No injector pulse

[77Datz]

I went through the EFI Bible, did everything it said and checked all the connections on the 35 pin ECU connector, everything checked out fine, i've checked the fusable links, relays, fuses, wiring, cleaned/replaced whatever corroded connections I can, checked the dropping resistors, replaced starter, new battery terminals, checked the airflow meter, cleaned fuel pump contacts and checked fuel pressure (43psi while trying to start, 39 afterwards) the issue is I can't hear the fuel injectors firing when I'm trying to start it, I've had a friend try to start it while i hold a screwdriver to the injectors, no click, no sound, no nothing. So I think I've narrowed it down to dead fuel injectors, but they click when voltage is applied to them via a AA battery, so the injectors work, they just aren't getting a signal to pulse and spray fuel. I'm thinking its a dead ECU but I don't know for sure, is there a way to check the ECU? or am I better off getting another one off ebay and seeing if it changes anything?

[FastWoman]

Your ECU might not be getting triggering pulses from the ignition. There should be connectivity between the negative terminal of your coil and the #1 pin of your ECU. Without this input, the ECU will not know that the engine is turning and will therefore not fire the injectors. You should get one injector pulse per every three sparks (i.e. one pulse per rotation of the engine).

FAIW, a miniature Christmas tree light plugged into an injector connector is a great way to check whether your injectors are getting current. The light will flash every time the ECU delivers a pulse. The flashes will be longer/brighter if you unplug the coolant temp sensor, which will create an extreme rich condition.

[77Datz]

I checked for continuity at the negative terminal and the #1 pin, and it checked out fine. I checked the voltage at the injector connector and ground, it read the battery voltage. Then, I manually grounded out an injector by connecting a wire to the #15 pin and the cars frame and the injector clicked, so the injector works, the wiring to the injectors is fine, its just not getting a pulse from the computer. I'm 95% sure at this point that I need a new ECU.

[Zed Head]

Here's something that I've seen on my car. I don't completely understand why but I've seen it on two cars and verified it on my 76 (the other was 78 so you're bracketed). If the tachometer is disconnected the ECU won't ground the injectors correctly. The circuit from the negative post is branched, so you'll still see continuity but it just won't work. I assume that the ECU sees two much current or voltage without the tachometer drawing things down.

For anyone that wants to verify it, easily, find the lump in the wiring harness under the dash over by the fuse box. That is the resistor that is in-line with the tachometer. Unpeel the tape, remove it, and try to start the engine.

I actually knocked mine partially loose while installing a relay, drove in to town, parked for a while and had a no-start when I came back. I remembered that I had just worked in that area, looked on the passenger side floor and the resistor was laying there. Put it back in and it started right up.

[SteveJ]

Actually I was just helping someone the other day with his injectors on his 77, and his final issue was that the tach wasn't grounded properly.

[77Datz]

Well, I think I found the issue, I took apart my computer to see what the components on the inside look like, and this is what I found, a fried power resistor. I have a new computer on order but I may try replacing the resistor and keeping the original computer.

[Captain Obvious]

That resistor is supposed to be a 15 Ohm wirewound. Before you yank it, measure it's resistance. Just because it got hot, doesn't mean that it's burned open. Might be, but check first.

You can measure it in circuit. Just probe right across it. In other words, you don't have to take it out to measure it.

[77Datz]

I measured it while it was still in the board and it read nothing, so just to make sure i un soldered it and checked it again off the board, it's toasted. I have a new computer on the way, but im going to get another resistor and try and fix this computer too.

[Captain Obvious]

Haha! Yeah, It's dead Jim.

As for replacing it... I don't have high hopes for that. I believe that resistor is part of a protection circuit indented to prevent damage to other parts of the ECU. If that resistor burned open, I bet other parts went as well. Worth a try, but I'm not holding my breath. Might be a good idea to put a couple amp fuse (2A) in series with the FI power coming off the battery.

With the injectors not firing, the ECU draws less than 2A, but the stock fusible links will pass much more than that.

[rcb280z]

Makes me think I should replace mine as a preventive measure. Let us know if the repair works and where you obtained the power resistor. Could probably find it at Radio Shack?

[77Datz]

I dont know if radio shack would have the exact one i'm looking for, I found it on ebay so I'll probably order from there, I'll let you guys know how it works out, i'll repair my computer first and see if it changes anything, then i'll try the new one thats coming soon too. I'd like to have a working spare in case anything ever happens to it again.

[SteveJ]

Here are a couple of sources for items like power resistors and transisitors:

DigiKey Electronics - Electronic Components Distributor

Mouser Electronics - Electronic Components Distributor