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blwilliams
07-21-2003, 05:24 PM
I'm running a 7AL with a 2 step on my dragster. I started having problems with what I thought was the two step in that it was limiting my low side RPM by a lot more then the module was rated. At first I replaced the module. This seemed to work for a while. Then I started getting the low side RPM problem again. I replaced the two step and the problem again went away.
Last Saturday, the problem returned. I had a 6000 module in the low side of the two step. My ear and the playback tach registered 5300 rpm. I tried a 6200 module. The rpm limit on the low side registered 5350. I installed the module directly into the 7AL. Did another rpm check, it first stuttered at 5300 then went up to 6200.
I made a pass, played the tach back and it showed a 6200 launch rpm.
Next round I put the car on the mat, I can tell by sound it is low rpm again. Played back the run, again it has left at 5300.
I once again installed the module directly into the 7AL and tried a low side converter test. With the 6000 module installed directly into the 7AL the engine would stumble at 5300. With the module removed completely the converter would stall at 6300.

My question is: Is it possible for the rpm limiter to go bad and or intermittent, yet the car runs well when not withing the rpm range of the limiter module? And if so is this problem repairable?

Thanks

Bruce Williams

msdtech2
07-22-2003, 09:21 AM
There are a couple of things that could cause this problem, first check that the MSD unit has a good ground. Then check the battery, if you are not running an alternator, make sure that the battery is at full charge, normally when a battery is under 12 volts, tach reading will read higher than normal and once the battery is low it can cause early rpm limiting, especially if you have other components running off the battery. After a run, check the battery to see if voltage is above 12 volts, if it is low it may be signs of a battery going bad. Also, if you are triggering through magnetic pick up, make sure that mag wires are routed away from any other power wires or plug wires, route them by them self if possible or use a shielded cable, PN 8862. This will insure that noise is not causing interference.
Thanks,
msdtech2

blwilliams
07-22-2003, 02:48 PM
I have done all that you have mentioned in your post. My battery is sufficient. plus I run an alternator.
I run a ground (#8) from the battery to the engine block, from the block to the MSD which sits above my legs with the rest of my electronics. My distributor pickup leads are run down the opposite side of the chassis from the rest of the wiring.
I run a mechanical fuel pump for the fuel injection. I believe the MSD is the largest amp draw in the car.
I have already sent the Biondo Mega 400 box to have the tech checked. They say it is OK.
I'm talking about 800 RPM not just a little bit of rpm loss or gain.

What's next?

Bruce Williams

msdtech2
07-22-2003, 03:32 PM
Other items that can cause the RPM to limit or simulate RPM limiting are valve's floating, converter, switches or relays that can vibrate causing to limit. If these check ok, and still suspect the MSD Unit, please send to us for testing.
Thanks,
msdtech2

blwilliams
07-22-2003, 03:43 PM
What I'm trying to say, is with out a rpm module installed in the MSD unit the engine runs fine. When you install one it limits the RPM at an incorrect RPM. The amount that is limited is around 800 RPM. This is enough that it is easly detected by ear as well as by watching the tach.

Thanks for your advice
Bruce Williams

msdtech2
07-22-2003, 04:03 PM
Then I would recommend to send in for testing.
Thanks,
msdtech2