Z dies after warmed up 77 Z Drives For About 10 To 15 Minutes,then Dies.

[Zed Head]

I'm still back on "engine dies tach goes to zero".  You're right though, I did not delve in to the details, I'm just posting things that might help.  Plus, I can't really figure out how exactly that combined relay works.  Is there not a diode between Pins 36 and 39?  It connects the two relays somehow, but what does it do?

 

No offense to crazy but the wiring diagram and the information you posted are in the same chapter as the tests he's run.  The trees and forest thing.  Maybe the fusible link you mentioned is the one overheating.  Check 'em all.

[Zed Head]

The fusible link that directly runs the fuel pump and the AAR is the driver's side rear link located in the fusible link blocks.

This is my new "most likely cause".  

[Captain Obvious]

No, no... It's great info. I was just adding some additional info to make things, ummm... "easier"? 

 

That FI combo relay system is a pain in the butt. I think it's a lot more complicated than it really needs to be, but I wasn't there when they designed it.

 

The diode is between pins 36 and 76. Pin 36 is the fuel pump control from the AFM and pin 76 is the fuel pump control from the START signal. The diode OR's those two signals together so that either of them can power the fuel pump. In normal use, the signal from 76 is used first while cranking, and then once the engine spins up to speed and you stop cranking, the signal from 76 goes away, but by then the signal from 36 takes over instead.

[Captain Obvious]

This is my new "most likely cause".  

 

I'm not so sure. I wouldn't think it would go intermittent. I would expect once and done. That link runs a bunch of other stuff as well including all the dash gauges. There would be other effects.

 

On second thought... Wait a minute... You said the tach dies also? 

[Zed Head]

I'm not so sure. I wouldn't think it would go intermittent. I would expect once and done. 

I was thinking more of dirty connections, not a melted link.  I mentioned early on in this thread about someone with a fuse, hidden in the trunk, that would open the connection when it got hot, then close and mysteriously fix itself when it cooled.  It's apparently a BMW problem that has happened to more than one person, but is not common.  The fuse is in a moist spot and after 25+ years the corrosion builds up.

 

He might fix the problem with just a wiggle of those connections.

[Zed Head]

The diode is between pins 36 and 76. Pin 36 is the fuel pump control from the AFM and pin 76 is the fuel pump control from the START signal. The diode OR's those two signals together so that either of them can power the fuel pump. In normal use, the signal from 76 is used first while cranking, and then once the engine spins up to speed and you stop cranking, the signal from 76 goes away, but by then the signal from 36 takes over instead.

Thanks for the explanation.  Believe it or not I've wondered about the Start power.  No more.

 

 

I might have enough in my head now to work through that maze of connections.  Hard to do though, without a problem to solve.

Edited March 11, 2015 by Zed Head

[Zed Head]

It's the directionality of the relay diagram that gets me.  There's some backward loops in there.

 

Anyway, back to crazy - the start-run for 10 seconds-then die is also a sign of a dirty AFM fuel pump contact switch.  I had that happen to me but once it got dirty, it never fixed itself.  I had to run power from the rear window defogger to get home.  You can see the switch under the black cover of the AFM.  Maybe yours was slowly losing contact and now it's totally open.  The Start signal runs the pump, the engine starts,and only runs until fuel pressure drops.  You can clean the contact with a match book striker.  You'll see a tiny spark with the key On when it opens and closes if it's working.  It's also another way to test the pump circuit.  Take the cover off and rotate the weight and you'll see the lever that opens and closes the switch.

[Zed Head]

There's a good picture of the fuel pump switch here - http://atlanticz.ca/zclub/techtips/afm/index.html

 

I wouldn't do any adjusting though.  

[Crazy477280Z]

Talk about the ghost in the machine!!! I came out and reinstalled AFM because  I took cover off last night to check fuel pump contact point, and it was clean and making good contact when flap closed, and every thing looked good when I opened flap. Potentiometer is making good contact and operates smoothly.[i bought it new two years ago.....] anyway, I could hear fuel pump operating, so I plugged solenoid wire up, and also connected a 12 volt tester light to outgoing end of #4 fusible link. It lit up fine and I left it hooked up so I could see if it lost power there when car died. It started right up, and ran for over half an hour wwwwwwhile I shook,twisted, and flexed every wire and wiring harness to try a..nd simulate condition...No dice...car kept running until Ishut it off. It started right back up so I timed it,and now Ime going to do FI tests again. Maybe it just had a cold and its all better now?? I sure wish it was that easy because now I have this problem that could pop up any time unless I fix it,and now how do I know when Ive actually fixed problem. Thats sure not going to leave me feeling too comfortable driving it when it could suddenly drop out from under me. Ime still going to trace down fuse link by battery just to check condition if nothing else. I will let you know what happens.

[Zed Head]

I didn't actually see anything on my AFM contacts and it was a new one from MSA also.  Out of nowhere it just quit.  I got home, took the AFM out, checked the pins for continuity (none), took off the side cover and fiddled around with the contact surfaces, and it started working.  I figured that a hair or something small and hard to see got stuck between the contact surfaces.  But I didn't have all of the issues you had before I got the "ten seconds of running after Start", so I went directly to looking at the AFM contact switch. 

 

Good luck.  The worry will fade away eventually.  With these cars there's always something else anyway.  I have my favorite tow shop's number in my cellphone.

[Captain Obvious]

Good luck.

 

Sounds like Murphy is luring you into his trap..... You absolutely know that this problem is going to come back again at some time in the future, don't you? At the most inconvenient and inopportune time, right? 

 

When that happens, take some more measurements and come back here for some help if necessary. In the meantime, work on the next hottest fire!

[Crazy477280Z]

Yeah,Murphy and I go back aways. I drove car for over half hour and it ran great with only issue being tach not reading under 1000 RPM. I havent been able to find tach resistor yet, I did not have a fusible link at battery,so P\O must have removed it, so I hooked one back in.When problem pops up again, I will let you know. Thanks for advice and help everyone. Its good theres people out there that are willing to share thier knowledge with some of us that are still learning about thier Zs,and care enough to take the time to help school us through some of these problems. I bet half of our cars wouldnt even be on road if it wasnt for sites like these. this is definitly the freindliest Z site.and I think,also the most informative as well. THANKS AGAIN.  Ime not going to dissappear like my car issue did. Ive got 2 280zxs that Ime sure something will go wrong sometime or other. If I can help someone else.I will allways pay forward also, so I will be around.