AIM Dash with MSD ignition box

[Gary_T]

Hey guys,

I have an AIM XG Log dash, and it's getting the tach signal from my MSD 6AL ignition box.

As I rev up, the dash RPM is reading way high - when I get to about 3-4k (real engine RPM) it reverts back to reading correctly up to redline (limiting rotor in the distributor at the moment).

I believe the MSD (Multiple Spark Discharge) produces multiple sparks below 3k'ish RPM, and then reverts to a single spark beyond that. I think this is the root of my issue.

What is the answer?? An AIM filter? Throw out the MSD (just for you Daryll E.!)?

Thanks!
Gary

[TimW]

pretty sure that AIM can't accommodate a varying number of multiple pulses per revolution. A filter won't help that likely. Have you thought of measuring RPM electro-mechanically instead of electrically? Essentially with a wheel speed sensor:

See http://www.apexspeed.com/forums/show...php?t=26463#12

Tim

[Matt King]

You can divide or multiply the spark signal pulse in the AIM software. I'm not sure if that will solve your problem, but it's easy to try.

[Gary_T]

Thanks Tim, reading through that now.

Matt, I saw the ratio for the RPM signal - the trouble is it changes with rpm (x sparks per rev under 3000rpm, 1 spark per rev above), I didn't see a way to factor that in.

This may be the impetus I need to finally pull the MSD off - I do like the soft-touch rev limiter, but with the RPM shift lights now on the dash - I shouldn't be doing that anymore anyway!

Does no one use an AIM dash with an MSD? I thought they were fairly common??

Gary

[cooleyjb]

Thanks Tim, reading through that now.

Matt, I saw the ratio for the RPM signal - the trouble is it changes with rpm (x sparks per rev under 3000rpm, 1 spark per rev above), I didn't see a way to factor that in.

This may be the impetus I need to finally pull the MSD off - I do like the soft-touch rev limiter, but with the RPM shift lights now on the dash - I shouldn't be doing that anymore anyway!

Does no one use an AIM dash with an MSD? I thought they were fairly common??

Gary

All the ones I've dealt with had Crane systems on them come to think of it. Part of the reason was the shop had horrible luck with MSD from what they said and when I was there everyone was on a Crane.

[RussMcB]

If you get stuck, Ellen at Veracity Data is extremely helpful and knowledgeable.

[sracing]

I am not sure where you are picking up the RPM pulse but the MSD box has a separate plug for your rpm signal. The MSD multisparks at below about 3500 RPM,so they have a external circuit in themthat provides a true RPM signal for Tachometers, etc. It actually works better than most ignitions for data.

You can't use the divide by programming on the aim since it will then only read correct at below 3500, and a filter won't do anything.

Here is a sheet that shows the pick-up point.

http://www.lextreme.com/ignition/msd6a.pdf

[Gary_T]

Thanks Jim,

I believe I am picking up the RPM signal from the tach output on the body of the MSD. I will double check it though.

Gary

[Garey Guzman]

Just curious why it matters - you're never that low in the RPM range on track, right?

[Gary_T]

I was thinking that as well... not typically, although getting the flash of all shift lights in the middle of a slow corner might be distracting!

[sracing]

Thanks Jim,

I believe I am picking up the RPM signal from the tach output on the body of the MSD. I will double check it though.

Gary

If you are at the correct connector, then the MSD box is defective. There is a single transistor circuit that provides a good square wave at the proper RPM (not multisparked). It has no impact at all on the ignition, but wont give a proper tach signal.

[Gary_T]

Thanks Jim - I didn't see this issue with my stack tach gauge last year - perhaps I just goofed on the wiring as I installed the AIM.

I'll check after work.

Gary