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

[Ωhm]

Since I've got a 87/88 schematics and we're dealing with a 86, things are sketchy.

Disconnect both TPS and MAP connectors. Connect DVOM to B- and on each connector one pin should have 5vdc on it. KEY ON. If true, note which pin_letter it on (A, B or C). One of the pins may show a lower voltage then that. Note which pin that is too.

[eaglescout526]

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

[Ωhm]

I'll check it out.

[Ωhm]

Between the three (3) schematic (86, 87 and 88), TPS is a mess for INPUT and GND circuits.

[eaglescout526]

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

[Ωhm]

Both 86 & 87 show TPS wiper circuit as going to GND, but 88 has it as INPUT. 88 is correct physically.

[SoCalManche]

33 minutes ago, Ωhm said:

Since I've got a 87/88 schematics and we're dealing with a 86, things are sketchy.

Disconnect both TPS and MAP connectors. Connect DVOM to B- and on each connector one pin should have 5vdc on it. KEY ON. If true, note which pin_letter it on (A, B or C). One of the pins may show a lower voltage then that. Note which pin that is too.

 

26 minutes ago, eaglescout526 said:

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

 

 

These are what I got:

 

MAP: A = 4.02V / B = 3.99V / C = 9.06V

TPS: A = 3.98V / B = 4.02V / C = 9.06V

[Ωhm]

That ain't right. I need to chew on this for awhile.

[SoCalManche]

1 minute ago, Ωhm said:

That ain't right. I need to chew on this for awhile.

Please do, this is why EagleScout and I have been going at these two connectors for some time now. Hopefully this will pick your mind more.

[eaglescout526]

14 minutes ago, Ωhm said:

That ain't right. I need to chew on this for awhile.

Can't tell ya how many times ive come close to beating my head on the wall with this one. Lol. Real brain teaser. 

 

12 minutes ago, SoCalManche said:

Please do, this is why EagleScout and I have been going at these two connectors for some time now. Hopefully this will pick your mind more.

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.

[SoCalManche]

35 minutes ago, eaglescout526 said:

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.

Does this help?

 

I went back out to try the (B) and (C) again. Before doing so, I checked each pin of both connectors with ground probe at Batt Neg post and power probe at each pin. The readings basically stayed the same; actually went up about 0.1V. Anyways, after that, I tried the (B) & (C) for both connectors (unplugged) and got 5V. Ground probe on (B) and power on (C).

[eaglescout526]

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

[SoCalManche]

3 minutes ago, eaglescout526 said:

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

But we still have 9V on (C) when at Batt Neg. Still weird, yes?

[eaglescout526]

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.

[Ωhm]

1 hour ago, SoCalManche said:

These are what I got:

 

MAP: A = 4.02V / B = 3.99V / C = 9.06V

TPS: A = 3.98V / B = 4.02V / C = 9.06V

The difference between B and C on both connectors is about 5vdc.

[SoCalManche]

11 minutes ago, eaglescout526 said:

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.

No love; it did fire, but only because I threw some fuel down the TBI. Sounds and smells like a damn lawnmower when starting up, lol.

 

Fuel pump did not engage, per normal, with fuel relay in circuit.

[SoCalManche]

I never see the injector "mist" any fuel into TBI.

[eaglescout526]

3 minutes ago, SoCalManche said:

Fuel pump did not engage, per normal, with fuel relay in circuit.

So no pressure was seen on the rail?

[SoCalManche]

Just now, eaglescout526 said:

So no pressure was seen on the rail?

Nope; no pressure. I dumped enough fuel into the TBI for her to maintain an idle briefly though. But no fuel being pushed.

[eaglescout526]

@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.

[Ωhm]

Pin 85 is strictly the grounding side for fuel pump relay (coil) control. Jumping pin 30 to 87 just does what the relay should do when controlled by the ECU.

[eaglescout526]

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]

8 minutes ago, eaglescout526 said:

Cause I wonder if the ECU isn't seeing the relay so it won't fire unless there's a connection there.

Other inputs determine when FP relay is activated. KEY ON will prime the pump. CRANK will keep FP relay activated. CPS will demand FP relay at all times.

[Ωhm]

1 minute ago, eaglescout526 said:

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.

That's when I first used the word "confusing".

[eaglescout526]

11 minutes ago, Ωhm said:

That's when I first used the word "confusing".

Lol which is why this post is 10 pages long.