'86 MJ 2.5L ECM....good?

[SoCalManche]

5 minutes ago, eaglescout526 said:

Yes, that will show you your output voltage with key one engine off at the sensor. Terminal 33 on the ECU should show the same output voltage.

Ground testing the MAP for the ECU is done with probe in A and probe in terminal 17 of the ECU. You can also check the Ohms of the ground at the sensor and the battery to see how bad the resistance is.

5V is on C and use A for the ground. Then check terminal 16 on the ECU for the same 5V.

 

For the TPS is that output or the 5V input?

 

How do I know which terminals are which? Like I said, kind of a diagnostic novice. If this info is online to reference, I can do that instead of wasting your time, lol.

 

17 minutes ago, eaglescout526 said:

Also the TPS input should be 5.0V with the throttle wide open and engine off at B and C. Jeez its been a while since ive done the TPS. I had them backwards. A is the output on the TPS and that should be 4.6V-4.7V. My bad. Good thing I have this book. 

Per this response, my TPS is reading 3.65V at Terminal A.

[eaglescout526]

10 minutes ago, SoCalManche said:

How do I know which terminals are which? Like I said, kind of a diagnostic novice. If this info is online to reference, I can do that instead of wasting your time, lol.

 

Per this response, my TPS is reading 3.65V at Terminal A.

I don't think the TBI book is a PDF yet. It is a goal to get my books into a PDF.

Ok here is the 86-90 2.5L* connector pin out: 

 

To adjust the TPS you leave the probes in the connector and loosen the screws on the sensor till you can move the sensor back and forth but not incredibly loose otherwise it will be hard to adjust the sensor. Once the screws are loose(again not all the way out just enough to move it with some effort), move the sensor back and forth until you achieve somewhere between 4.6V-4.7V. I have mine at 4.64V but yours may differ. 

 

 

*book shows our 2.5L as a 2.46L which is the total Litres of our engines but AMC rounded it up to save breath. 

[SoCalManche]

13 minutes ago, eaglescout526 said:

I don't think the TBI book is a PDF yet. It is a goal to get my books into a PDF.

Ok here is the 86-90 2.5L connector pin out

 

To adjust the TPS you leave the probes in the connector and loosen the screws on the sensor till you can move the sensor back and forth but not incredibly loose otherwise it will be hard to adjust the sensor. Once the screws are loose(again not all the way out just enough to move it with some effort), move the sensor back and forth until you achieve somewhere between 4.6V-4.7V. I have mine at 4.64V but yours may differ. 

Well this is definitely a piece of information that I've needed! Haha

 

Do you have super long leads bought separately from the multimeter? My leads definitely won't reach...

[eaglescout526]

Yes I do. My dads fluke leads are really really long so that's my bad for thinking you had some that are of good length. 

[SoCalManche]

Just now, eaglescout526 said:

Yes I do. My dads fluke leads are really really long so that's my bad for thinking you had some that are of good length. 

No worries! I'll go buy some tomorrow and then do some tests and get back with the info. 

[eaglescout526]

Sounds good. Hopefully we can get to the bottom of this issue of yours.

 

Also fun little note. The book strongly recommends the ECU be the last thing to replace as they are reliable. 

[SoCalManche]

 

On 5/1/2020 at 10:24 PM, eaglescout526 said:

Sounds good. Hopefully we can get to the bottom of this issue of yours.

 

Also fun little note. The book strongly recommends the ECU be the last thing to replace as they are reliable. 

Okay, so I got a different multimeter (needed a more legit one anyways as I'm slowly learning the ways of the 2.5L electrical gremlins), and wanted to get a baseline check with everything we did two days ago just to make sure we were good to go. Well even though I wasn't worried about the MAP sensor, now I am confused.

 

If I test for voltage for MAP at Terminal C, with ground probe at battery post, I get 10V. If I throw the ground probe in Terminal A and retain the power prove in Terminal C, I get 155mV. If I disconnect ECU and retain ground prove in Terminal A, and put power probe in Terminal 16, I get a baseline reading of 83mV with a slow, but steady increase; I'm sure if I continued to hold it there, it would continue to rise.

 

I also tried both multimeters to verify it wasn't a bad multimeter.

[eaglescout526]

7 minutes ago, SoCalManche said:

If I test for voltage for MAP at Terminal C, with ground probe at battery post, I get 10V. If I throw the ground probe in Terminal A and retain the power prove in Terminal C, I get 155mV. If I disconnect ECU and retain ground prove in Terminal A, and put power probe in Terminal 16, I get a baseline reading of 83mV with a slow, but steady increase; I'm sure if I continued to hold it there, it would continue to rise

Ok lets recap MAP sensor

A-Ground

B-Output voltage

C 5 Volts.

 

Since the ECU is a computer and all computers love 5V systems, the sensor should get 5V

Terminal 16 is MAP supply voltage

Terminal 17 is MAP ground.  

 

So you tested the ground to the supply voltage, which means you got a reading but not the right reading we are looking for.

 

Try A and T17

Then try C and T16 and see what comes up.

[SoCalManche]

5 minutes ago, eaglescout526 said:

Ok lets recap MAP sensor

A-Ground

B-Output voltage

C 5 Volts.

 

Since the ECU is a computer and all computers love 5V systems, the sensor should get 5V

Terminal 16 is MAP supply voltage

Terminal 17 is MAP ground.  

 

So you tested the ground to the supply voltage, which means you got a reading but not the right reading we are looking for.

 

Try A and T17

Then try C and T16 and see what comes up.

A + T17 = ~63mV reading.

C + T16 = 0mV reading (Auto'd out to mV).

[eaglescout526]

And you're using VDC right? Test Grounds by using Ohms, we don't need to see voltage on ground circuits just resistance and open circuits. Buuuuuuttttt if we see voltage on any of the grounds then we have a problem.

[SoCalManche]

7 minutes ago, eaglescout526 said:

And you're using VDC right? Test Grounds by using Ohms, we don't need to see voltage on ground circuits just resistance and open circuits. Buuuuuuttttt if we see voltage on any of the grounds then we have a problem.

I used DCV for the readings. If that is wrong, I apologize. 

 

If I switch to ohms, the readings for either remains at "0.L"

[eaglescout526]

Just now, SoCalManche said:

DCV

VDC DCV still DC voltage.

 

Hmmm. This is interesting then. Try A to the Neg battery post and see what Ohms we get. We shouldn't get an open loop between A and T17 but we should be getting something at C+T16. Now we do have the key on engine off when testing right?

[SoCalManche]

10 minutes ago, eaglescout526 said:

VDC DCV still DC voltage.

 

Hmmm. This is interesting then. Try A to the Neg battery post and see what Ohms we get. We shouldn't get an open loop between A and T17 but we should be getting something at C+T16. Now we do have the key on engine off when testing right?

Let me clarify so that if I'm testing wrong, you can point it out. All testing has been done with key ON, engine OFF, ECU unplugged (since we've been doing ECU terminal testing too).

 

I tried testing ohms from Batt Neg to Terminal A, and got "0.L" again. This was key ON, engine OFF, ECU plugged in. I don't know if ECU plugged/unplugged makes a difference.

 

 

[eaglescout526]

Youre doing it right. I just wanted to make sure that you have been testing with key on.  I'm not sure how much of a diff the ECU unplugged or plugged in makes but it could help rule the ECU out. How does the dipstick ground look on the block? Clean and shiny? Most if not all the sensors ground out at that grounding point. 

 

Just for giggles ohm test the ground post to the engine ground strap on the fire wall and see if it will say O.L. I will admit it can be tricky to get good contact while back probing the connectors. 

[SoCalManche]

1 minute ago, eaglescout526 said:

Youre doing it right. I just wanted to make sure that you have been testing with key on.  I'm not sure how much of a diff the ECU unplugged or plugged in makes but it could help rule the ECU out. How does the dipstick ground look on the block? Clean and shiny? Most if not all the sensors ground out at that grounding point. 

 

Just for giggles ohm test the ground post to the engine ground strap on the fire wall and see if it will say O.L. I will admit it can be tricky to get good contact while back probing the connectors. 

Yes, the dipstick tube stud is clean. I refreshed it last year while I was initially trying to trace down this problem. I will go check the firewall-block ground, but I want to be transparent and mention I did replace the strap with a 4AWG wire (per Cruiser's tips) after refreshing the contact points.

[SoCalManche]

Welp, looks like we're getting somewhere. Ohms readout:

 

- Firewall: ~12

-Rear engine block: ~10.5

-Dipstick stud: ~10.3

[eaglescout526]

2 minutes ago, SoCalManche said:

I did replace the strap with a 4AWG wire (per Cruiser's tips) after refreshing the contact points

That's fine, it will help for better grounding.

 

Thinking to myself, you won't have the C101 cause its an 86 so no tar to clean out. Getting O.L.

Ok I'm going to type verbatim what the books says.

 

MAP Sensor test:

 

Inspect the MAP sensor vacuum hose connections at the throttle body and sensor. Repair as necessary

 

Test the MAP sensor output voltage at the MAP sensor connector terminal B(as marked on the sensor body) with the ignition on and the engine off

 

Output voltage should be 4 - 5 volts.

Note: The voltage should drop to 1.5 - 2.1 volts with a hot, neutral idle speed condition(not applicable cause you can't get the engine to run)

 

Test ECU T33 for the same voltage described above to verify wire harness condition. Repair as necessary.

 

Test the MAP sensor supply voltage at the sensor connector terminal C with the ignition ON. The voltage should be 5 volts(+or- 0.5V)

 

Five volts should also be at T16 of the ECU wire harness connector. Repair or replace the wire harness as necessary.

 

Test the ECU with diag tester MS-1700 if necessary.(Wish we could find one)

 

Test the MAP sensor ground circuit at sensor connector terminal A and ECU connector T17. Repair the wire harness if necessary.

 

Test the MAP sensor ground circuit at the ECU connector between T17 and T2 with an ohmmeter.

 

If the Ohmmeter indicates an open circuit inspect for defective sensor ground connection on the flywheel/drive plate housing(bell house) near the starter. So the dipstick ground is what this is for us. Book covers three engines with the same TBI set up.

 

If the ground connection is good, replace the ECU. 

 

If T17 has a short circuit to 12 volts correct this condition before replacing the ECU. 

 

9 minutes ago, SoCalManche said:

Welp, looks like we're getting somewhere. Ohms readout:

 

- Firewall: ~12

-Rear engine block: ~10.5

-Dipstick stud: ~10.3

 

So there's a little bit of resistance after all.

[eaglescout526]

See if you can back probe the ECU connector. If you can I would plug the ECU back in and do some back probing.

 

Oh wait never mind its a giant plastic plug that can't be back probed if I remember correctly.

Edited May 3, 2020 by eaglescout526

[SoCalManche]

10 minutes ago, eaglescout526 said:

That's fine, it will help for better grounding.

 

Thinking to myself, you won't have the C101 cause its an 86 so no tar to clean out. Getting O.L.

Ok I'm going to type verbatim what the books says.

 

MAP Sensor test:

 

Inspect the MAP sensor vacuum hose connections at the throttle body and sensor. Repair as necessary

 

Test the MAP sensor output voltage at the MAP sensor connector terminal B(as marked on the sensor body) with the ignition on and the engine off

 

Output voltage should be 4 - 5 volts.

Note: The voltage should drop to 1.5 - 2.1 volts with a hot, neutral idle speed condition(not applicable cause you can't get the engine to run)

 

Test ECU T33 for the same voltage described above to verify wire harness condition. Repair as necessary.

 

Test the MAP sensor supply voltage at the sensor connector terminal C with the ignition ON. The voltage should be 5 volts(+or- 0.5V)

 

Five volts should also be at T16 of the ECU wire harness connector. Repair or replace the wire harness as necessary.

 

Test the ECU with diag tester MS-1700 if necessary.(Wish we could find one)

 

Test the MAP sensor ground circuit at sensor connector terminal A and ECU connector T17. Repair the wire harness if necessary.

 

Test the MAP sensor ground circuit at the ECU connector between T17 and T2 with an ohmmeter.

 

If the Ohmmeter indicates an open circuit inspect for defective sensor ground connection on the flywheel/drive plate housing(bell house) near the starter. So the dipstick ground is what this is for us. Book covers three engines with the same TBI set up.

 

If the ground connection is good, replace the ECU. 

 

If T17 has a short circuit to 12 volts correct this condition before replacing the ECU. 

 

 

So there's a little bit of resistance after all.

So right out the gate, the first test is failing. Ground probe at Neg Batt Post, power probe at Terminal B = 10V.

 

Then, in order to probe E33, I took ground probe to E33 while leaving power probe at Terminal B. This gave me 0V.

[SoCalManche]

Ha, yeah. Can't be back-probed.

[eaglescout526]

Just now, SoCalManche said:

Ha, yeah. Can't be back-probed.

 

Yeah not like the 4.0L when I pulled an ECU for another member

 

Lets do the last step and see what Ohm testing T17 and T2 gets.

 

I want to ask, what kind of shape is the engine harness in in it entirety?

[SoCalManche]

7 minutes ago, eaglescout526 said:

 

Yeah not like the 4.0L when I pulled an ECU for another member

 

Lets do the last step and see what Ohm testing T17 and T2 gets.

 

I want to ask, what kind of shape is the engine harness in in it entirety?

Can you specific how I am supposed to probe? Am I probing between the two at the same time? If so, it reads O.L. 

 

I'll send pictures of how the harness looks. In my opinion, it seems in pretty good shape for being 33 years old.

[SoCalManche]

[eaglescout526]

7 minutes ago, SoCalManche said:

Am I probing between the two at the same time? If so, it reads O.L.

Yes and if you did it right and according to the book O.L. indicates defective ground. Ok this is good, meaning the ECU isn't being fried by backfed 12V. 

 

So we can say there definitely is some sort of ground or wiring problem. So I know from goofing around the engine won't run without the MAP or the CPS(obviously the CPS). Theres something funky going on and you upgraded and cleaned the grounds per Cruisers tips. Did you by chance get the ground from the post to the alt bracket I believe? I know on my 88 the ground goes from the post to the alt bracket.

 

And you have confirmed spark too right? Just engine won't run.

[SoCalManche]

1 minute ago, eaglescout526 said:

Yes and if you did it right and according to the book O.L. indicates defective ground. Ok this is good, meaning the ECU isn't being fried by backfed 12V. 

 

So we can say there definitely is some sort of ground or wiring problem. So I know from goofing around the engine won't run without the MAP or the CPS(obviously the CPS). Theres something funky going on and you upgraded and cleaned the grounds per Cruisers tips. Did you by chance get the ground from the post to the alt bracket I believe? I know on my 88 the ground goes from the post to the alt bracket.

 

And you have confirmed spark too right?

I don't have a ground going to alt bracket, but rather from neg post to radiator bracket.

 

Yes confirmed spark. I can dump some fuel into throttle body and it will fire until all fuel added was burned off.