Renix Confusion

[Francesco]

Hey guys,

My truck has me stumped. I pulled apart the engine harness this morning to see if I had any issues and do the Cruiser54 ground fix, even though my harness was ohming out totally fine, and the harness looks perfect inside. My issues is I'm getting spark, but no injector firing. Injectors all have continuity to ground, and I tested their continuity to the switches at the ECU harness and they were fine. My ECU has a burnt diode on it, and am in the process of getting a new ECU, however I need to make sure that there wasn't something that caused the ECU to go bad in the first place. Plus, when I connect the ECU, all of the wiring in the car instantly gets weird, as far as continuity between ground and power, etc. I'm hoping it's all an ECU issue, because once I unplug it goes away. Car used to run fine and I've had it for several years so I know there isn't some issue that it's been improperly rewired. Here is what I've done so far:

Tested all sensors (CPS @ 220 ohms and 0.55V)

Distributor, fuel pump, FPR, CPS, and all sensors have been replaced within 2 years (fuel pump and dizzy last month)

Has just under 40 psi fuel pressure cranking

Starts and runs on VERY rare occasions, so I know it's not a timing issue, etc.

I have done all Cruiser54 fixes (add grounds, clean ECU, clean connectors, etc)

Let me know if you guys have anything else I can test!

[cruiser54]

You've done a great job so far. 

I've seen ECUs lose individual injector drivers. Not ever all of them. 

I'll add this ECU pin out.....

 

 

[MiNi Beast]

Awesome info there.  thanks for adding that. 

 

Should help to test things for sure. Id almost source another ecu to swap out for testing, but only cause I'm not good at the testing procedures. can't say I wouldnt be surprised if its a bad ecu. 

[cruiser54]

It could be. 87 to 90 will work. Stick or auto. 

[scaleless]

When a diode or transistor burns up it can fail open-circuit or closed-circuit.  If the latter it'll probably cause unintentional paths to ground which will likely cause failures elsewhere and testing may give very weird readings.

[JMO413]

Sounds to me like a ECM can't hurt if you know yours is burnt. I updated mine too the 89-90. Got it from AutoZone of all places and it was $150 if remember right.

[Francesco]

Hey everyone, I got a replacement ECU off of Ebay with no fix of my issues. I tested all injector wires, all have continuity to ground on one side and proper continuity with the ECU connectors on the other end. Not sure what to test next, I'm stumped. I ripped apart the harness, plan on soldering the crimps and fixing all that ugly, but found zero issues with the harness itself as well. 

[Ωhm]

C201_C11 & C201_D10 both are INJ FEED circuits for the ECU. B+ from the Fuel Pump Relay.

 

 

 

[Francesco]

Okay, I will test the wiring from the B+ relay to the ECU, and make sure it's getting power. 

[Francesco]

Do you have the entire route? Key on from ignition switch goes to b+ relay to fuel pump relay to ecu, etc? 

[Ωhm]

2 hours ago, Francesco said:

Okay, I will test the wiring from the B+ relay to the ECU, and make sure it's getting power.

I'm going to stop using the term B+. My meaning was for Battery Voltage. The ECU receives Battery Voltage from the Fuel Pump Relay for the INJ Feed circuit.

 

 

 

1 hour ago, Francesco said:

Do you have the entire route? Key on from ignition switch goes to b+ relay to fuel pump relay to ecu, etc? 

The 88 Electrical Manual should have the same set up as your 90.

 

 

 

_mj1988electricalmanual_1.pdf

[Francesco]

Thank you, I will try and test that today / tomorrow and see what comes from it. 

[Francesco]

So the B+ Latch relay wiring tested pretty solid, it had some resistance when connected to ground, however I tested the wiring from ECU to relay and it was solid. I will continue to follow that wire, as well as test the fuel pump relay. 

[Francesco]

So I should key on and check for voltage at the INJ Feed pins? 

[Francesco]

So while cranking I have 1.6V on my voltmeter on hold on both INJ Feed pins, and have continuity to the fuel pump relay pin (87 I think). 

[Francesco]

So after the ecu swap it turns out I do have injector pulsing now, however still a no start. I have spark confirmed as well, just went to TDC And checked I didn’t put the distributor on backwards. 

[Francesco]

Hey everyone! So I got the truck running, turns out it was the ecu after all, however now it is running a bit rough, and you can feel it misfire every few seconds and stumble when giving it gas. I put new plug wires and plugs on there as well, because the old ones were old, and the plug wires weren’t seating well. I am going to index the distributor this evening. 

[MiNi Beast]

Awesome glad to hear that.  

Id inspect the crank sensor (cps) as well, as they are a common culprit on these things. 

Gotta love this site eh?