eaglescout526

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.

That's probably right, I just wonder why the couldn't label it more precise than just G110. But yes you have the ground post to engine block by alt bracket, then from bolt head to strap and then the dipstick ground, you also have that one on the drivers side fender but that does the blower motor, wiper motor and etc.

T8 goes to D2_6 to WOT then to splice D which grounds out at G110. Where ever that ground is at. 88 replaces the WOT switch for the PS pressure switch.

Hang on, uploading Pt II of the engine and fuel controls diagram that has the WOT, ISA and a few others.

Ok so the switch itself is good, low resistance at WOT and it should be infinite but I believe O.L. is the same, I think. Volt meters get weird around there. It passes the closed throttle voltage test but fails the WOT meaning there's something wrong with the harness which leads to testing for an open circuit in the wire or connector between the switch and ground.

Correct. To test the WOT you need to unplug it from the harness. Test the on off operation using a DVOM Digital volt ohmmeter Operate the switch The resistance should be infinite when the throttle is closed. A low resistance should be indicated when at the WOT position Test the switch several times Replace if defective(I don't even know if that switch is available) Reconnect it to the harness With the ign on test the WOT switch voltage at D2_6 to ground/D2_7 There should be no voltage reading at WOT position: greater than 2 volts if not at the WOT position If no voltage is present in either position, test for a short to ground in the wire harness or switch Check for open circuit between T8 of the ECU and the switch connector Repair or replace harness if necessary If the voltage is always greater than 2 volts, test for an open circuit in the wire of connector between the switch and ground.

Ok. Theres not a ballast resistor anywhere is there? And we have confirmed the pump runs for both one second and while cranking with jumped and relay?(just double checking) Do you have a Wide Open Throttle switch?(located on the throttle body by the bonnet and linkage) Book says to check MAP sensor but we can't even get 5V to it.

Yup that's factory.

If the MAP went bad it would struggle to accelerate right and idle either high or low. I wouldn't replace the TPS until the proper voltage gets delivered to both sensors. Like I said, they aren't getting the 5V and the sensors need the 5V to "run" themselves to send data to the ECU. I don't think throwing new sensors will solve the issue. Just for giggles, how do the spark plugs look?

I mean honestly the only person in the world I know who has the MS1700 is nickintime and I happen to have an ECU tester and so do 30 people on eBay. But the only absolutely last thing is the ECU and wiring harness.

That’s why we never did the adjustment because of the MAP sensor 5V. Something is bizarre so I am not certain if a new ECU will rectify the 5V problem.