Injectors not firing; ECU problem?

[Lani Kai]

My injectors are not firing, so I've done some tests...

TL;DR: Has my ECU gone bad?

Tested at the ECU harness, and each injector connection reads the proper voltage when grounded.

Tested at the individual injector connectors, and read the proper voltage as well.

Manually fired the injectors by grounding each corresponding terminal on the ECU connector. Turned ignition to "ON" and heard each injector click.

Checked the ground circuits on the ECU harness, and all were fine.

Basically, wiring from ECU to injectors tested OK, and wiring from ECU to ground tested OK. I've read that ECU failures are pretty unlikely, but all evidence seems to suggest that it may be at fault. Are there any flaws in my logic, and is there anything else I should test before buying a new unit?

[grantf]

At least you have done some testing. I am not an expert by any means so I am just throwing out ideas. How about power to the ECU with the ignition on?

Never mind reread youre post, You have already done that. I would not rule out something else just yet though. Do you have the means of borrowing a spare ECU from a friend?

Edited June 15, 2012 by grantf

[Zed Head]

Are you getting spark? The ECU uses the voltage from the negative post of the coil to determine when to fire the injectors.

Also, and apparently this isn't the same for everyone, the ECU on two cars I've worked on, a 76 and a 78, needed to have the tachometer connected to fire the injectors. Not exactly sure why, although I have a theory, but I have confirmed it.

Anecdotally, the resistor in the circuit to the tachometer fell out of my 76 and the engine wouldn't start until I found it and re-installed it.

[Lani Kai]

Do you have the means of borrowing a spare ECU from a friend?

[ATTACH=CONFIG]54152[/ATTACH]

I'm looking around, but haven't found one yet. Anyone in Orange County, CA that can help me out?

Are you getting spark? The ECU uses the voltage from the negative post of the coil to determine when to fire the injectors.

Also, and apparently this isn't the same for everyone, the ECU on two cars I've worked on, a 76 and a 78, needed to have the tachometer connected to fire the injectors. Not exactly sure why, although I have a theory, but I have confirmed it.

Anecdotally, the resistor in the circuit to the tachometer fell out of my 76 and the engine wouldn't start until I found it and re-installed it.

Yes, I'm getting spark and the tachometer is connected and working.

[Zed Head]

Have you checked continuity on Pin #1 at the ECU connector to the coil negative post? In case you missed that test. Looks like you covered the others.

[Wade Nelson]

Confirm that you have B+, IGN, and ground at your ECM. If you have 12.8V across the battery terminals you should have at least 12.5 from IGN to GND and B+ to GND (voltage drop of .3V or less) I'm not aware if the Z ECM has multiple B+ and/or ground connections (to allow greater total amperage), if so, check 'em all.

If you have a voltage drop of .3V or more you need to correct THAT before proceeding, by cleaning up grounds, etc.

See if you get signal to cold start injector when cranking.

Then confirm that the signal from the distributor is the same at the distributor as it is at the ECM pin. A scope is preferable but you may be able to do it with a DVOM. Most likely you're not getting rpm / tach signal to ECM and ECM simply doesn't think the motor is spinning.

Borrowing an ECM or buying one is risky business, you need to determine WHAT if anything CAUSED yours to fail before frying a brand new one or a loaner.

So the NEXT steps are to go down ALL your ECM pins, looking for B+ or Ground where IT SHOULD NOT BE.

After that, I would crack the case on the ECM and give it the sniff and eyeball test, looking for burned components, and smelling for burn smell.

Only after that would I even start LOOKING for a replacement ECM.

I have diagnosed well over 200 vehicles with ECM problems of various sorts and only had a handful that were actually a failed ECM, TCM, PCM, etc. It's invariably a wiring or connector fault NOT getting a necessary signal to the ECM, or something sending 12V or ground up an ECM pin where it shouldn't be.

GL

[FastWoman]

If you have continuity from the (-) of the coil to the #1 of the ECU, voltage/ground to the ECU, and a firing ignition coil, the ECU should ground the injectors to fire them. Literally nothing else about your vehicle would have to be right.

A quick tip: You can plug a christmas light bulb (miniature variety) into an injector connector to see if it flashes when you crank the engine (easier than listening for an injector click). It should flash longer/brighter if you disconnect the coolant temp sensor.

I'll be very interested to know whether your ECU has died. Sometimes it seems the 1978 ECUs are particularly prone to drift. I've not yet heard of one going out, but it wouldn't completely surprise me.

FAIW, I bought a spare '78 ECU for $20 off of ebay.

Edited June 15, 2012 by FastWoman

[Lani Kai]

I checked for continuity from #1 to the coil negative and I had continuity.

Also, what does "B+" refer to?

And thanks for the tip about the Christmas light bulb. I've been testing with a spare injector that I can just hold up to my ear, but a light would probably be a little easier.

[Zed Head]

B+ is the battery positive post, or any wire connected to it.

If you have the connection from Pin 1 to the coil negative and you're getting spark, then the injectors should be firing. The ECU "counts" 3 sparks then fires the injectors. You can actually test the system by grounding the coil negative three times and listening, or looking if you have a christmas tree light, for injection. You might try that before you decide to try another ECU. Connect a jumper wire to the coil negative, turn the key to Run and tap the jumper to ground. Tap, tap, tap, etc. Every three taps you should get a click or light from the injector circuit. It's an easy test and takes out the noise and voltage drain of the starter. All you're doing when you tap the jumper to ground is energizing and discharging the coil, which creates a spark and gives the ECU one count through Pin 1.

Don't leave the jumper wire touching ground or you'll overheat the coil.

Edited June 16, 2012 by Zed Head

[Lani Kai]

Was doing some testing just now and I'll post what I measured at the harness in a bit, but now everything completely dies when I turn the key to "Start"

It won't turn over at all at the moment.

[Zed Head]

If the voltmeter shows 12 or more volts but drops to zero when you turn the key to Start it might just be dirty battery cable connections. Pretty common.

[Lani Kai]

With ignition to ON, ~12.7V at connectors:

1, 10, 14, 15, 30, 31, 32, 33

1: Coil trigger input

10: Fuel pump? The EFI bible has a test from 10 to 20 as AFM to fuel pump but it's for a 1975 and mine is a 1978.

14, 15, 30, 31, 32, 33: Injectors

With ignition to OFF, continuity at connectors to ground:

4, 5, 16, 17, 21, 35

4: Start signal circuit

5, 16, 17, 35: ECU to ground

21: Cold start

With ignition to ON, continuity at connectors to ground:

4, 5, 16, 17, 21, 35

Is there anything above that suggests something is awry?

Sticking a Christmas tree light in the injector connector and drawing a wire from ECU connector to ground causes the light to turn on.

Drawing a wire from coil - post to ground and connecting to ground causes a spark at the wire on every tap, but does not seem to make injectors fire even after more than three taps.

Coil is new. Cold start valve is also having problems (always has). It's constantly getting a current and keeps continuously spraying fuel. Previously, when the injectors were working, I had unplugged it because I don't really need it. Right now, the only way the car can start is with the cold start plugged in (the car is running on fuel from the CSV only)