No-Start: Fuel Injectors Not Firing 1978 280z

[Zed Head]

I almost added a comment about a transistor's usage as a a switch or relay but that's a whole other forum, with lots of discussion possible about nomenclature and terminology and what's actually happening.

Regardless, the two pins at the injector plug will always both show battery voltage against ground in the stock Nissan ECU setup with the key on. I just wanted clear that up to help the OP along in his quest. I even went out and measured, again, to be positive I was living in the same reality now as last time I did it, and just to see those comforting numbers on the meter.

[superlen]

Zed,Sarah

There is a weak pullup on the transistor output inside the ECU. That is what keeps the voltage up at 12V when the injectors aren't being energized. I'm not sure why Bosch decided they needed/wanted a pullup there. It's not like any spike or noise would accidentally fire an injector. I don't remember offhand, but I think it was like 40k (may have started as a 47k in 1975. ) Note: If you see +12 (reference to battery ground) at both sides of the injector connector UNPLUGGED, you can be sure that the dropping resistors are seeing power as well as the ECU. One side gets powered from the dropping resistor, the other from the weak pullup inside the ECU.

Rossi,

Just keep following the advice given on here and even a nuts and bolts guy like yourself can diagnose this and trace down the problem.

Len

[Zed Head]

Thanks superlen. I see pull-up references all the time on the Megasquirt threads. Forgot about them. I thought the purpose of the pull-up resistor was to hold the transistor in a certain logic state. Otherwise it floats erratically.

Anyway, you've filled in another hole in my own logic state (pun stretch!!, p's and n's....never mind)...

[FastWoman]

I'm not sure why Bosch decided they needed/wanted a pullup there. It's not like any spike or noise would accidentally fire an injector. I don't remember offhand, but I think it was like 40k (may have started as a 47k in 1975. )

Interesting! Of course you and I know those are 6 resistors that serve no function, except possibly to put 12V at the injector plugs for testing. Or possibly the pull-up resistors were used for test-benching the ECUs.

I suspect, and could be wrong, that this little detail harkens back to a better time when executive bean counters didn't pitch hissy fits over 10 cents extra in parts, and would condone letting their designers have the luxury of putting in an extra part here or there to shape an output however they saw fit. I suspect there is also robust input/output protection on this pair of boards, as well as reverse-polarity protection. Only a guess. Perhaps that's why these ECUs have lasted as long as they have.

BUT... ROSSIZ... This would explain why you read 0 VDC across the injector plug. You really need to measure both sides against ground to draw any conclusions. Both sides should measure +12 VDC as Lenny indicates.

Edited April 16, 2014 by FastWoman

[superlen]

Zed,

There are definitely pullups on the INPUT side of the transistor to do exactly as you said....hold the transistor in a known stable state when you aren't firing it. The pullups I noted earlier are on the output side of the transistor. These pullups aren't really needed in normal operation, in the OFF state the output of the transistor would just float if the injector wasn't connected....which is fine. Basically these pullups are in parallel with the injector/dropping resistor series circuit.

Lenny

[Captain Obvious]

Lenny, Another data point... My spare 77 ECU has no such internal pull-ups. I've got a snubber to ground to squelch inductive kickback from the injectors and I've got 1.5Ks that go from the emitters of the output Darlington to (and only to) pins on the main ECU connector. Pins 11 and 26? I'm thinking they are test port connections to probe the action of the outputs without connecting directly to the injectors.

[rossiz]

ok gang... i spent yesterday afternoon fiddling and i believe i have the problem identified.

i just read through all your posts and can't tell you all how grateful i am for your time and attention to my little z and it's quirky issues, as well as impressed by the knowledge base on this forum. kinda cracks me up when you start talking amongst yourselves and the conversation slowly drifts from "the car won't start" to quantum physics (or at least that's how it sounds to an electrical simpleton)

at any rate, with the help of some basic deductive basic logic and a good deal of luck, the issue revealed itself.

and the winner is.... (wait for it)..... FW AND THE RELAY PLUG!!!

i just used the engine itself as a tester and while it was running i started as far back on the harness as i could get to and started carefully wiggling wires until i got to the plug itself. i couldn't effect any change wiggling individual wires, even at the plug, but when i push on the plug itself, normal to the spades in the relay, the motor would die. the connections were all clean and greased, all fit tightly, but because it's a 6-spade plug, i figured they weren't being held in contact properly or one was not correctly seated in the plastic housing. so i took the entire plug apart, examined each crimped connector and plugged each lead directly to it's respective spade on the relay.

here's the odd thing: i get exactly the same result - if i push sideways on the connector/spade (again, at 90 degrees to the spade) even very gently, the thing cuts out, and as soon as i let go, the connection is back. i did this on both the new relay and the old one, and both act exactly the same. i haven't taken one of the relays apart yet (will tonight) but it seems odd that they would be so very sensitive to any movement like that. usually relays are pretty simple beasts. does this sound familiar/right with how the relay spade connections should act, or do i somehow have 2 equally bad relays - made 35 years apart and by different manufacturers??

anyway, i have all the plugs on the relay and need to order a new 6-plug connector along with new crimp-on connectors and a good crimping tool (sources, anyone?) so i can re-build the plug and keep all the wires in the right places. btw - all the wires are blue except one, which is green. if i'm not mistaken, at least 2 (maybe 3?) of the blue wires get 12V+ which seems funny...

oh, and thanks for the info on the injector testing - i cut up a string of xmas lights and just started testing every possible combination and came up with the following: light will light (and burn out!) testing either pin in the plug to one lead and the other to the engine block (ground), both leads to the pins and i get flashes when the engine is running. i now have a few bulbs in my emergency kit, along with a multimeter (which at some point i will have to figure out how to actually use).

[ZCurves]

Rossiz, can you post some pics and location of the relay and offending plug. From what you are describing, I may have a touch of this too. I think that this whole thread is very valuable because it is one of those very subtle mind numbing issues that we all have to deal with from time to time. As for a source for a replacement plug, you should try Vintage Connections, they have a bunch of that stuff.

[Zed Head]

Beware that outside of the relay may not represent the condition of the inside. They're easy to take apart, just bend the little metal tabs.

[FastWoman]

Wow, that's a NASTY carcass of a relay, Zed!

Rossiz, I'm glad you located the offending relay/connector. As far as crimping goes... well... I really, really hate crimped connections, especially in marine applications (we have boats on salt water). What I do is to crimp first and then sweeten the crimp with solder. You have to use the solder sparingly!

And yes, just a little wiggle or movement can indeed kill the engine if you've got bad connections. In the future, the best thing you can do when you have an intermittent problem is to go through your wiring, wiggling connections, BEFORE you do anything else. The reason is that you might make the problem go away when you wrestle with a part or connector here or there, and you might attribute the "fix" falsely to the part you replaced, when the real culprit is a bad connection waiting to strand you on the side of the freeway a couple of weeks later. As a rule of thumb, whenever I'm working on an electrical issue, I never replace parts in any area I haven't wiggled first.

Anyway, good job! (Don't forget to do the Snoopy dance around your car for good karma.)

[Captain Obvious]

Beware that outside of the relay may not represent the condition of the inside.

I don't know Zed... I don't see any misrepresentation there. That relay clearly says "No Good" on the outside of it. Truth in advertising!

PS - Holy crap!!

[rossiz]

so what i discovered is a weakness in the relay design that actually can cause it to lose connection at the contacts if the spade is flexed at all. the spades are held in a plastic base, which is flexible. the other end of the spades (inside the box) are the stationary contacts - this is where the problem is.

the gap between the stationary contacts (spades) and the moving contacts (the ones pulled by the solenoid) is fairly precise, as the solenoid throw is quite short. if there is any sideways pressure on the spade, it pivots about the plastic base and the contact gap changes. with my relays, the moving contacts had such a close tolerance to the two static contacts that a fairly light push would open the gap enough to break the connection.

i figured this out by fiddling with the brand new relay with the cover off and the engine running - i was able to pinpoint the exact spade that flexed causing the engine to die.

since there are 6 fairly thick wires going to the plug, they are pretty stiff when bundled/taped together and if they aren't bent so the plug attaches to the relay straight on, there's some "spring back" to the wire bundle and in my case it was pulling just enough to cause the relay to be intermittent.

to ZCurves: it's the EFI relay - located in the engine bay under the fusible links. i moved all my relays to inside the cabin, on the firewall under the fan on the passenger side to get them out of the elements. it works great, but the space is a bit tighter than the stock location so my wires were a little bit contorted - hence the strain on the plug and subsequent issues. i'm not sure if this would be an issue in the stock location unless the wires get messed with/pulled on for some reason.

to FW: i shall take full responsibility for this problem, as i did the re-wire - so the car gets a pass this time

i will take back my previous rant (well, at least some of it) and keep an open mind about the EFI system. for now it seems to be functioning pretty well. i still plan on replacing the injectors and fuel rail when i put in a header, hopefully later this summer when cash allows. this should clean up the engine bay and hopefully reduce the hot-start issue.

thanks again to all for the prompt and helpful replies.

Geoff