Stock 280z fuel pump relay too much for MS v3 output

[HizAndHerz]

Just a heads up, but there may be an issue with driving the stock fuel pump relay from MS, at least on a 78'z. I know that at least a couple of folks are driving the stock relay from the FPUMP# signal.

 

After just a couple of weeks of driving with MS-II v3.0, the car would start and die within a minute. If left alone for at least 5 minutes, it would start just fine, then die within a minute again. I logged the failure and saw the NB O2 voltage dropped just before the MAP, pulse width or RPM changed. Fuel was being cut even though MS was driving the same pulse width. Figured it was either the injector drivers (probably not both at the same time) or the fuel pump relay driver. Removed the FPUMP# wire to the relay and put the old wire back. Ran fine.

 

When I first installed MS, I was concerned about the fuel pump driver being able to drive sink enough current for the relay, but never measured the current through it. Should have checked that first.

 

I'm going to replace the transistor on the MS board, but leave the wire to the relay disconnected for now. Soon, I'll be replacing the fuse links and relays with a power distribution box from a Dodge Neon. I've had a couple of the stock relays go out on me - time to get rid of those 30 year old parts.

[Daeron]

I did this write-up on my subaru website, but the information is as appropriate to Zs as anything else. I realize you already said you have a power box from a neon, but check this out anyhow...

 

http://www.ultimatesubaru.org/forum/showpost.php?p=598608&postcount=1

[mpcapps]

I'm actually using the 78's stock fuel pump management (fuel pump control relay and fuel pump relay) and it works fine. since it doesn't rely on the AFM for cutoff purposes (it uses oil pressure and alternator voltage) it's compatible with an MS installation. I simply don't use the FPUMP pin on MS, and have left the rest of the fuel pump system electronics in the car intact.

[Daeron]

Hiz and herz was kind enough to point out a flaw in my relay write up; i intend on fixing it as soon as possible but just in case I forget, may as well mention it here that apparently, terminals 87 and 87A are NOT parallel outputs; it appears 87 has power output to it with the relay energized, then when the relay is not energized power is output to terminal 87a.

 

disregard the post if you didnt read my relay write up linked to above; but I wanted to make the note here for posterity's sake in case (as I said) i dont get around to fixing the write up before i forget.