Was working, now no RPM on Sim or injector output

[kccampro]

I was hoping to actually get this car out of the driveway before running into a problem with my ECU but for a yet unknown reason my injectors stop firing. I put the ECU back on the Sim and there's not RPM signal and neither injector LED is responding. So, I assume I should start with diagnosing the RPM issue. I'm trying to retrace my steps in the assembly guide and work through the schematics but this is probably the area were I am the weakest. I tried searching other threads but didn't come up with an answer, or even a starting point. Any insight on where to look first is GREATLY appreciated. Thanks.

[trakkies]

The injectors won't inject with no tach signal, so the first thing is to find out why that isn't working. What tach input configuration are you using?

[kccampro]

Well, I was wired for a hall sensor / optical sensor per the instructions shown on The DYI autotune page for running an HEI as shown below:

Remove D1 and D2, if they are installed. Replace them with jumpers.
Remove the jumper between XG1 and XG2 and any jumper leading to TachSelect, if fitted.
Run a jumper wire from TachSelect to XG1.
Run a jumper wire from OptoIn to one of the two +5V holes in the Proto Area.
Remove C30.
Replace R12 with a 1.3k resistor. You may substitute a resistor from between 1 to 2 k.
Jumper OPTOOUT to TSEL.

HOWEVER, I'm wondering if I've cooked something because I inadvertently overlooked the fact that I'm first getting the car running with a coil and mechanical distributor before converting to the HEI. Essentially, I converted my tach input circuitrty to the HEI set up when i assembled the ECU several months ago but got the car started using the negative side of a traditional, old school distributor. In re-reading the assembly instruction I see that the "the pulses pulled from the negative side of the distributor can spike to very high voltages"'. Could I have cooked something by wiring the tach input for an HEI but using a traditional coil by mistake?

[kccampro]

Also, I reverted the jumpers back to the initial setup for a non HEI distributor (XG1 to XG2) etc. still no luck. I think I cooked something.

[trakkies]

Could well be U3 is fried. It's basically a special type of LED which is driven from the tach signal and that operates a photo transistor all in the U3 IC.

There are loads of different ways to mod the opto circuit for the same input out there. I tend to stick to the MegaManual ones unless I actually have problems.

The 4N25 opto isolator (U3) isn't expensive - about a dollar. If you decide to change it, cut each pin with fine cutters as far up from the board as you can. Once it's removed, use a hot iron on the reverse side of the board to melt the solder then pull out the remains of the pin with small pliers. Clean things up with solder braid.

[kccampro]

Could well be U3 is fried. It's basically a special type of LED which is driven from the tach signal and that operates a photo transistor all in the U3 IC

That's what I was thinking also. I've ordered a new one and the holder so if I have to swap it out again I can justly it put without having to unsolder the pins. I'm also going to check the other components in the circuit to see if anything looks odd.

[trakkies]

It's usually age that requires attention to injectors - they get gummed up with the 'varnish' in petrol. I'd not expect this to happen to newish ones - provided they have the correct filter feeding them. They could of course fail totally, or have a partial short in the winding, in which case a resistance check should show this. Compare to a known good one.

[kccampro]

Yup, blew U3. Replaced it and we are back in business