A/F Meter Controller lacks RPM information
 

Problem: I'm rying to record A/F ratio data and rpm for review on a computer. The A/F data is fine but I can’t ‘see’ the rpm data. (the stock tach works fine.)

My ignition uses MSD 6AL-2, Blaster 2 coil, Crane optical points with XR-700 box and a ballast resistor. An MSD tach adapter 8920 got the stock tach to function. All of this has worked well for years.

To this I added a Zeitronix A/F meter which is supposed to record data from the Bosch exhaust sensor and the Porsche’s tach output. The data is then downloaded to software which displays it. The A/F data is coming in loud and clear, but I can’t see the rpm information.

Zeitronix suggested I check the wiring and connections; all are fine.

Aseem in Oslo (http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/564110-zeitronix-connection-rpm-map-2.html#post782000) had the same problem and eventually solved it by splicing into the tach lead at the tachometer. I’ve done that; it hasn’t helped my problem.

It’s my understanding that running a lead from the low voltage side of the coil to the Zeitronix isn’t an option; it will fry the A/F controller. Aseem found this out; most A/F meter instructions say don’t do it with CD ignition.

I’m suspecting the Crane optical points, the related XR-700 box, and/or the ballast resistor. Can someone who knows way more about electronics than I do (there are a lot of you) confirm or rule out any of these out as a culprit, or offer any other advice?

Add on: After this message I bypassed the MSD tach adapter. THis got me an rpm signal, or at least the carrier, but no response as rpm changes. So it sounds like the Zeitronix is getting a weak or incorrect signal.

I have a Daytona Sensors AFR instrument that records AFR, RPM, and MAP on my 911 1973.

It will record 2 hours of data that I can read out on my laptop. I can even make Excel tables of the data.

I have had it in my car for 4 or so years.....great for tuning the MFI.

By the way you can buy this unit from Pelican.

Take the RPM signal from the 1/4" male faston on the end of the 6AL. Problem solved.

Thanks. I looked at the Daytona instrument but prefer the Zeitronix setup or at least I will when I get it running. Tried to get the signal directly from the 6AL tach wire directly. I'm getting noise, or a carrier, but no modulated rpm signal.

It's impossible to have a carrier but no signal on the tach line from the MSD 6AL box- it's a +12V square wave with a 25% duty cycle. As such, it goes from ground, to +12v, to ground. Of course this is the same waveform as a set of points, albeit without the longer dwell period (63%). Carrier would be ground.

OK, so you are using the Crane XR700 to drive the optical pickup. Of course the XR has the ability to switch a low-resistance coil directly, you don't need the MSD, and you will probably have better ignition without the MSD.

The optical trigger itself has a "points output" or square wave so you could drive the MSD directly off that without using the XR700 box. see page 4

http://www.cranecams.com/uploads/instructions/9000-2006_.pdf

But that is another story. So how do you have it wired, the output from the XR700 triggers the white wire on the MSD?

What is the stock tach wired to, the 8920?

What is the 8920 wired to?

Understanding all this will help figure out where the tach signal is dropping out.

Many thanks for your interest.
After volts and amps and ohms all this electrical stuff is pretty much nuclear physics to me.

Here's the stetup, wire by wire to help my feeble brain:
Optical trigger has 3 leads with molex connector which go to the XR700 molex plug. Crane literature shows a switched red lead, a black ground, and a white to an MSD or in my case (the yellow lead on a Crane XR700.)

Crane XR700. Red lead goes to switched hot line; black lead is grounded.
XR700 gets the molex lead above. The yellow Crane lead goes to the white lead on the MSD 6AL (per Crane, this yellow lead is used for, among other things, input from a points amplifier like the XR700.)
There's a ballast resistor on the red lead between the hot switched source and the Crane.

MSD 6AL. Heavy red to battery plus; heavy black to battery minus.
A small red goes to a switched hot line.
Has pickup from the distributor,
Orange to the coil positive, a small black to coil minus.
Grey output to the connector on way to a white lead thru 8920 to the tach.

8920 Tach adapter. Red to switched hot line; black to ground.
White line is input from the grey MSD tach output.
Violet is output to the tach.

So if I'm not seeing a carrier but no signal what am I seeing? There's no doubt I'm seeing something; a line/trace comes and goes when I bypass the 8920.

JIm

Jim,

Here is what I would do if I were you.

Optical trigger has 3 leads
Red (Switched +12V)
White (points output)
Black (Ground)

Remove the XR700 from the system. This is a transistorized electronic ignition module that is intended to drive the coil directly. Since you aren't using it for that purpose, it just adds complexity, noise and weight.

Connect red from optical trigger to switched +12v small red for MSD
connect black from trigger to ground
connect white from trigger to white points input on MSD

remove 8920 adapter. connect a wire to the male faston on the side of the MSD directly to the tach and to the rpm signal input for the AFR gauge. (I have actually done this using a factory tach which was converted by NH speedo to take the MSD output and an LM-1, so I know it works)

***

back to your setup

the yellow lead on the crane goes to the coil negative. the other lead is red, this is switched +12v. The way the crane triggers the spark is by switching the yellow in a square wave. You're connecting this square wave (with its own, probably variable, dwell) to the points trigger input of the MSD (white wire).

I don't really like this setup because of the possibility of a mismatch in the dwell periods of the XR box with the MSD input.

Here is why: unlike a CDI which charges a capacitor and then dumps it into a low-impedance transformer (our shortie Bosch 001 coil or the HVB blaster you have), inductive systems like the XR700 charge the coil, then break the connection to the charging current which triggers the spark. As such, there is a dwell period that they use.

What's going on with the optically triggered XR is, the finned box switches battery current into the coil, then when the shutter passes the IR LED on the trigger, the box switches off. This happens for different lengths of time based on engine RPM, and the engineers probably designed a feature to stop the coil charging at low RPM so it doesn't burn up. As such, the dwell is variable.

The white input line on the MSD expects a fixed dwell period and a trigger on the falling edge of the square wave-- e.g. points close, voltage at points wire goes to +12v, points open {WHAM coil fires} points close and charging starts again.

Next: what is your output to the 8920? Is the grey wire your tach output? My 6AL-2 has a grey wire for the tach output, does your 6AL have the same? On my 6al (not the -2) there was a faston terminal.

The 8920 is an amplifier. It listens on the white line and sends out a higher power signal on the violet. Pretty simple, but probably not necessary.

Try it out with just the crane optical trigger and the MSD per the diagram on page 4 and see if that doesn't solve all your problems- it will eliminate a lot of complexity and you will have spare parts left over.

It's deja vu all over again!
My current setup has been working well for maybe 8-9 years and I wouldn't mess with it if I didn't want to get the A/F meter running.
When I first put this stuff in I couldn't make the Crane optical trigger work with the MSD. It's all lost in time now, but I had a really foul running car... sputtering, no power. At that point someone here said I needed the XR700 box. I asked Crane. One tech said I did. Two days later another said I didn't! I put one in, the car ran fine, and I've stuck with it ever since.
So I approach throwing all that away with trepidation, while knowing that a pound less electronic circuitry and its associated additional complexity would be a good thing.
Before I take that step, I wonder if there is any possibility I'm dealing with a grounding problem? Any point to making sure all the relevant grounds are combined? Someone here in an earlier thread thought that might be helpful.
Thanks for the explanation and taking the time.
Just noticed your question about the MSD tach output. Mine is grey (on an AL-2 like yours.)
JIm

Good luck getting this working. Not sure the Daytona would work right out of the box.....I just like mine.

The MSD setup you have sounds complex.

Hope it all works soon.

Ok 304065. I've pulled the XR700 to get the optical sensor wired directly to the MSD. Car won't start. Have been over the wiring a couple of times and it seems ok. A timing light detects no signal in plug wires while cranking. The MSD and the coil check out fine. There is 12v current at the optical sensor and the msd. No ballast resistor. I'm now betting I fried the LED sensor at some point along the way. It's about the only culprit left. Not sure how that happened with battery disconnected but it's possible. Anything quirky to be aware of?

Well this has taken longer than planned but we are finally operational.
I replaced the Crane optical trigger and related Crane xr-700 with a Pertronix. Both pieces were more than 12 years old and I was beginning to wonder when they might fail. I also pulled the 8920 tach adapter which had been in the system. All this as suggested above. At that point the car ran and the tach was good, but I still lacked an rpm output from the Zeitronix. On their advice, I ran a ground from the Zeitronix (black power ground) to the point at which the MSD box was grounded. Instant success!
I think it was worth the trouble. One drive around the block showed me that the PMO accelerator pump is dumping way more gas into the engine when suddenly floored at 2-3k rpm than it needs. The air/fuel ratio goes to about 10.5 under these circumstances, way rich and the reason for an obvious stumble at that point.

Good to hear that it is working. Good going and I am sure it feels great to hear it running again.

I guess the only way to test the output of the Crane is to hook up scope that will show you the square wave signal output. Or maybe a meter that reads frequency. I have a meter that reads frequency on order and want to test my Pertronix someday to see what it is putting out.

I am going to start testing my Daytona Sensors CD-1 this week. My Bosch 3 wire box is still OK, but I would like to do the advance and rev limiting electronically and not use the distributor advance. I plan to post my progress and installation.

Enjoy your carb tuning with the AFR......an AFR instrument sure helped me sort out my MFI!

JHtaylor, glad to hear you got it working. Double check the polarity of the optical trigger, of course it's an LED, they only work one way.

Dick,

Even the factory used an advance mechanism to prevent cross-cylinder firing, all the way up to the 993. While you can certainly control the advance with electronics (the factory did, starting in 1984) you still need the phasing mechanism. Search here for "rotor phasing"

ROTOR PHASING.......very interesting discussion thanks for the tip.

I read many of the posts and I now get it. And I spoke to Chris Shroeder the designer of the CD-1 at Daytona-Sensors, and he suggested sticking with the mechanical advance if all is working well.

If I do want to run and advance curve in the CD-1 then I would be best to go to crank fire. That way the distributor is only doing 1 job and that is pointing to the right cylinder. All I would need is a toothed wheel and CPS, and mounting.

The CD-1 has a rev limiter, so I will use that and maybe go to a non-advance rotor. The other nice thing I like about the CD-1 is that it has data logging, so I can log RPM, timing, and MAP.

thanks for the information. Very interesting.