I've been driving the Z to work pretty regularly (as long as no rain is forecast), and the car has not missed a beat since replacing the original relay. Many thanks again to Zed Head for his input - it would have been a bitch to find otherwise.

Congrats on the quick fix. You are lucky to have a Bosch 0 322 514 127 Fiat relay handy. These dual units are getting harder to find since Bosch stopped making them around 2020.

Something I found years ago. If you are looking for a Bosch 0 322 514 120, try Beck Arnley 203-0053. It's a Bosch relay, but they just took it out of the Bosch box and put in a Beck Arnley box. It is also hard to find, but the more options increase your chances of finding one.

Yesterday, I drove the car with the Mrs., and it acted up - no throttle response & cut out a few times, stalled. Restarted every time. I thought maybe I was actually out of gas, as the (cheap) new gauge seemed pretty inaccurate. Anyway, that wasn't it. Today I took apart the old relay & cleaned the contacts to retry the relay. It didn't a make a difference, the coil is the problem, it overheats & cuts out as before.

Took a bunch of pics of the relay, so I could try replicating it. resistor for AFM circuit is 2ohms. I'll probably just make a new relay setup, just didn't want to cut the factory harness at this point.

pitted contact

after

resistor value 2ohm

tried to cleanly remove the coil, cracked the board in the process

I also added an extra heat sink for the HEI module, as it did seem to be getting pretty hot

So, in the end, I didn't find anything conclusive, and after the initial glitch/hiccup in the driveway, I took a  short drive this evening & flogged it hard to see if I could make it act up. It ran just fine.

 

 

 

I think I found a problem today, car hiccuped & then died, and this time it was dead - the maxi fuse melted. I'm assuming it's just poor quality. Got to bobby pin from a student, but that melted right away, clearly not good for 30A . So I broke a keyring in half and shoved that in there so I could drive home. This was the replacement for the fusible link between shunt and ignition switch.

Edited June 13, 20241 yr by HusseinHolland

30 amps is alot of Draw. Any ideas?

On 6/13/2024 at 10:16 PM, Patcon said:

30 amps is alot of Draw. Any ideas?

I don't know. I did determine that the fuse holder I used was actually the version rated at 20A (based on the AWG) - I must have swapped out the 20A for a 30A fuse, so maybe the combined draw of all the consumers off the switch was just too much, it definitely overheated.

So, I've re-wired the Ignition switch feed (from Shunt) via the Midi fuse block. Using a 40A fuse to match the value of the fusible link previously replaced with the inline blade that melted.

Sectioned the supply plate

Approx 3mm wga Red/Blue to match original ignition switch feed (Red-White) from shunt to 40A fuse

MTA Midi fuses & power supply crimp terminals

Replaced (White) feed wire from fuse block to Shunt, approx 3.5mm wga Brown - mine looked iffy. Sliced it open to look at where the wiring was kinked. Actually in better shape than the insulation appeared. I had bought some 7.8mm Female spades of the type used on the Datsun, thankfully.

Used 60A Midi fuse - original link was 80a, according to chart. Feed line from main fuse block is 80A

labelled so I don't forget what feeds what (have to figure out what the 3rd one is for) 

l

 

Edited June 15, 20241 yr by HusseinHolland

Never realized that the 280Z pumps don't seem to be on a fuse.  They are powered directly from the pump relay, which is protected by a fusible link.  Seems like they could have run it through the fuse panel, since it has a long run plus exposure to the elements.  Kind of odd.

Here's an interesting chart showing some typical pump draws.

https://aftermarketindustries.com.au/fuel-pumps-accessories/fuel-pump-current-draw-chart-walbro-vs-bosch/

7 hours ago, Zed Head said:

Never realized that the 280Z pumps don't seem to be on a fuse.  They are powered directly from the pump relay, which is protected by a fusible link.  Seems like they could have run it through the fuse panel, since it has a long run plus exposure to the elements.  Kind of odd.

Here's an interesting chart showing some typical pump draws.

https://aftermarketindustries.com.au/fuel-pumps-accessories/fuel-pump-current-draw-chart-walbro-vs-bosch/

Good point - that load alone accounts for more than half the rating of the fuse I had installed, coupled with under-rated wire gauge. I'm planning on re-wiring the EFI relay to use a couple of standard cube relays for the crank & pump circuits, with a (more readily available) Volvo EFI system relay to govern it. The pump relay will have a separate fused feed. Since I won't have the floating ECU ground normally used to control it, I'm going to use a VW relay that requires a tach pulse to switch the main relay ground. With that, I can do away with the AFM circuit.  The goal is to shift the load from the complex relays to the (Volvo/Bosch "J"  cube relays that are intended to carry up to 50A loads, and are easily available. I'm driving this to a Fiat meet in Ohio in August - I don't want to drive that far relying on a relay that is essentially unobtanium.

along these lines

 

Edited June 16, 20241 yr by HusseinHolland

That was the first image I came across but another search makes those numbers seem high.  Especially for the relatively low pressures the 280Z system runs.  Although you did raise your pressure from 2.5 to 3.0 bar.  Pressure drop across the filter and fuel lines is a factor also.  But other charts show about 10 amps.  If you have the meter you could measure.  Fun project.

https://help.summitracing.com/knowledgebase/article/SR-05245/en-us

https://www.autoperformanceengineering.com/html/fpspecs.html

 

I decided to go a easier softer route - used the factory base, adding standard cube relay sockets that will be secured in the factory relay cover, so easy to replace the part that could wear, without dealing with rewiring the harness

 

Plugged in to test - make sure my wiring is correct before dealing with the other side

Cutting the cover

Removed the 2nd coil, more to come.

Finished the 'adaptor' socket

Ran the switched coil feeds through to the underside - not enough room to solder them on the component side

After I tested it in the car, I epoxied the base after - I had cracked the base when I pried off the first coil, so I wanted to make sure nothing was going to move. Put the cover back on to retain the new relay sockets 

 

Edited June 18, 2024Jun 18 by HusseinHolland

nice fix! interesting about the diodes, when I looked at the schematic. At 1st I thought they were for flyback voltage spike protection from the coils, but the way they are wired it seems more like just a reverse polarity protection (looks like coil current is blocked if reversed voltage applied). If that diode works I wonder how come peeps keep frying ECU's when connecting up backward? Maybe I am not looking at it right...

Ok so Pin 4 on the ECU is NOT diode protected (gets 12v when in "start") and Pin 47 also gets a shot from "Start" CSV which goes back to the ECU as well on pin 21. this is a 1975 fyi. 

 

 

on my DIY 300zxt harness I  did NOT use them (diodes) as polarity protection but did wire in P with the coil for flyback protection. Just have to not get stupid when connecting the battery. 

 

Edited June 19, 2024Jun 19 by Dave WM