eaglescout526

Ok so the note in the book says that the CTS test is to be done with the plunger fully extended as it would be after normal engine shut down. If it is necessary to extend the ISA plunger to test the switch, an ISA motor failure can be suspected. So by watching your videos, is the ISA motor tip in contact with the throttle plate? If not then the motor is jammed, but the motor can be taken apart, cleaned and lubed up. We can get back to testing the CTS in a second. Was there any change with the jumped relay and opening the throttle?

Does D2_13 show any voltage or resistance? T25 on the ECU should show the same thing at D2_13

Yeah just a little bit. See we missed something because my dumbass thought that your ISA is functioning. Welllll one of the possible causes is the ISA plunger is not extended thus causing a no start condition. I am wondering if the closed throttle switch in the ISA is not triggering something in the ECU that energizes the fuel pump relay. Yes the CPS can still tell the ECU to send spark.

This makes me want to do mine now. Yall gotta stop giving me ideas. Did you wire wheel them?

I just realized something after watching those videos for the 5th time. After shut down or failure to start while cranking, the ISA should return to open throttle position for the next start. Not once did I see the ISA do that. What if you jumped the relay but used the accelerator pedal to open the throttle just enough to fire. I am almost wondering if during crank the injector is pulsing the fuel that is being pumped from the jumped relay and its not enough fuel to get down in there because the throttle plate is closed. But when putting enough fuel down the throttle body yields a successful start but rich condition because of the lack of O2 from the closed throttle plate.

So 1986 used the ECU with AMC number 8953002099 87-90 uses 8953004763 or Mopar 53004763. Both these numbers are for manual trans. 86 auto used AMC 8953002100 87-90 auto used 8953004764 or Mopar 53004764 Nothing is mentioned on both switches just model year.

Ok I got my parts books. I think 86 only had the power steering relay as I can find that in my 86/87 parts catalog. Let me get the ECU numbers.

So that switch probably traces back to the power steering relay then. So I wonder what the purpose of the power steering relay is, especially with the amount of wires that go into it.

Yup that's the switch. In the 88 electronic manual it shows that switch in place of the WOT switch but both my hard copies of the 86 and 87 electronics manual shows only the WOT switch instead of the power steering pressure switch. But I never found anything on the PS circuit.

Well I want to say the 84-86 but mostly 86 had the WOT switch. 87-90 that switch was replaced with the power steering pressure switch. It gives the ECU a better reading on if the engine is at WOT or not via the pressure of the power steering fluid. I will go check my schematics to see if that switch is in the circuit for 86. Can you trace the wire and see where it goes? I kinda am thinking it could be contradictory.

I saw the fuel pressure for both, so that's definitely weird. One thing I did see was that you have the Power steering pressure switch and the WOT switch.

Lol which is why this post is 10 pages long.

Then in theory the engine should still run while jumped, but it does not, but runs with relay in socket with fuel poured down the throttle body.

@Ωhm, what would happen if he jumped 30 to 87 and 86-85 on the FP relay? Do we think the engine would actually run or is the relay regulating a safe load on 86 to 85? Cause I wonder if the ECU isn't seeing the relay so it won't fire unless there's a connection there.

So no pressure was seen on the rail?

Eh yes and no. Yes cause it should be 5V no matter which ground the reading is from but now we get 5V through the ECU ground and 5V supply, so that part of the ECU is not bad. Makes me wonder if it will fire.

So we have 5V now? Hey MJ! Make up your mind! Perhaps we jostled a connection in the harness back to life?

Can't tell ya how many times ive come close to beating my head on the wall with this one. Lol. Real brain teaser. The only absolute thing I can think of if the ECU has gone bad is something in the 5V rail has gone bad and is sending more voltage than it should down the line. But even with negative lead in B and checking for 5V at C we still don't get anything close, seems like it is high or it is low, never at 5V.

TPS seems like a bigger mess in 88(C101) than the 86 being a direct line to the ECU.

I have an 86 schematic book and I did post pics of that on like page 6 or 7 of the engine management section.

Id be kinda surprised if nothing showed it self. This kind of ordeal makes me want to have a few 2.5L ECU's for testing/back up.

I see why you were never looking forward to tearing into it....you got your work cut out for ya.

Hmm, just wondering if maybe a small rodent got to a wire somewhere, alrighty. Lets try again tomorrow and see what we get.

Do you know if the truck has been attacked by rodents before?

An open circuit of some sort. So somewhere is an open circuit of either corrosion, damaged wire, loose or disconnected wires.