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Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go hereXJ (84-01)
All OEM related XJ specific tech. Examples, no start, general maintenance or anything that's stock.
Ok. I have a 1990 Renix XJ. 255k on the clock. All sensors are new, new ignition system, cooling system, basically I’ve replaced or rebuilt everything except the engine and trans. The other day driving it home from smog it shut off on the freeway. No Spark. Ended up being the B+ Latch relay. Got that sorted out. Now I have good spark, compression at 100 in all cylinders. I have 40 pounds at the fuel rail while cranking it. It’ll fire if I use ether. The injectors just won’t fire. I do not know what else to test or look at. I’m tempted to just go order a reman ECU right now and throw that at it. I checked the orange fusible link at the starter relay and that was good….I haven’t checked the MAP sensor. Could that cause no injector pulse? Injectors are good, too. Less than 1k miles on them. Flow matched 746’s. CPS, distributor and coil are all good. They checked out, but I pulled them and had them replaced on warranty yesterday anyway (AutoZone limited life time). Any ideas on what to look at would be appreciated. No injector pulse was confirmed with a noid light.
There is a common ground for all 6 injectors. There are two large splices that are prone to failure along that ground. This particular ground starts at the dipstick, this is also the path that provides ground for the ECU. It may be prudent to refresh that ground, or at least check your Ohms between the injectors and the negative on the battery. In the diagram you will see the injector grounds in pink. The wagon wheel things are the factory splices. Following the pink to the far right you see the two grounds on the dipstick.
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.
All the components listed below ground at the dipstick tube stud:
Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition Control Module, Fuel Injectors, ECU main ground (which other engine sensors ground through, including the 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. If the whole stud turns, you can use a 7/32″ six point socket or wrench to hold it so the nut can be removed. Worst case, cut the wires and remove the stud and nut. Install new terminal eyelets on the wires when going back together. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Surfaces 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. Apply a liberal coating of
, which is available at Lowe’s and other stores. Reinstall all the wires to the stud and tighten thhttps://i0.wp.com/cruiser54.com/wp-content/uploads/2015/10/dipstick-stud.jpge 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, apply
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 its intended use and subject to corrosion and poor connections at each end.
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. Apply
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 with a liberal coating of
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 tail lamp 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. Add a 10 gauge wire, with an eyelet on each end, from that grounding point to a bolt on the frame. Better yet, on both Cherokees and Comanches, complete Tip 29 for the best fuel pump grounding. Be sure to scrape all mounting points to bare metal and apply
also.https://i0.wp.com/cruiser54.com/wp-content/uploads/2015/10/Fuel-Pump-Ground-Comanche.jpg
If you want to UPGRADE YOUR GROUND AND BATTERY CABLES with custom made parts, contact Paul at www.jeepcables.com
I had an intermittent misfire on #3 for a long time. Finally tore open the harness along the valve cover to uncover that common ground for all the injectors. Quite a bit of corrosion. I replaced all the injector connectors, ran a fresh ground wire from #3 and redid the splice. All good now.
Thanks for all of the reply’s. The grounds are all cleaned and upgraded. I even have additional grounds from my auxiliary battery. Injector grounding is good. I’m getting no power to the injectors from the ECU. I did check the MAP just now. I have 5V incoming and 4.6 out. That’s good. All relays are good. Spark is good. Compression is fine. Just no power to the injectors. I guess I’ll order an an ECU. It’s the only thing left unless there is a relay or fuse I don’t know about.
Yeah. You’re right. The more I thought it over I didn’t think it could be the ECU out put to the injectors. I started thinking the distributor stator input to the ECU was bad. I cut open the wire harness while waiting for the new ECU to come in and found injectors one and two had the wires melted and were shorting it out. I’m repairing those now and then I’ll check the remaining 4. In let this thing sit in the street until this morning when my neighbor came over to tell me “it’s a lawsuit waiting to happen”. So, he towed me up my driveway. This is the first real night I’ve had to put my hands on it.
anyway, yeah. Looks like it wasn’t an ECU problem. I may have created one. We’ll see. But the problem was the injector wires melted and were shorting. Hopefully in starts in the next hour.
Yep. Still not starting. I guess that wiring may have sent a short. I opened up the whole harness last night and couldn’t find anything else. See what happens with the ECU tomorrow
Shorted wires to the injectors typically just blow the individual drivers on the ECU. Curious that you didn't have spark to any of them, indicating all six were blown. It could be having two shorted at the same time burnt something upstream of the injector driver inside the ECU.
Any indication what caused the short? Frayed wires that heated up, chaffing, damage, etc? One common theme with damaged wires in the harness, especially where it crosses from the injectors to the firewall is bad motor mounts allowing excessive movement.
10-05-2023, 08:00 PM
No Injector Pulse
