Trying to help a friend w/FAST

[jpwalt1987]

I am trying to help a friend of mine with his BBC with a FAST system. Here is the deal, Big Block Chevy, BDS blower with a BDS fuel injection hat. He is running an MSD 7AL and MSD crank trigger. The computer is seeing RPM as 12,000rpm. We all know that is wrong but we arent real sure why. The engine is obviuosly Not at that high of an rpm. The car has been changed around abit. He had this fuel injection system on there, then a guy who claimed to be a tuner leaned it out and popped some head gaskets. Thats when the blower and injection system came off and the carb and nitrous went on. Well now he has the blower back on but now he has a crank trigger in addition to the distributer. It ran good back when he had the blower, fuel injection and just a distibuter. It is obvious that the PCM is seeing too many "rpm pulses". BUT WHY?? I know on the Buicks when you go to a dist. you have to grind off some of the reluctors in the dist. My friend says that there are no wires except the coil wire and the plug wires going to the Dist. so I wouldnt think that would be the case. Anyone have any ideas on a direction to start looking? I told him to convert back to just a Dist without the crank trigger to try to isolate the problem to the crank trigger. Thanks for any help you guys can give. Jeremy

 

[Dusty Bradford]

The FAST is seeing 12,000 rpm??

The FAST gets it's rpm signal from the crank trigger. Generally when the pick-up goes bad you get no rpm signal. I'd check the connections to make sure the purple and green wires of the pick-up are wired to the fast connector properly. Also check the grounds on this wire to make sure it's properly grounded. If all this is correct, I'd try another pick-up before sending the FAST box in.

 

[Craig Smith]

check to see that the number of cylinders is still set to 8. There's an old bug in that code that once every great while would set the number of cylinders to 0 on powerup, and the result was that scaled RPM would show as maxed out.

 

[jpwalt1987]

Craig , that sound like a very likely cause!! I have been looking at the wiring diagram for the MSD 7AL-3 and a crank trigger. It looks like all you do is unhook the dist. and plug in the crank trigger so I dont see him hooking it up wrong. What wire would go to the FAST from the 7AL3 or vice versa? I cant see what terminal it should hook to by looking at the wiring diagram. Does the crank trigger need to go to the MSD or to the FAST? I think we might be onto something here. Looking at a wiring diagram for the fast system it seems that the only wire would be the white "points "wire R3-X3 white, pin W. Would that hook to the points terminal on the MSD and still have the crank trigger going to the M+ and M- terminals? I might be over thinking this !!!! What do you guys think?

 

[HighPSI]

The crank trigger goes to the FAST and NOT to the MSD. The points wire from the FAST triggers the MSD.

 

[jpwalt1987]

Hey Cal........

Cal, I talked to my friend and he said that he had talked to you about this on the phone. I figured that since he ran the way it is wired by you it has to be right and from the way he told me it was wired, it is. He told me that the BDS wiring harness that was supplied with the kit he bought is all gray wires!!! I told him about the bug in the code that is mentioned above and he is going to check that. Looking at the wiring diagram for the FAST system I dont see where the crank trigger would be hooked to the ECM. Duane told me that it hooked to a black and red wire but I didnt see it on the wiring diagram that FAST has online. Do you know what number pinouts on the ECM the crank trigger should hook to? The points wire is PIN W. Is there any kind of "ignition divide" setting on the FAST system that would be different for a crank trigger versus a distibuter??? That was mentioned on YellowBullet.com. What do you think? Thanks. Jeremy

 

[HighPSI]

If we are talking about a Classic FAST:

The white points wire is in FAST pin X-3 and connects to the Points input of your MSD

The Crank trigger is in FAST pins A3 and B3 and connects to your crank trigger. A3 is Red and connects to crank trigger Green wire. B3 is black and connects to crank trigger Purple wire

 

[jpwalt1987]

Any idea what would happen if you had the purple and green wire wired backwards?? Do you think that it would give the crazy rpm signals we are getting? Thanks for all your help. Jeremy

 

[HighPSI]

It might. I assume you checked that the number of cylinders was correct in the fuel calc table. If you are running without a cam signal, make sure nothing is plugged into the cam plug.

 

[jpwalt1987]

Update

UPDATE:::

I talked to my friend today about his MSD/FAST problem.
He wound up rerouting the crank trigger wires away from all the other electrical stuff such as plug wires and the like. Problem solved. It was getting electrical interference from that stuff. The tach now reads correctly and there is no false rpm signal causing it to hit the rev limiter when it is not actually at redline. Thanks to all that gave input and ideas. Jeremy