I have what seems to be an ignition problem causing massive misfiring, generally above 3K RPM under moderate to hard acceleration. When it happens, the tach jumps around wildly, sometimes goes from 3 to 6 or if it happens at 4500 it will drop to 2000 then bounce back up over 6K etc....

Recent repairs that were necessary new cap, rotor, distributor drive gear. I have an MSD box but it is out of the loop until I figure this out.
Unnecessary, new distributor that came with module, capacitor and pick-up.



As far as I can see there are 3 paths to ground for the coil primary control circuit. I feel the problem lies here because of the way the tach reacts in conjuction with the miss.

We have the noise capacitor (new), the tach circuit, and the circuit controlled by "electronic switch B" (new). The EST output and the Distributor Reference output from the ECM both hang at 1.4V at idle and climb steadily with RPM. They do not drop out or spike when the problem occurs. Am I missing something in the operation of the circuit here?

I will test out the tach circuit next, not sure how that could effect things other than having an intermittent short, perhaps something in the filter. I don't know how the filter works, what the signal going in is supposed to look like, or coming out. I will also double check grounds for the ECM.

How does the tach signal work? If I put a DVOM in the circuit, what should I see?

I'm trying to get as many ideas as possible. My time to work on the car is limited so I need some help brainstorming, then I can come up with a plan of attack.

Sorry so long, just trying to give a clear picture.... of mud

 

Here's a bump for ya.

 

Did the car run ok before? I realize you had a dist gear going south, but did it exhibit the same syptoms? If it ran ok, I would swap out the module for the old one and bypass the filter for the tach. Just cause something is new doesn't mean it's good. It just means no one else has used it, especially electronics. The tach filter will make your tach go backwards with the MSD, so when you accelerate hard, the numbers head back to zero. A lot of the factory tachs in GM cars do this and removing the filter makes things normal. If the new distributor came with a new module, swap out your old parts, you have nothing to lose.

 

The tach signal is a pulse and your DVM can't respond to the short duration pulses. You need an ignition analyzer or an oscilloscope to find out what is going on. Misfire under load is usually due to insufficient spark. First, at night when it is dark, look at your idling engine for sparks along each plug wire. Plug wires are held away from grounded metal by the wire guides, so check that your plug wires are not resting on any metal. Next, measure each wire for resistance, they should be about 5000 ohms per foot. Wiggle the ends and see if the ohmeter reading stays constant. Check your center graphite button on the cap. MSD spark systems burn out the center button due to the larger current pulses. MSD uses a copper button.

 

Should have mentioned this before...the MSD has not been run on the car for more than a few minutes and the problem started long before it.

You know, maybe I am reading too much into this. I will go back to basics and try out the things you mention here. I've been caught up in what the tach is doing.