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740 No Pulse to Injectors, Has Spark

Yeah no kidding. Slap some dielectric grease in there to prevent future oxidation and to promote a good connection. There was a bulletin to slap grease in the ecu connector on these cars.


Like I said in your other thread, these mid 80s 740s have the biodegradable wiring. It may have shot wiring under the intake or at the firewall.

Have you checked continuity between the injector ground wire terminal and the ecu!? Do so and wiggle test the engine wiring harness and see if the resistance goes open or high.


One MORE test....use a test light between injector ground and a known good ground. Make sure is does NOT light up=short to voltage. Do it with key on.
 
Tried ether just now... didn't start, but there was definitely ignition... I probably just used too much. All I got was a pop, which nearly ignited what dripped into the intercooler hose. Crisis averted. Tried again with the same result. If I cranked longer, it probably would have started but I didn't want to take any chances.
Tried the Y jumper relay bypass, no difference at all.

The male terminals from the ballast resistor are good and clean, that's where I tested the resistance because I didn't want to pull the spades off the resistor pack. That was the issue with the previous resistor I had on the car and I was unable to fix it at the time.

I have already checked continuity between the ECU connector and the injector connectors, all good. I moved around the harness and there was no change, so we're good there.

I will try the test light in a little bit.
 
Just because some voltage is getting to the relay doesn?t mean sufficient current is being supplied to the injectors

Jumping the fuel pump relay doesn?t work on a 7/9 because it also provides power to the RSR. So your pumps will cycle but injectors won?t fire. At least on the ones I?ve driven.

This takes a 2 pronged approach. Power needs to be jumpered to both the pumps and the rsr.
 
If it doesn’t start on ether you have more than just a fuel problem. It (probably) won’t blow up in your face..I’d see if you can get it to somewhat idle on ether. But that’s just me
 
Okay... pardon my total stupidity... don't know how I didn't catch this sooner.
When I tested the injector ground with the test light, it DID light up. I quickly realized what I completely missed before... there is power on both injector terminals. That led me to once again test resistance from the injector connectors to ECU pin 13. 0 ohms on ground, and anywhere between 9 and 15 on power side. Didn't seem quite right. Then I realized that I have 12 volts on pin 13... and I'm pretty damn certain there isn't supposed to be any voltage there! Maybe I didn't fry the ECU, but I'm betting I did.

Anyway, I'll be pulling the wiring apart under the intake manifold to look for where the wires are making contact... because that must be where it is. ZVOLV has only mentioned it a few times...

In the meantime, I appreciate everybody's help and for sticking with me through this!

What's the best way to pull that harness out? Just pull it forward? It doesn't seem to have much flexibility without removing the manifold.
 
I gotta look at a diagram, but the i think the ecu powers up the RSR, therefore, one could power up the RSR circuit manually and see if there is still a short to voltage on that injector ground circuit.

You gotta figure out if the short is internal to the ECU, or in the wiring.
 
YES

NO the ecu powers up the FPR which powers the RSR.



That's what I believe to be the case also. That's why I changed my test recommendation to what I just said in post 49.

Unplug the ECU and then power up the injector positive side of the circuit with one of several methods such as a jumper wire, a power probe, or my preferred method: open the relay and pinch the contacts closed.
 
Well, I tested the resistance to the injector positive wire with the ECU connector removed, so I assume it's wiring-related.
I opened up the harness and found the covering on the part where the injector ground wires combine was split and the bare wires were exposed. That doesn't explain why they were getting power, though, since none of the other wires were damaged. However at that point, there was no continuity between the injector positive and ECU pin 13.
Tomorrow (if it isn't raining AGAIN) I'll pull apart each individual injector harness and make sure there isn't any damage.
 
Pinched the fuel injector relay and I have voltage with the ECU disconnected.
After putting the harness back, I still have ~9 ohms on the injector control to ECU pin 13. I realized after disconnecting ALL of the injectors, it went to open circuit. Once I plug in one injector, it goes to 13-14 ohms, and decreases as I continue to plug in each injector.

I tested resistance on the cylinder 1 injector (I'll test the others once I have some specs) and it showed ~2.5 ohms. What kind of reading am I supposed to see?
It seems that maybe my injectors are the reason there is power at both terminals.
 
Okay, so there isn’t a problem! I was just driving myself nuts trying to figure that one out.
So maybe it’s just my ECU? I don’t know what else it could be.
 
Well you also said it doesn?t run off of starting fluid right? So there?s more than one issue at stake here.

That may be because of the ignition timing. I haven't adjusted it since replacing the distributor, for obvious reasons. I'm really not concerned too much with that right now... I'm concerned about the lack of injector pulse.

Now... I'm just realizing, shouldn't the injectors have less than 12 volts because of the resistor?
 
Bypassed the RSR, no changes.
I?m really thinking I just have two dead ECUs. There?s nothing else I can think of that could be wrong. Although it seems weird that injector pulse is the only thing not working on this ECU.
 
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