would a pickup coil cause the car to miss badly and the tach to go wild....I've replaced the ignition module and cap and rotor... the rotor was burned through the small metal tab that makes contact with the cap.... help thanks Doug

 

Plug wires and spark plugs should also be replaced. A jumpy tach is typically caused by the secondary side of the ignition system.

RBob.

 

The burned rotor speaks of high secondary ignition amperage. Check plug gaps and wire condition for sure.

GD

 

thanks guys but what is or where should I look in the secondary ignition .. oh and what is a secondary ignition... these electronic jobs throw me a bit.. lol im thinking that maybe the pick up coil???

 

Nothing to do with "electronic jobs"......

Primary and Secondary ignition exists on all ignition systems - way back in the mechanical distributor days with points and capacitors.

The "Primary" side of the ignition consists of the coil control - the distributor module or the points, etc, as well as the winding of the ignition coil that is connected to the distributor (tach signal, or negative terminal on the coil), and the ignition switched power source (positive side of the coil).

The "Secondary" side of the ignition consists of the secondary winding of the ignition coil (one end of which is connected to the "tower" for the coil to distributor high-tension lead), the cap, rotor, plugs wires, and plugs. This is the "high voltage" side of the system that generates the spark. When we refer to the "Secondary" side of the ignition system we are referring to all components involved in generating and carrying the high voltage current to the spark plug including the plug itself. As misfire in these components (such as a short to ground from a cracked plug or a wire without proper insulation), can cause the tach to jump. I'm not clear on exactly what mechanism causes this behavior, but it must cause noise in the tach signal (the coil primary ground signal) that runs the tachometer.

GD

 

oh wow,,, now that's useful information.,, explained in terms I understand... thank you.... that all makes much sense.. yes I agree that plugs or wires could do this but wouldn't that be fairly even ,, in this case the miss is random as heck … and the tach goes bonkers like even all the way around... yet not all the time ,, in any of the symptoms. they are sporatic more pronounced in high rpm... have not yet changer out the pick up coil can it be tested?

 

The pickup coil and module can't really be tested without special equipment. If it's running you can do some checks with a meter and more with an oscilloscope. The problem is that the distributor pickup creates a sine wave that is then used by the module to generate a square wave for the ECM. Without the ability to generate the input, or measure the output it is not possible to test the module. This really requires a scope and known good patterns to properly test. The input from the reluctor can be measured with a scope and so can the output from the module to the ECM and these signals inspected for drop-outs, noise, etc. I have seen a distributor/module that had drop-outs in the square wave at low RPM and this caused the ECM (which looks at falling edges) to interpret the drop-outs as additional falling edge signals - the timing of these falling edges caused a mis-calculation of the RPM such that a significantly higher RPM (like 2000 RPM) than the engine was actually running at (about 400 RPM) was calculated. The ECM then changed the fueling and timing and caused a near-to-actual stall situation. This occurred whenever the RPM dropped during a change from park to drive, or drive to reverse, or coming to a stop at a light, etc. About half the time the engine would actually stall. I was only able to find this with a scope. I do not know if this was the module, coil, or the distributor reluctor - both were aftermarket and I needed the vehicle on the road so a known good used distributor was installed and the problem was corrected.

The *usual* failure mode of the module is to stop working when hot. Often working again when allowed to cool.

GD