TPain Posted February 12, 2012 Share Posted February 12, 2012 Hi guys Car: 1985 GL-10 4x4 Turbo Wagon Automatic; 51000+ miles I'm not getting a ground signal INTO my ECM which is preventing my fuel pump from working; as a temporary fix, I grounded the fuel pump at the source. I'm about to check all the visible ground locations under the hood but wanted to get you're thoughts too before I begin trouble shooting. Thanks! Link to comment Share on other sites More sharing options...
MilesFox Posted February 12, 2012 Share Posted February 12, 2012 The fuel pump wires come into the car from under the seat, and run along the trans tunnel, across the foolr in front of the seat, and join the harness at the passenger door sill up towards the dash. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 12, 2012 Share Posted February 12, 2012 (edited) The fuel pump wires come into the car from under the seat, and run along the trans tunnel, across the foolr in front of the seat, and join the harness at the passenger door sill up towards the dash. The ground wire from the pump to the ECU has been tested and is apparently good. But pin #50 on the ECU where that wire connects has no ground. So one of the grounds for the ECU itself is bad. Different ECU's were tried and that is not the problem. The problem is with a ground supply for the ECU not for the pump. I have an '85 FSM and have looked for information on ground point locations and I cannot find anything to help him. Sadly the car is three+ hours from me so I can only help over the phone and online. I have never had this problem nor have I worked on many running 85/86 MPFI/Turbo systems so I don't know where those ground points would be... it's a vexing problem to be sure. GD Edited February 12, 2012 by GeneralDisorder Link to comment Share on other sites More sharing options...
TPain Posted February 12, 2012 Author Share Posted February 12, 2012 I'm gonna check all the obvious grounding points. I believe there's a main one on the manifold? Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 12, 2012 Share Posted February 12, 2012 I'm gonna check all the obvious grounding points. I believe there's a main one on the manifold? Generally speaking the ground(s) for the ECU are located on the intake manifold sub-harness to insure that the 5v powered sensors on the manifold have a very tightly controlled ground loop back to the ECU. But I only have experience with SPFI systems (86+ throttle body non-turbo injection) and new stuff from the 90's. I would also be checking for grounds under the dash in/around the fuse panel, steering column, and ECU. GD Link to comment Share on other sites More sharing options...
naru Posted February 12, 2012 Share Posted February 12, 2012 (edited) Hi guys Car: 1985 GL-10 4x4 Turbo Wagon Automatic; 51000+ miles I'm not getting a ground signal INTO my ECM which is preventing my fuel pump from working; as a temporary fix, I grounded the fuel pump at the source. I'm about to check all the visible ground locations under the hood but wanted to get you're thoughts too before I begin trouble shooting. Thanks! Just put your meter between battery negative and the ECU plug ground points.Voltage measurment under load is preferred,but,resistance unplugged may do. The failure is more likely inside the ECU,IMO. oops, missed the multi ECU thing,were those multi known working ECUs? Edited February 12, 2012 by naru missed the multi ECU thing Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 12, 2012 Share Posted February 12, 2012 The failure is more likely inside the ECU,IMO. Three different ECU's were tried by his mechanic. Same problem. Seems very unlikely that it's the ECU itself. GD Link to comment Share on other sites More sharing options...
TPain Posted February 12, 2012 Author Share Posted February 12, 2012 If i opened up the ecu would i be able to tell if something was wrong? all the visiblle grounds under the hood appear in tact Link to comment Share on other sites More sharing options...
TPain Posted February 12, 2012 Author Share Posted February 12, 2012 Itd be nice if i knew 100 percent if my ecm was good. is there a way to test it? Link to comment Share on other sites More sharing options...
naru Posted February 12, 2012 Share Posted February 12, 2012 Probably nothing to see inside.Never know though. If the ECU fuel pump ground pin is open to all the ECU ground pins when using the diode test function of a multimeter,you can be pretty sure the ECU is bad.This assumes the ECU switches fuel pump ground w/a transistor rather than a relay.A safe assumption I think.Compare to a known good one to be sure. Link to comment Share on other sites More sharing options...
TPain Posted February 12, 2012 Author Share Posted February 12, 2012 Is there a way to test the ECM itself? I'm not an electrical whiz or anything but is it possible to test it with a tester? I don't trust Jeff at all. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 12, 2012 Share Posted February 12, 2012 Probably nothing to see inside.Never know though. If the ECU fuel pump ground pin is open to all the ECU ground pins when using the diode test function of a multimeter,you can be pretty sure the ECU is bad.This assumes the ECU switches fuel pump ground w/a transistor rather than a relay.A safe assumption I think.Compare to a known good one to be sure. The weird part is that it still uses a fuel pump relay for the 12v supply to the pump. And switches it via the relay coil ground as normally seen on any other Subaru. I really don't know why the ECU has the fuel pump ground going into it.... but that's the way it's wired. It may not be switching the ground with a transistor at all - it may just be going to a ground junction that, for whatever reason, the engineer's put inside the ECU. GD Link to comment Share on other sites More sharing options...
TPain Posted February 12, 2012 Author Share Posted February 12, 2012 Yes very bizarre. Fuel pump works fine, ground signal to fuel pump does not. The ground is sent via the ECM but I don't know absolutely 100% that it works and I'm not taking Jeff's word for it; the ECM works in every other way but the ground signal does not. Is there a way I could test the ECM itself? It opens right up and just looks like a circuit board. All the main grounds to the engine, body, transmission, etc. are all there. I don't know what other grounds there are besides the ones that I can see. The fusible links all work and appear fine. The fuses under the dash are all good. Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 12, 2012 Share Posted February 12, 2012 Is there a way to test the ECM itself? I'm not an electrical whiz or anything but is it possible to test it with a tester? I don't trust Jeff at all. You will need the ECU pinout and someone knowledgeable with a multi-meter to effectively test it. GD Link to comment Share on other sites More sharing options...
TPain Posted February 13, 2012 Author Share Posted February 13, 2012 Pinout as in what each part on the circuit board does or are you referring to the pins on the plug in? Link to comment Share on other sites More sharing options...
TPain Posted February 13, 2012 Author Share Posted February 13, 2012 http://www.ultimatesubaru.org/forum/showthread.php?t=76906 What do you think of this? Link to comment Share on other sites More sharing options...
Cougar Posted February 13, 2012 Share Posted February 13, 2012 (edited) TPain you can indeed verify if the ECU is working correctly or not. You need to find out which pin the return (ground) side of the fuel pump circuit ties to the ECU. When you find it then all you need to do is check the voltage on that pin when the fuel pump is supposed to be ON, using a good ground point as your meter refence point. The voltage on the pin will drop to nearly zero volts when the pump is on since it is supposed to tie to ground. Your meter reference is ground so the meter should see very little voltage difference if things are working correctly. If the voltage is more than a couple of volts then that means either the circuit inside the ECU is either bad, the ECU circuit it isn't working for some reason, or the ECU has a bad external ground. The reason the ECU is included in the circuit is to provide a safety for the fuel delivery system. The ECU monitors the engine firing pulses and if it doesn't see the firing pulses when the ignition is on it turns off the fuel pump circuit by removing the ground connection to the fuel pump. The real trouble may be due to a problem with the connection to the disty. Check to see if there are any code errors coming from the ECU. If that is ok then I would question the grounding for the ECU. There are multiple grounds tied to it. To make sure they are ok use your meter to measure the voltage on the wiring for each ground connection while connected to the ECU and power is ON. With your meter using ground again as the reference point a good ground connection will show nearly zero volts, or no potential difference. Ground is ground. A bad ground will have voltage on it due to the resistance in the lead to ground. Usually the resistance is due to a dirty connection. Pretty simple, huh. Edited February 13, 2012 by Cougar Link to comment Share on other sites More sharing options...
GeneralDisorder Posted February 13, 2012 Share Posted February 13, 2012 TPain you can indeed verify if the ECU is working correctly or not. You need to find out which pin the return (ground) side of the fuel pump circuit ties to the ECU. When you find it then all you need to do is check the voltage on that pin when the fuel pump is supposed to be ON, using a good ground point as your meter refence point. The voltage on the pin will drop to nearly zero volts when the pump is on since it is supposed to tie to ground. Your meter reference is ground so the meter should see very little voltage difference if things are working correctly. We know it's pin #50. If the voltage is more than a couple of volts then that means either the circuit inside the ECU is either bad, the ECU circuit it isn't working for some reason, or the ECU has a bad external ground. Since multiple ECU from running vehicles have been tried - the problem is almost certainly an external ground for the ECU. The reason the ECU is included in the circuit is to provide a safety for the fuel delivery system. The ECU monitors the engine firing pulses and if it doesn't see the firing pulses when the ignition is on it turns off the fuel pump circuit by removing the ground connection to the fuel pump. So why does the ECU control both the power and ground to the pump? All the newer systems only control the power side via the fuel pump relay. This controls both? To what end? The real trouble may be due to a problem with the connection to the disty. Check to see if there are any code errors coming from the ECU. Apparently there are not codes present. He still doesn't have a CEL and the previous mechanic surely checked for codes - he's an idiot but he did work for Subaru and surely he knows enough to check that. If that is ok then I would question the grounding for the ECU. There are multiple grounds tied to it. To make sure they are ok use your meter to measure the voltage on the wiring for each ground connection while connected to the ECU and power is ON. The problem is.... finding those ground points. FSM doesn't say.... With your meter using ground again as the reference point a good ground connection will show nearly zero volts, or no potential difference. Ground is ground. A bad ground will have voltage on it due to the resistance in the lead to ground. Usually the resistance is due to a dirty connection. Pretty simple, huh. I agree that a ground somewhere leading to the ECU is dirty or disconnected. That is almost certainly the case. The problem is - he doesn't know where all the ground points are located and the FSM isn't helping. I would find out which pins on the ECU are supposed to lead to ground - and check them all. Any that are suspect can be spliced and new ground points added near the ECU. GD Link to comment Share on other sites More sharing options...
TPain Posted February 13, 2012 Author Share Posted February 13, 2012 Keep in mind thought that only one other ECU has been tried. Frankly, I don't even know for sure if he did that. What I really want to do is to test the ECM that I have. What the best way to test it. I'm not an an electrical whiz and I could use some help. Then there was that post I found of someone else's ECM working fine but the ground for the fuel pump did not work; which is why I have my doubts over it not being an ECM problem. Link to comment Share on other sites More sharing options...
Cougar Posted February 13, 2012 Share Posted February 13, 2012 (edited) I told you how to check the circuit for the fuel pump circuit inside ECU in my previous post. Appearently the ECU pin you want to check is pin 50, going from what GD stated. One thing I forgot to mention is you need to check the voltage as soon as the ignition is turned on. If the engine isn't running after a few seconds of power turn on even a good ECU will shut down the circuit. The testing needs to be done with everything connected normally. If another ECU was actually installed as a check I have to wonder if it had the same trouble. I have my doubts there is an external ground problem causing this trouble but it is possible I suppose. I take it that you can't hear the pump turn on even briefly when the ignition is turned to ON. EDIT: I just had another thought about this issue. It may be a good idea to verify that all the connections to power for the ECU are ok. I believe one source passes though the ignition relay and there are a couple more connections also if I remember correctly. Edited February 13, 2012 by Cougar Link to comment Share on other sites More sharing options...
naru Posted February 13, 2012 Share Posted February 13, 2012 A test light grounded thru pin 50 would be better as it would draw some current. Link to comment Share on other sites More sharing options...
Recommended Posts
Please sign in to comment
You will be able to leave a comment after signing in
Sign In Now