree of notoriety as iPad spell check corrected my misspelling of Arduino-just did it again :-).

Best wishes.

Hello dc42,

Cruising my ipad this afternoon and came upon your reply to my post from February.

Here's the latest.

A retired control engineer (Bob) has been involved and now has an operating ignition ECU on his workshop bench. He has discovered that at 1 ATM, there is a 70k "messy" sine wave (3V PP) at the IECU/VSD connection, which goes toward a lower frequency as vacuum sensor pressure is reduced. He has a leaky VSD and was only able to change the frequency to about 25k. We're getting a functional unit to him next week for full testing of VSD characteristics. Bob has already designed a turbo controller for older Saabs and has a regulated 12 V source from that device.

We have also sourced a reasonably priced GM MAP sensor Bob is going to use to drive a microcontroller based inductance emulator, as you suggested. Another discovery was that the IECU has an RPM based map that functions independently of "fine tuning" from the vacuum sensor.

You mentioned aircraft autopilot adapter circuits from the 80's and I have not been successful searching for additional information. Do you have any source material you can share?


Hi Steve,

We really need a diagram showing what both ends of the VSD are connected to. According to they both go to the IECU. So we need to trace the circuitry inside the ECU that connects to the vacuum sensor.

Designing electronics to simulate an inductor is not difficult provided that one end is grounded or decoupled to ground in the application, but unlike a real inductor it will be sensitive to polarity and will pass current in one direction only. A bias current could be added to overcome this. I can think of a couple of alternative solutions:

  • Start with a 2.5mH inductor (which will pass any DC component of the current), then add negative inductance electronically in parallel with it

  • Do a voltage-to-frequency conversion in the microcontroller, and drive the ignition computer at that frequency via a low-impedance buffer.

But I don't think we can get much further without knowing more about what is inside the IECU.


Thanks for your words of wisdom!

I'll pass them along to our control engineer, who I'm confident is familiar with the challenges you have outlined but will nonetheless welcome your thoughts.

Bob has spent considerable time exploring the IECU with an oscilloscope. All efforts to source schematics or patent information have been unsuccessful, so he's doing it the same way that has made the Chinese so successful...reverse engineering :-).

Thanks again, and I'll let you know how things develop.