RenX Files #2

Time for a new one. Renix 89 no start today. We have spark, we have fuel in the fuel rail and pressure. B+ latch relay and Fuel pump are good and working and they have power. Ballast resistor good.

Jeep started, and died and never fired back up today. Got two flashes using a NOID light in first test, but none since. No sign of the continuous 12 volts at the fuel injectors that should be there all time. Power to them is lost between the fuel pump relay (per the FSM wiring layout) and the injectors. FSM seems to indicate the ECU fires the injectors through a solid state ground in the ECU, but the 12 volt battery power is not getting to the injectors from the fuel pump relay.

Ideas???? Past experience with the weak spot locations?

Injector harness used to rub at the rear of the engine at the valve cover.

Unplug the ECU and check for injector feed at D10 and C11?

The book says the injectors fire on the plus side and share a common ground. I've been through that harness, though not recently, seem to remember a bunch of splices in the ground circuit and many possibilities for mischief. I'd start out by testing the injector grounds, front to rear.

Many years ago I found a wire at the bend of the injector harness at the firewall, it had a fatigue break under the insulation. They even put out a TSB about fatigue breaks at this point in the harness. That was my 87 and the wire broke after only three years. You can feel the fatigue break under the insulation if you squeeze the wire. I got lucky and found mine pretty quick, the frayed wires under the insulation were black, blue and slightly melted.

I had a power issue with my MAP, no power to the MAP no injector pulse.

Dipstick tube stud.

Which book? Haynes electrical schematic shows 12 V + to the injectors from the fuel pump relay on 87-90
Pg 52 of the Renix MPFI manual shows pin 4 as power to the fuel injectors from the fuel pump relay. I have power to pin #5 from the ignition switch and power to pin # 1 from the battery fuel link, and the relay does click. I guess I need to test the relay for bad contacts, I forgot to verify the voltage out at pin 4 to fuel injectors, but I did try new relays, and got no change, so I suspect it is +12 volt wire to the injectors.

Also the Renix ECU repair thread, IIRC (OK I was wrong on this I guess http://www.naxja.org/forum/showthread.php?t=953382&page=6 seems they are hot switching power to the injectors) the ECUs were repaired by replacing the ground path transistor on the ECU board for each injector that was not firing, which confirms that Renix ECU fires the injector by completing a ground path, not by supplying the voltage to the injector.

Did I miss something on that?

OK, page 55 of the MPFI manual it contradicts this. It says the ECU fires the injectors by supplying power to the injector momentarily, and claims a permanent ground to the injectors. NOTE: I found two pin out errors in the Fuel pump relay drawing in that same manual so I trust nothing in these now , page 52 pins 1 and 5 are miss-labled, back wards.

Page 91 ECU wiring shows a common ground to the injectors. I guess I will check my running 87 Renix to see where the faulty data is.

8Mud said:

The book says the injectors fire on the plus side and share a common ground. I've been through that harness, though not recently, seem to remember a bunch of splices in the ground circuit and many possibilities for mischief. I'd start out by testing the injector grounds, front to rear.

Many years ago I found a wire at the bend of the injector harness at the firewall, it had a fatigue break under the insulation. They even put out a TSB about fatigue breaks at this point in the harness. That was my 87 and the wire broke after only three years. You can feel the fatigue break under the insulation if you squeeze the wire. I got lucky and found mine pretty quick, the frayed wires under the insulation were black, blue and slightly melted.

I had a power issue with my MAP, no power to the MAP no injector pulse.

Click to expand...

Ecomike said:

Which book? Haynes electrical schematic shows 12 V + to the injectors from the fuel pump relay on 87-90
Pg 52 of the Renix MPFI manual shows pin 4 as power to the fuel injectors from the fuel pump relay. I have power to pin #5 from the ignition switch and power to pin # 1 from the battery fuel link, and the relay does click. I guess I need to test the relay for bad contacts, I forgot to verify the voltage out at pin 4 to fuel injectors, but I did try new relays, and got no change, so I suspect it is +12 volt wire to the injectors.

Also the Renix ECU repair thread, IIRC (OK I was wrong on this I guess http://www.naxja.org/forum/showthread.php?t=953382&page=6 seems they are hot switching power to the injectors) the ECUs were repaired by replacing the ground path transistor on the ECU board for each injector that was not firing, which confirms that Renix ECU fires the injector by completing a ground path, not by supplying the voltage to the injector.

Did I miss something on that?

OK, page 55 of the MPFI manual it contradicts this. It says the ECU fires the injectors by supplying power to the injector momentarily, and claims a permanent ground to the injectors. NOTE: I found two pin out errors in the Fuel pump relay drawing in that same manual so I trust nothing in these now , page 52 pins 1 and 5 are miss-labled, back wards.

Page 91 ECU wiring shows a common ground to the injectors. I guess I will check my running 87 Renix to see where the faulty data is.

Click to expand...

When I was trying to chase down the broken injector wire on my 87 I noticed all those ground splices in the injector harness and again when I was trying to chase down the ground fault in the TPS on my 88. I remember thinking that all those splices looked screwed up (like an abortion) and recipe for disaster. I could be wrong, I'm getting old and the brain locks on occasion (I call them brain farts).

8Mud said:

When I was trying to chase down the broken injector wire on my 87 I noticed all those ground splices in the injector harness and again when I was trying to chase down the ground fault in the TPS on my 88. I remember thinking that all those splices looked screwed up (like an abortion) and recipe for disaster. I could be wrong, I'm getting old and the brain locks on occasion (I call them brain farts).

Click to expand...

They look like they're held together with bands you'd use on a bird leg and then covered with a scrap of duct tape.

What, no chewing gum, just duct tape?

All the injectors are hot fired and share a ground and are spliced to ECU ground pin B11 and shares the same ground screw with B12.

The fuel injector ground also has a spliced wire that leads to the diagnostic connector 6 pin at pin #3.

Digger87xj said:

All the injectors are hot fired and share a ground and are spliced to ECU ground pin B11 and shares the same ground screw with B12.

The fuel injector ground also has a spliced wire that leads to the diagnostic connector 6 pin at pin #3.

Click to expand...

Correct. And they all ground at G105, engine dipstick stud.

cruiser54 said:

Correct. And they all ground at G105, engine dipstick stud.

Click to expand...

I wondered. Thanks.

The FSM diagram does not call out the ground point, just that they share that point.

Digger87xj said:

I wondered. Thanks.

The FSM diagram does not call out the ground point, just that they share that point.

Click to expand...

That's one reason I wrote the following:



Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
　
Revised 03-04-2013

And this one:

Cruiser’s Renix Sensor Ground Test
　
This sensor ground circuit affects the CTS, TPS, IAT, MAP, ECU and diagnostic connector grounds. It’s very important and not something to overlook in diagnosing your Renix Jeep as it is common for the harnesses to have poor crimps causing poor grounds. If any or all of the sensors do not have a good ground, the signal the ECU receives from these sensors is inaccurate.
Set your meter to measure Ohms. Be sure the key is in the OFF position. Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.
Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.
You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.
Revised 06/12/2012

Ecomike said:

Which book? Haynes electrical schematic shows 12 V + to the injectors from the fuel pump relay on 87-90
Pg 52 of the Renix MPFI manual shows pin 4 as power to the fuel injectors from the fuel pump relay. I have power to pin #5 from the ignition switch and power to pin # 1 from the battery fuel link, and the relay does click. I guess I need to test the relay for bad contacts, I forgot to verify the voltage out at pin 4 to fuel injectors, but I did try new relays, and got no change, so I suspect it is +12 volt wire to the injectors.

Also the Renix ECU repair thread, IIRC (OK I was wrong on this I guess http://www.naxja.org/forum/showthread.php?t=953382&page=6 seems they are hot switching power to the injectors) the ECUs were repaired by replacing the ground path transistor on the ECU board for each injector that was not firing, which confirms that Renix ECU fires the injector by completing a ground path, not by supplying the voltage to the injector.

Did I miss something on that?

OK, page 55 of the MPFI manual it contradicts this. It says the ECU fires the injectors by supplying power to the injector momentarily, and claims a permanent ground to the injectors. NOTE: I found two pin out errors in the Fuel pump relay drawing in that same manual so I trust nothing in these now , page 52 pins 1 and 5 are miss-labled, back wards.

Page 91 ECU wiring shows a common ground to the injectors. I guess I will check my running 87 Renix to see where the faulty data is.

Click to expand...

Me thinks the guy that did the schematic had the hiccups.

piss poor scan. originals look much better.

8mud, what year was that manual the scan came from? The 89 does not have all the dots showing all the wires connected to everything else and shorting it all out, LOL, like that one does. That one shows a direct short to ground, LMAO.

The 89 MPFI manual drawing is tiny and impossible to read with out a magnifying glass.

Ecomike said:

8mud, what year was that manual the scan came from? The 89 does not have all the dots showing all the wires connected to everything else and shorting it all out, LOL, like that one does. That one shows a direct short to ground, LMAO.

The 89 MPFI manual drawing is tiny and impossible to read with out a magnifying glass.

Click to expand...

It is from a PDF. I got on another site many years ago. I've found more than a few screw ups from various sources over the years. There is occasionally a credibility gap between what the books says and what is actually there.

Which direction the electricity actually flows is still up for debate as far a know :looney:. It really makes no difference, unless you are trying to chase down a power supply problem and the fault seems to be a system fault with separate power supplies and a common ground. Getting off on the wrong track can be time consuming, but IMO it is all a learning process anyway and I usually learn as much from my mistakes as I do from successes.

Up for debate?

RENIX uses a common ground and switches voltage + individually to each injector when in sequential mode. If it defaults to non-sequential mode, all injectors are fed + voltage at the same time.

The HO uses a common 12v feed and switched grounds. This is most common throughout EFI systems.

How is that debatable?

Digger87xj said:

Up for debate?

RENIX uses a common ground and switches voltage + individually to each injector when in sequential mode. If it defaults to non-sequential mode, all injectors are fed + voltage at the same time.

The HO uses a common 12v feed and switched grounds. This is most common throughout EFI systems.

How is that debatable?

Click to expand...

WHO wanted to debate it?

I tried to remove the ground nut at the dip stick to engine block location, but the stud came loose instead of the nut. Cycled it about 8 times, and tried pen lube, but the nut will not come loose, just the stud in the block. No room to work in there. I got the ground path back to the test injector #3, and the noid light flashes when I crank now, but still no ignition, does not try to run at all. Tried cranking at WOT many times last night and today, pulled one plug earlier and it was dry. No smell of gas at the tail pipe. Battery is still giving me great carnking speed, but will need charging before long at this rate. It has hit 100 F in the shade now and I am melting. :firedevil

I am seriously thinking of cutting the ECU etc ground wires at the dip stick and extending them to the battery, and leave the starter-block ground where it is. The starter works great, it has a solid ground there.

But now I am not sure what the new problem is. Time to recheck everything top to bottom.

Conclusion, the ground path must have been part or all of the problem for the injectors. I did get about 25 seconds of steady Noid light flashes while cranking at WOT the last try, and a great ground signal path on the one injector pigtail I was using to do my diagnostics tests. Best guess the dip stick ground stud had come loose.