Renix WOT cutout (long)

OK. THis is going to be a long winded post...
The back story:
A week after buying the MJ I had to have it towed. It had melted the fusible link to the fuel pump/injector circuit. With the help of Langer1 I got it running in the wee hours of the night (thanks Langer, still owe you lunch)
I replaced the fusible link with a fuse holder, and I had been using a 20A fuse. I put another fusible link in and it does the same thing. RIght nw I've got the fuse in for troubleshooting purposes.
THe problem:
At WOT the motor will stop running at 3500 RPM. It always blowes the duse/link on the fuel pump circuit. I can rev past 3500 RPM if I'm below 80% throttle. I don't think it's anything mechanical. Replace the fuse, it will start right up.
WOT is fine below the 3500 RPM mark.
Troubleshooting:
I pulled individual injectors and ran on 5 injectors until the cutoff point. No change.
I don't have a fuel pump ballast resistor, for what that's worth.
Wiring harness seems to be in ok shape. I pulled back all the loom on the injector harness and there's no damage.
I know the fuel pump wiring is good, in the troubleshooting when it died I inspected every bit of it.

I'm at a loss as to what to look at next. I'm thinking it might be the fuel pump, but that doesn't make sense. It always runs at the same current draw and the regulator returns the extra fuel.
Someone with more experience just point the newb in the right direction. I'm handy with a multimeter.
Thanks.
1987 MJ 4.0 Engine is most likely from a 90, ECU is orginal 87 unit. Injector year unknown. Fuel pump unknown.

I know that the computer at WOT gives the fuel pump full voltage over the ballast resistor. But your not running yours, so that makes no sense. Unless maybe it switches to full voltage after 3500rpm, in that case id trace that circuit. I believe theres two, one with the ballast resistor and one without.

You could have a injector going bad, or there gummed up.
Try some good cleaner like Sea Foam.
They draw high current when they get sticky so at lower speeds they’re off more that on and the fuse holds.
At higher RPM they are like on more than off so the fuse blows.
They pull about 5 amps each and even more when they stick.
Also you want to use a time delay fuse until you get it fixed like MDL 30.

summitlt said:

I know that the computer at WOT gives the fuel pump full voltage over the ballast resistor. But your not running yours, so that makes no sense. Unless maybe it switches to full voltage after 3500rpm, in that case id trace that circuit. I believe theres two, one with the ballast resistor and one without.

Click to expand...

Mine never had a ballast resistor.

langer1 said:

You could have a injector going bad, or there gummed up.
Try some good cleaner like Sea Foam.
They draw high current when they get sticky so at lower speeds they’re off more that on and the fuse holds.
At higher RPM they are like on more than off so the fuse blows.
They pull about 5 amps each and even more when they stick.
Also you want to use a time delay fuse until you get it fixed like MDL 30.

Click to expand...

Well, I had disconected each injector one at a time. When revved on 5 injectors it still did the same thing. I will put the fusible link back in and re check. Just to double check, fusible link should be a piece of wire 2 gauges less than the circuit wire correct?

Normally a fuse link is 2 sizes smaller than the circuit it protects.

The fat size is just soft rubber insulation that wont melt like plastic
when the link gets hot.

You can buy fuse link wire in bulk to replace one if needed. Be aware
that you have to use crimp connectors on the ends and it needs to be the
same length as the stock one or about 8 long. Solder will ruin the
thermal properties of the link.

If you replace a fuse link with regular wire or with heavier wire, you
will cause a fire to start when whatever blew the first link goes nuts
again.

good to know. I will source some bulk fusible link wire, and use crimp style connectors. Hopefully advance has the wire. I'd like to get this solved this weekend. WinterFest is just around the corner!

Happen to have a schematic here.
At all times that fusible link powers the fuel pump relay, TCU terminal 14, and ECU terminal B7 (main battery voltage input, and, presumably, each one of the ECU's outputs, the most significant being the injector drivers)
The switched side (ign on) of the fuel pump relay powers the coil of the O2 sensor heater relay (primary side), the injector drivers in the ECU (C-11 and D-10), the EGR solenoid, the A/C clutch relay (primary side), and of course the fuel pump. The EGR solenoid and the O2 sensor relay and A/C relays are operated by completing their circuits to ground through the ECU so I would suspect nothing with them. I can't find anything that says what happens at the TCU but I suppose you could eliminate anything there by unplugging it.

In looking for loads present on the circuit only at WOT I see nothing.

I suppose it's possible, though unlikely, to have a fault in one of the ECU injector drivers, but I doubt it because whichever of the drivers was presenting too much load to the circuit would probably be nonfuncional so it's respective injectors would probably not be firing at any time and the overload would probably not be dependent on engine speed.

Since you Jeep has reached voting age it is more likely you just had a tired out fusible link that just popped. From what I can tell, that one is the one that carries the biggest load (other than the larger one going to the alternator) so it stands to reason it would be the first to fail.

I'd suggest putting in a new fusible link (schematic only says it could be orange or green, doesn't say what amperage value, but I think that will be marked on the original molded black insulator piece at the harness end) and powering the circuit through an ammeter to see how high it goes. Other posts here seem to suggest going above 30 amps could indicate a problem. I don't think the 20 amp fuse you had in the circuit would be big enough. You can check the draw of the fuel pump itself by jumpering across diagnostic connectors D1-5 and D1-6 with an ammeter.

good info Dave. I appreciate it. I'm sourcing the correct fusible link wire now. No place in town has it in stock. Online we go. For future reference my fusible link was green.
No TCU as it's a 5 spd.
No AC, I got buckets and a sunroof, but just an idler puley to cool me off in the summer.
Good tip about reading the current. I'll wire it up so I can see it from the cab and then see if it's a steady increase in current draw or an instant jump like something shorts.

One other thing is the primary side of the AUX fan and the ALT there is no fuses on these connections.
My schematic shows green or Red heck if you have power door locks that is a 30 amp fuse just for that and it is also feed by that one link. You just need a bigger fuse.
It's also the supply for all the lights so your looking at more like 50 amps minum.
Use a piece of 12awg rubber covered wire.

no aux fan either.
I will grab some wire at the parts store to see if it fixes it tomorrow. Then replace it with the proper fusible link when it gets here.

87manche said:

no aux fan either.
I will grab some wire at the parts store to see if it fixes it tomorrow. Then replace it with the proper fusible link when it gets here.

Click to expand...

You should overheat without it.

don't have one, doesn't look like it was ever there. RUns at the proper temp. DOn't know if it's because it's a 5sp without AC or what. It's on the list of things to add on.

I just wonder if it was always a 4.0l

vin number says it was.

87manche said:

don't have one, doesn't look like it was ever there. RUns at the proper temp. DOn't know if it's because it's a 5sp without AC or what. It's on the list of things to add on.

Click to expand...

The 5-spd without AC didn't come with an aux fan.

The only things I saw in the FSM loading that particular fusible link are the things that I mentioned. Without the auto trans or the A/C there's that much less. Still there is more than a 20A fuse will handle.

And just because I said I don't think there's an overload don't go sticking a 12 ga wire in there. If that's not a correct diagnosis the wiring can go white hot in about a second.

Oh and though it's not relevant to the issue you have today, you are right the aux fan that's not there was probably never there.

Some didnt come with the efan. Lots of base models didnt have them

Dave in Eugene said:

The only things I saw in the FSM loading that particular fusible link are the things that I mentioned. Without the auto trans or the A/C there's that much less. Still there is more than a 20A fuse will handle.

And just because I said I don't think there's an overload don't go sticking a 12 ga wire in there. If that's not a correct diagnosis the wiring can go white hot in about a second.

Oh and though it's not relevant to the issue you have today, you are right the aux fan that's not there was probably never there.

Click to expand...

Definitely not just sticking wire in there. I've ordered the proper fusible link in bulk. That way I get lots of tries in sorting it out. I will update when I get te fusible link, and it's installed.

ok, so I'm back on this today.
I got te proper fusible link wire. 2 guages smaller than the guages I need to replace.
the fuel pump/inector circuit got a 16 GA fusible link, the wire strippers measured the harness wire as 14GA. Still does the same thing. It will stay runing now, ust because the fusible link is more tolerant, but it still cuts out around 3K RPM. Any more ideas?