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Still Can't Get this Thing to Start #2

My 78 242 ran like absolute crap and I did a lot of troubleshooting for 3 months, never found the problem. Comprehensively ruled out fuel and spark problems. When I was pulling out all the unnecessary wires from the loom for my LS swap, I found the culprit. One wire was broken right where the main harness goes though the firewall, solid blue wire I think went to the Lambda system (hard to remember now). All the other wires looked perfect. So 1 broken wire could be your issue.

Before that, I had a lot of problems with the fuel pump relay burning up. I wired a modern 30A relay next to it to run the fuel pumps, just used the original relay to turn the new one on.
 
Do you have a test lamp?

Does it run on starting fluid? (click my sig)

Spark, injector pulse, fuel pressure, compression, ignition and cam timing. Something is off.
 
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Yes, it runs on starter fluid. It sat for approx 6 months, and before that, ran fine. It's got to be corrosion or something...

I'm just not sure why it would run so nice for 20 minutes the other day and suddenly shut off. It was idling while I was airing up the tires, so I wasn't moving anything around.
 
Now that you have a Bentley Bible say what it lists for the test for the spark signal going to the computer. Remember, I described it to you in a PM. If the ECU is not getting that signal it will not turn on the injectors. If it is getting that signal and you’re not getting any injectors to fire, that indicates a bad ECU.
 
Now that you have a Bentley Bible say what it lists for the test for the spark signal going to the computer. Remember, I described it to you in a PM. If the ECU is not getting that signal it will not turn on the injectors. If it is getting that signal and you?re not getting any injectors to fire, that indicates a bad ECU.

The Bentley says to test that the ECU is receiving RPM signal, to connect the test probes to connector pin 4 and ground. It should receive ~8v when the motor is cranked.

During my inspection of all the interior wiring, I did notice what looked like corrosion on the ecu. I removed the board from the casing and found that moisture had made it's way into the case during the time the 242 sat. I gently cleaned the corrosion off. So, I wouldn't be surprised if it ends up being the ECU.
 
ECU is receiving RPM signal

Jumper wire test at fuel-relay's receptacle that I mentioned above does not require a RPM signal. Relay is removed, and jumper wire inserted between Red and Yellow-Red wires. Fuel pumps will engage. I'd have to see this receptacle again, but I think I did this test to check fuel pump engagement.
 
Jumper wire test at fuel-relay's receptacle that I mentioned above does not require a RPM signal. Relay is removed, and jumper wire inserted between Red and Yellow-Red wires. Fuel pumps will engage. I'd have to see this receptacle again, but I think I did this test to check fuel pump engagement.

Yes, if the bridge is gapped between the YR & R main power wires, the fuel pumps engage. Everything is receiving power as it should. The injectors are not receiving ground signal from the ECU (WH-GRN wire) as they should.

ZVOLV, I might have a test lamp somewhere, but I've mainly been using Fluke DMM, which is plenty sensitive enough to give me indication of issues.
 
Ok...earlier it was about zero voltage at injectors.

6/12/24V CIRCUIT TESTER
ET112X
; used it for years....may be cheaper elsewhere.

Yeah, you were right in saying that the engine has to crank for the ECM to trigger the relays open.

Thank you for the link. That's a nice one, especially because it has the extra lead required to be hands free.

Does anyone know if the LH ECUs are repairable? I opened mine up and cleaned small corrosion spots off it, before I got it running. And the car isnt super watertight, so I'm thinking maybe moisture got back in or something. I'll have to test the RPM wire next weekend to make sure.
 
So the test in the Bentley calls for the gray wire (RPM signal) to carry +8v to the ECM, but I did already test it at as 2manyturbos described, to check for continuity, then I also checked for resistance. Less than an Ohm, if i remember correctly. When I have time, I'll check for voltage.

It would honestly make sense, being the ECU, though. To go more into depth, the car had ants crawling through it. I actually saw what looked like a tiny ant nest in one of the holes of the ECU casing. So I bent back the tabs and removed the board. There was corrosion spots and moisture in a few small spots. I did my best to gently clean it with QD cleaner and a microfiber cloth. After putting it back together, new relays and spinning the fuses, it ran. I let it sit for the week, and I was going to take it home. Last week there was the ice storm in OR, and my 242 was in the open. There was moisture in the car again. The car ran sitting still for 20 minutes then died. So that's why I'm pretty sure the ECU is faulty.

If I get it running on a different ECU (fingers crossed), I'm interested to take the old one apart again and take a closer look at what's happened.
 
The gray wire connects to the - spade on the coil. It's driven by the ignition box. It will be less than 1volt when the coil is charging (during the dwell time), and 12volts (battery voltage) when the coil is not charging. It will pulse between the 2 voltages twice per rev (i.e. 2 sparks per rev), so your meter may show something other than a simple 8volts.

If you have a tachometer, the gray wire goes to it too. If it's easy to get to, you could try unplugging the tachometer to see if the signal improves enough for LH ECM to detect cranking.
 
The gray wire connects to the - spade on the coil. It's driven by the ignition box. It will be less than 1volt when the coil is charging (during the dwell time), and 12volts (battery voltage) when the coil is not charging. It will pulse between the 2 voltages twice per rev (i.e. 2 sparks per rev), so your meter may show something other than a simple 8volts.

If you have a tachometer, the gray wire goes to it too. If it's easy to get to, you could try unplugging the tachometer to see if the signal improves enough for LH ECM to detect cranking.

Good to know. I suppose I didn't relay the correct info. The manual says a minimum of 8v.

I don't have my tach installed yet.
 
Injector pulse

Install a test lamp between a positive voltage source and an injector ground terminal. Crank. Blink?

Put a screwdriver handle to your ear and the tip to the injector. Crank. Click?
 
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The injectors are out of the intake so I know they are not pulsing.

As for fuel, I did open the tank a few weeks ago to check condition of the in tank pump. It looked to have a good amount of fuel, but I can double check.
 
It sure sounds like the ECM isn't working. If it was working, it would turn on power to the injectors and fuel pumps, via the fuel pump relay #15, when cranking was detected. The problem could be the ECM is powered up OK, but not detecting the coil pulse. Or, it could be that the ECM isn't powering up.

With key on, have you checked for +12v power at the MAF? The ECM should see Key on, and should turn on the System Relay #9, which supplies power to the MAF. You can also check the intake manifold ground wires to make sure the ECM is properly grounded (not corroded).
 
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