eaglescout526

Sounds like fun doesn't it? Otherwise the book doesn't really say what else to do other than repair the harness.

So the book says it should be zero volts after engaging the starter motor. Then says repair the harness as necessary. Then it says to check the MAP sensor and repair as necessary. And of course the last test I wish I had is the MS 1700 ECU scanner.

I mean you could hunt down a special test bulb but something like a courtesy light would also work for the test bulb. The only other ground would be the one from the engine to the fire wall which you have cleaned and replaced in your pic. The injector pulses, that's all I know. My guess is it pulses and the fuel enters and the injector pulses it down into the throttle body in mist like fashion so the fuel atomizes when it gets to the combustion chamber. But yes there needs to be a good 14-15PSI of pressure for the fuel system. Everything except the CPS is VDC.

When I got my MJ my dad and I towed it with a car dolly with the front wheels on. Course we took back roads to get home.

Ok so here's a couple troubles from the TBI book Engine starts but won't idle: Fuel pump inoperative or ignition system malfunction. Check fuel pump. Check the ign system. Repair as necessary. Check the start signal after engaging the starter motor. Voltmeter from D1 pin4 to D1 Pin3 should indicate zero volts. Repair harness as necessary. Check MAP sensor. Repair as necessary. Engine will not start: Defective Wide Open Throttle swtich(you might have this if theres no pressure switch on any of your power steering lines.) Low Bat voltage Fuel supply low Low fuel pressure Fuel pump inoperative Defective secondary ignition circuit Primary ignition input to the ECU defective No bat voltage applied to the injector(12V from one pin top chasis ground) Injector Ohm resistance too high(less than 10 Ohms) ECU is not switching the injector Leaking injector No start signal voltage ISA motor plunger not extended Coolant temp sensor inoperative. These are the possible causes the book lists.

Good as in the pump turns on and runs. But you say you don't hear it for that one second with and without relay correct?

Me, I wouldn't rush out and get one just yet. We know the ECU will receive the voltage signal and tell the coil to spark which we can confirm by pouring fuel down the TB. So the CPS is not the entirety of the issue. We need to figure out why the pump won't turn on for one second and during crank/run. Has the pump been confirmed good before?

If you do that youll start to see your dream MJ.

What kind of MJ rabbit hole have we plunged into? Hopefully this won't be like the one where the hood lamp wire was plugged into the coil, but this is confusing to say the least. Yes check the CPS side. I think .3 is a good number. I know Ohm said 2.5VAC . Ok here is tip No.7 from cruisers website: "Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS itself as shown in Figure 2 as Connector A, crank the engine over. It won’t start with the CPS disconnected. You should get a reading of .5 AC volts." "If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from NAPA or the dealer." .32VAC is getting on the low end for the CPS output.

Well that's odd seeing how with the relay in you get spark and engine fires when pouring fuel down the throttle body. The volt meter is on the Volts AC setting right? This is the only component that requires the VAC setting.

No, he means you can unplug the sensor from the harness and test for Volts AC while cranking. I think he wants to make sure the CPS is sending out the proper signal. here's the 86 engine and fuel management diagram.

Lol no worries. We will find it. We will just keep running down the diag list until we get somewhere.

Lol good we have continuity there which means no breaks in the harness between the ECU to the relay. Now my 86 electrical manual shows the same as the 88 above but a little bit different but still the same for T6 to the relay. 86 shows to go to the B splice and is triggered by the ignition switch. 30 on the relay is power from fusible link F. 87 goes to splice K which seems to power a few different things. I will have to get a good pic of it in the morn.

It should be pin 85. TBI book says to check for continuity not necessarily voltage but it didn't hurt to see if the ECU is sending any down the line. I think I have an 86 electrical manual.

Does your voltmeter have a continuity test setting? Should be a symbol represented as what looks like sound waves by the dial. That would confirm weather theres a break in the wire(no beep) or not(beep) by the multimeter. But testing for volts is a good thing to check too.

Looking at the diagram I think you are just looking for to see if there is a break in the line between T6 and relay terminal 85. So once you get your probes in place, that's when you would do the key on test and see what the volt meter shows for voltage if any voltage goes to the relay.

So see where the relay is in the third pic of the second half? Looks like 86 jumps to 85 and goes into T6. I just realized I screwed up the pic alignment but where the relay goes down to the ECU in the diagram, it shows the EGR but it really goes to the area of the ECU diagram marked fuel pump relay which is T6.

I think that's why the book says to test T6 and the relay to make sure there is continuity there. T6 has the power to trigger the relay for that one second and then stop. While the relay itself waits for power from the start up procedure to run the pump. I only saw one associated with the fuel pump relay and a couple other things. Below is the top image split into four to see the words better. First half. Second half. This is from the PDF electrical manual. Hopefully this will help give an idea as to what we are looking at.

One would send power to the fuel pump and the other is the fuel gauge wire. I think the fuel gauge was C. Well I did some looking at the electrical book and saw a couple things. First I saw is there is a fusible link tied to the fuel relay harness. Second the connectors would be at the tank(1), second would probably go into the cab behind the kick panel then probably into the engine bay wiring harness that routes back to the ECU. Well I am learning more about the diagrams but ill admit the last two after the kick panel are a shot in the dark.

Yes and no. I am not sure if any one of the fusible links is even apart of the fuel pump harness, but i was just thinking about the contact tabs and such on the starter relay as those can corrode too and cause resistance or loss of power. Yes you can. No, I was thinking more using the connectors as breaks in the harness. Rather than getting really long leads and putting one in the relay harness and the other at the pump itself, I was thinking of using the connectors as testing sections so that way you aren't trying to dig into the entire harness when hunting down the problem. So that way when testing from one connector to the next and suddenly an open loop comes up in one section between two connectors, the problem could lie there. Ohm will probably correct my thinking if I'm wrong.

Lol fair enough. I just typed what the book has. But I am not great at explaining things to others in simple formalities but here goes. So when you turn the key, the pump comes on and primes. Common knowledge with Renix. Since its not coming on there probably an open circuit somewhere in the fuel harness between the pump itself, the ground and the relay to the ECU. Or possible corrosion of a contact or wire. So you already jumped the relay harness and still got nothing which means there is something going on in the harness. Which makes me wonder the condition of the starter relay connectors.(not entirely related but apart of the start up procedure per the ECU) Since the pump doesn't operate we can't check for voltage. Well you still could to see if there is any power at the relay contacts. Checking the continuity of the circuit to see if there is any power between the two ends. As for checking continuity between the pump and the relay, if I had the wiring diagram for the pump to relay, they probably mean for you to check each connection so like the fuel pump to its first harness connector, then from that connector to the next one until you reach the fault or relay contact. Then checking the ground circuit would be the same deal, making sure there are no breaks or corrosion between the connector and the ground behind the lamp. I think I explained it well enough. If not let me know.

That is correct! I forgot all about that. 80's to 90's GM coolant bottle caps do indeed fit. I know a guy who has a Fierro and he had that coolant cap and I tested it and sure enough it fit as it should.

Ok so the component diag section states this Fuel pump will not operate for one second with the ignition switch on Possible Cause: Open circuit or fuel pump ballast resistor defective, which Jeep didn't get the later till 87 from the factory. So you have already done the first part, connect a jumper wire across the fuel pump relay If the pump operates, check for battery voltage at the relay. Check the continuity betwenn the relay and Pin 6 at the ECU. Repair the wire harness as necessary. Replace fuel pump relay. Test ECU with MS-1700. Test for continuity between the fuel pump and relay contacts(long enough leads much?) Check the pump ground circuit. Repair the harness as necessary.

Well its doing something. But we couldn't get a clear picture on why the ground has an open circuit.

RPMs fluttering would be a possible MAP sensor issue or something with the EGR and its solenoid.