PWM outputs on Haltech ECU

Can some one explain PWM outputs.

I am repinning the ECU harness to fit a new Haltech (Elite 1500) ECU. The old (Haltech E6K) ECU has two PWM outputs, and according to the very basic car wiring diagram one of these outputs is shown to be going to the coil +

I have a GM HEI dissy with the coil in the lid, which uses a 7-pin GM ignition module. The 12v power to the HEI is via ignition switching to a connector on the HEI lid (BAT).

I don't understand what this existing PWM output is being used for, as I thought PWM outputs close to ground. The wire that connects to this output which is erroneously labelled Coil+ appears not to be grounded nor does it have any current output. I am at a loss to understand what this (pink/black 20 AWG) wire is.

That will be the ignition bypass or override. The ecu grounds this pin during cranking (say below 400RPM) which makes the HEI module control the timing at a fixed 10deg BTDC instead of the ecu controlling timing. Since there is only 1 tooth per TDC, when in bypass mode the HEI just fires a spark at the instant it sees a tooth rather than trying to predict engine position for controlled advance. This makes the timing more stable when the cranking RPM is unstable. Once engine is running this pin needs to be pulled high (~5V or more) so the HEI then switches back to ecu controlled spark advance.

I expect you will still find an "ignition bypass" function in your Elite to perform the same function, otherwise you could do it with some sort of GP output or RPM switch type function.

Thanks Adam.

I have located the ignition by-pass and Haltech suggested this wires to a 5v sensor power supply so that the ECU is in charge of timing even at cranking. So, I am still left unsure what the wire to the old ECU is connected to the PWM output, but I guess this may go to one of the DPO ‘s to pull to ground when wiring the new ECU

Can you attach this diagram? There's zero reason to connect any output of an ECU to the coil positive...

So, I am still left unsure what the wire to the old ECU is connected to the PWM output

It is called a PWM output but it is not PWM, it can do either PWM or just on/off as it would be for this function. I dont know which Haltech outputs have pull-ups etc so the choice of output you use may or maynot be important on the new ecu.

Haltech suggested this wires to a 5v sensor power supply so that the ECU is in charge of timing even at cranking

I highly recommend you keep the bypass function operational, the reason it is there is because with only 8 teeth in the distributor your ecu effectively only gets an update on engine position/speed once every 90 degrees. For the ECU to "guess" engine position it has to use the timing between the two teeth that occurred up to 180 deg in the past - if for example you had 1 cylinder fire in the last 180deg then the crank speed is now going to be much different than it was 180deg ago and your timing is going to be a long way out. This will result in difficulty starting sometimes in the best case or a big kickback breaking the starter motor in the worst case (I have even heard of a bent conrod from kickback before).