Purpose of dist pickup going to ECU...

Hi-

I'd like to know why the pickup coil in the dist runs to the ECU.

Does the computer vary timing, or is fuel mapping based on RPM, and this is based on the pickup's input to the computer?

Thanks

xj_convert

The computer does vary the ignition timing but it uses the crankshaft position sensor for that. The sensor in the distributor (usually but not always referred to as the camshaft position sensor) times the injector impulses.

Just to add to the previous thread, If you think about it, the distributor rotates at half the speed of the crankshaft. You've no doubt heard about getting the distributor in 180 degrees out, well the sensor inside the distributor is the reason that is a problem. It syncronizes the spark to the position of the cam on the compression stroke.

Are you guys saying "sensor" figuratively or literally?

IOW, there is a sensor in a 4.0 (91) distributor other than the stator and reluctor (the pickup set)?

Thanks xj_convert

Yes, the sensor in the distributor is called the "camshaft position sensor" it is also called the "sync sensor". It is a Hall effect sensor which tells the engine computer if cylinders 1-5-3 or 6-2-4 get fuel this turn of the crank. It is an important sensor.

I have read posts where guys have unplugged it and their XJ still ran. The computer and work around if the sensor is not working. If for some reason you have it disconnected you could get 180 degrees out and not run.

Hope that helps.

xj_convert said:

Are you guys saying "sensor" figuratively or literally?

IOW, there is a sensor in a 4.0 (91) distributor other than the stator and reluctor (the pickup set)?

Thanks xj_convert

Click to expand...

What on most distributors would be an ignition-related pickup set is the cam position sensor used for injection. There is no ignition timing performed here - that's all done at the flywheel and the ECU.

The purpose of asking these questions is to find out what I have to do to convert the 4.0 to HEI. I have tons of experience with vintage musclecar (big block with carb application), but only know how the ECU/MPFI works in theory- particular apps, and the way things are actually accomplished I'm still learning about.

For example, in the case of the carb'd 258 with ECU, it appears that folks have bypassed the ECU, and let the timing be dictated by the advance in the distributor.

Later HEIs have some convienient modules and wiring harnesses to allow me to split off the pickup coil (provided it's in the same range as the 4.0) and send that to the ECU.

...but it sounds like I'd have to address the "cam sensor" in the 4.0 distributor. Not a show stopper, but now I have an idea what I'd be up against.

Thanks

xj_convert