MSefi.com read only forum (Jan.15,2018)

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.

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?

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?

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.

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.

trakkies wrote: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.

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.

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