New problem - Fuel pressure drops off - 75 280 L Jet, w/ composite injectors, ProtunerZ rail, 3 bar FPR

[HusseinHolland]

45 minutes ago, Dave WM said:

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. 

 

Odd that the diagrams all only show 2 diodes - the one for the crank only enrichment circuit, and the one on (+) 86c - there is another on the combined ground leg (85 terminal)

Horizontal mounted diode is on the (50) crank only enrichment /pump signal circuit. Diode on top right center is on the ground leg (85). Diode on bottom right center is on the power relay coil feed (86c)

 

 

 

[HusseinHolland]

I repositioned the fuse sub panel - to somewhat conceal all the wiring

Edited June 19 by HusseinHolland

[HusseinHolland]

Car started exhibiting the same symptom as with the original relay - dropping the pump feed & monetarily cutting out. I was afraid that using the original base could lead to issues, so I made a new new from a couple of Volvo relays.

I wired in the 3 diodes and a 2ohm resistor. 

Had incorrectly marked the 86c feed to the coil on the power relay (tied it to 86 as well, NOT good), so I had to redo that

corrected wiring

Male pin I'm using for 86c is also on the outer left instead of inner left, so a bridge on the relay socket was needed. There are male pins in the 85 location (bottom right) on both bases. I had already made a bridge on the relay sockets, to allow the use of the Fiat version L-Jet relay, so both are connected to the ground circuit.

The next problem was that the male pins are shorter length than the ones used for the Datsun/Bosch L-Jet relays, which meant they were not fully seating in the relay sockets. This meant the connections would come & go, took me awhile to figure out that was the problem. I shaved back the base on the relays - that wasn't enough, so I had to shave the small rise around each pin also.  

That made the overall male pin depth close enough to the factory length. With that, the bases stay properly seated and no connectivity / functional issues with the adaptor after driving it this afternoon.

 

 

[HusseinHolland]

So, car started acting up & cutting out again, of course when my wife was with me. I was trying to get everything ready for the trip to Ohio tomorrow. It seemed like it was the relay block connections again, however I tested the grounds & switched feeds & that was not the issue. I put a test light on the feed signal from the AFM pump switch, and that would come & go in keeping with the car dying.  

With this, it is quite possible the earlier adaptor base I made from the original relay is in fact OK, and the recent problem has in fact been the AFM switch.

So, I tweaked the AFM switch to put a little more tension on the point contact. In addition, I added a LED to the circuit so I can see if the problem recurs. I also added a switch to the circuit, to bypass the AFM pump switch if it fails altogether.

Edited August 15 by HusseinHolland

[Dave WM]

so the AFM shows open then dies or dies and then shows open? like in comedy, its all about timing. The bypass should show the problem if indeed the afm swtich. 

Edited August 17 by Dave WM

[HusseinHolland]

Car has run beautifully all the way out to Columbus, OH. about 600 miles so far, mostly 65-70 highway speeds. It has consumed about 1/2 quart oil, which I haven't seen in local/mixed highway driving. 

Couple of the Fiat race cars at the Fiat Meet

One of the older techs at the shop was previously a Volvo guy, and has a 90 740 16V wagon he wants to move out of his garage where it has been stored for th past 18 years. I may have to buy it 

Needs the head re-installed - he is now physically unable to do the work, hence the desire to see it go to another Volvo guy.

 

[HusseinHolland]

On 8/17/2024 at 11:35 AM, Dave WM said:

so the AFM shows open then dies or dies and then shows open? like in comedy, its all about timing. The bypass should show the problem if indeed the afm swtich. 

Dave - the switch contacts were weak - I could start the engine (since the AFM is bypassed when cranking) - hear the pump relay click off & the engine begin to stall, then quickly apply pressure to the AFM contact (cover removed) - and the engine will resume normal operation. I retensioned the AFM contact post to make sure the connection is more consistent. I can observe the switch function now by way of the LED I added. I'll add a pic of that for reference.

 

Edited August 20 by HusseinHolland

[Zed Head]

I had a similar problem with the AFM switch once.  There was a hair or a piece of dust or a bug wing stuck on one of the contact points.  They looked closed but there was no current through them.  I didn't find the actual offending item but the problem disappeared after cleaning the points like you'd clean a set of ignition points.  It was a head scratcher for a while.  Also had a problem with low idle RPM dropping air flow low enough to shut off the fuel pump.   

Probably why Nissan switched to the oil pressure/alternator switch later.

[HusseinHolland]

Only pic of the Z I have from the trip - in one of the PA tunnels