Modern Tachometers

There's a guy that did an engine swap in a car. The new engine has 3 pickups on the crankshaft whereas the old engine has just 2.

New: Nissan VQ35
Old: Nissan QR25

The guy wants me to make a circuit that will convert the 3 pulse per rev signal to the 2 pulse per rev signal. I already have a board laid out that uses a micro to read the input and then it will give the appropriate output. I've come up with 3 scenarios and would like to know if any of the 3 are correct. What I don't know is if the signal is mainly a duty cycle thing, or if its a pulse count, or if it is timing between pulses.

1. Read first two pulses on input and mirror them to the output. Ignore third pulse.
2. Read pulse high time and low times. Multiply high time and low times by 1.5 (almost the same as 1, just that the pulses are evenly spaced)
3. Measure asserted time period, output asserted time period as a pulse with 2/3 of the length

If the signal comes from the pickup, it would make sense that the overall time that the signal is asserted in any given time period remains constant. The faster it spins, the shorter the asserted period but the less time until the next pulse arrives. Given this, I would assume that the tach is not driven directly from the pickup sensors but electronically, maybe based on simply the time period between pulses. If this is right, it seems that option #2 has the best chance of success.

Hang on a mo , lets see if I got this straight..

This guy swapped engine blocks but NOT the engine management system?

He's using a stand alone managment system, but for whatever reason it will not run the AC or the tach. The board I made for him is already capable of controlling the AC, but it was after I completed the prototype board that he told me about the tach. So, I wasn't able to try out a few ideas on it and I'm hoping to get it right on the first shot.

I think what I'll do is wait for two pulses, measuring the high time and the low time, have a multiplyer for the high time (set to 1.0), and output a low time of so that the low + the high is equal to 1.5 times the period of the output from the VQ35.

If I had access to a scope at his shop, this process would be much simpler. Hmmm.... I think I could use my microcontroller development kit as a low quality logic analyzer

This is all dependent on the manufacturer I suppose, but I've never seen a cluster tachometer where duty cycle matters, it's usually driven by time between rising or falling edges. I think it's fairly safe to assume this, though, I wouldn't bet my first born on it.

For number 2, you don't need to calculate high and low times. If you are using a micro, you just time between falling edges (or rising edges, doesn't matter), you should have an interrupt that triggers on these. Use the timer value to constantly update your out pulse to the cluster tach.

its an interesting project , can you tell us just exactly what hardware we are looking at ? We know the engine model , but what of this stand alone management system , after all there can only be a dozen or so types on the planet.

The particular engine managment system doesn't matter, the board I made for him doesn't interface with it. I think its a SAFC II (I thought those only did fuel???) though.

**broken link removed**

SAFC II isn't standalone engine management; it's just a piggyback that spoofs MAF/MAP signals to modify stock fuel tables.

Pretty board. Nice work.