Engine crank...no start...no power to fuel pump....

Ok, before you say main relay...hear me out....

-91-lovefab LF800, AEM EMS
-Car has been starting/running fine up until a week or so ago when I went to move it and it wouldn't fire...would just crank.
-A year ago I replaced the main relay and ignition switch. The old ones still worked fine...just replaced them to be safe.
-I determined the fuel pump was not getting power. I couldn't hear it prime when switching the ignition on.
-The fuel pump fuse is fine, I replaced it anyways. I have checked all other fuses in the car repeatedly.
-I tried my old main relay and there is no change...I even tried re-soldering it and again...no change.
-I tried unplugging the fuel pump resistor and jumping the connection....no change.
-I then determined that the fuel pump relay(small 4 prong relay behind drivers seat was not transferring power.
-There are 4 wires: Yellow-12V switched on by ignition, Black/red-goes to fuel pump, Black/Yellow-goes to main relay, Solid Red-??? I am not sure
-If I remove the relay and place a jumper wire between the Yellow/12V wire to the Black/Red/Fuel pump wire, the fuel pump comes on when I switch on the ignition and the car will start/run just fine. The only difference is that it doesn't just prime and then shut off after a couple seconds when the car is just switched on. The pump will just keep running.
-I ordered a new fuel pump relay and installed it....no change. I can feel and hear the relay clicking on for a couple seconds when I turn the key on...and then feel/hear it again when I turn the key to start...as it should, but it isn't transferring the power.
-So, the relay is getting power, has a fine connection to the fuel pump, and seems to be getting the correct signal from the main relay to kick on and off.

Any ideas? Where does the red wire to this relay come from? Is that a ground? I can at least start/drive the car with the jumper wire in place, but it just isn't "right". There were no changes to anything between the time that is was starting fine and then all of the sudden the fuel pump wasn't getting power.

Thanks.

IMG_1623 by blodi, on Flickr

The red wire from the relay goes to the ECU. The ECU switches that wire to ground to energize the relay and start the pump. If your fuel pump relay is clicking, then the connection to the ECU and the ECU are probably OK.

The yellow wire that you jumpered to the black / red wire normally just supplies the coil in the fuel pump relay, not the fuel pump. The power for the pump comes from the black / yellow wire which goes back to the main relay. The black / yellow wire is supplied through the main FI relay by the #2 15 amp fuse. Make sure the fuse is OK.

Take a voltmeter and use a back probe to measure the voltage on the black / yellow wire during the start sequence. You should measure 12 v there (only briefly if the car does not start). If you are not getting 12 v, then, assuming the fuse is good, the connection between the pump relay and the main FI is faulty, your new main FI relay is faulty or the signal from the ECU which energizes the pump contacts in the main FI relay is faulty (faulty wiring or a faulty ECU). You cannot check the main FI relay operation by listening for clicks. There are two relays within the same package, one can be operating and the other not.

I have attached the wiring diagram for the main FI relay and fuel pump relay for reference

Thank you very much. That is really helpful. I'll dig back in this week when I get a chance and check out all of those connections. I will report back.

Ok, so I did a bit more testing today. The red wire does show as ground when I switch on the ignition for a few seconds, so that is fine.
There is no power from the main relay at all from the black/yellow wire at the fuel pump relay plug. So...either somehow both my main relays went bad at the same time, or, more likely, there is an issue farther up the line...either ECU (AEM) or connection.
I did test the Black/Yellow wire between the main relay and fuel pump relay and it's good. I did also try bypassing the fuel pump relay one more time...no change. I also double checked the #2 fuse...still good.

I'll dig in more later, free time has been lacking lately.

blodi said:

Ok, so I did a bit more testing today. The red wire does show as ground when I switch on the ignition for a few seconds, so that is fine.
There is no power from the main relay at all from the black/yellow wire at the fuel pump relay plug. So...either somehow both my main relays went bad at the same time, or, more likely, there is an issue farther up the line...either ECU (AEM) or connection.
I did test the Black/Yellow wire between the main relay and fuel pump relay and it's good. I did also try bypassing the fuel pump relay one more time...no change. I also double checked the #2 fuse...still good.

I'll dig in more later, free time has been lacking lately.

Click to expand...

Check to see if you are getting a ground on the green / black wire at the main relay. Should start at 12 v and then drop to ground when you attempt the start. IIf you have a failed start, I don't know whether this will just temporarily drop to ground like the fuel pump relay red wire or whether it will stay at ground after the ECU is energized. The ECU connects the green/ black wire to ground to energize the portion of the main FI relay that supplies the fuel pump. If that portion of the main FI relay is not getting grounded it would explain why you are not getting 12v on the black / yellow wire.

The good solution would be that the green/ black wire between the main FI relay and the ECU has a bad connection. Do you have one of those interface connections between the AEM ECU and the vehicle harness? If so, that would be a good place to start. The bad solution would be a blown output transistor in the ECU. Not a disaster and should be repairable; but, a definite hassle.

Once again, very helpful. I'll try that out.

I did just have the AEM sent in about a year ago and tested out and it came back with a clean bill of health. I did also try last week just unplugging all of the harnesses from the AEM and re plugging them in just in case of a bad connection...no change.

Thanks again!

Ignition? Not signalling the ecu? Ecu Then not powering relay? Would check aem earths and joins hope it's simple

I did put in a new iginition switch a year ago too when I put in the new main relay. I'll do some digging tonight more, just haven't had a chance. Also, going to take my two main relays to another primers tomorrow to test them out...I have a hard time imagining they are both bad though. It must just be a simple connection somewhere I'm missing. Just odd that it stopped starting out of nowhere.

Ok, so I tested the green/black wire at the main relay...no voltage/no ground...nothing. I tested the wire from the main relay connection to the AEM connection and it was good. So...the EMS isn't sending out a signal it appears. I am going to take the EMS tomorrow and try it on another NSX to see if it works or not. If it is the AEM that failed it kind of sucks, since I just learned that AEM no longer services the series 1.

Quick double check. With the ignition on, jumper the green black wire to ground at the EMS terminal. If you hear your main relay click, then you can be pretty confident that the problem is with the EMS. Since everything else on the EMS seems to work (i.e. the engine runs when you jumper the fuel pump relay), it is likely just the output transistor that switches the main relay to ground that has failed. A 0.15 cent part! AEM should be able to repair even if they don't support the software anymore. If AEM says no go, check around to see if you know any electronics whizards or perhaps some local tuners who may have contacts with said whizards.

Well, I already pulled the AEM out to take tomorrow, but I can try that tomorrow back in my car if all else fails. You've been a great help, Old Guy! Once/if I narrow it down tomorrow to the AEM I'll see if I can pull strings at AEM and see if they'll repair since there shouldn't be much diagnosis. Otherwise, I'm handy with a soldering iron if anyone can tell me which resistor that is exactly.

Tried out my EMS on another Primers car yesterday and... no fuel pump. So, I guess we've found the problem. When I have a chance I might crack open the EMS and see if I can locate the faulty transistor. Worst case, I'll just buy a new/used EMS. It really is too bad that AEM no longer services these. Maybe I am better off just getting a newer one as who knows what other transistors or issues mine may have on the horizon.

If you are going to pull the AEM case open, check for a broken wire between the terminal plug for the green/black wire and the printed circuit board. That should be an easy fix. Even if you get a replacement ECU, repairing your existing unit has the advantage that it gives you some back-up for what is essentially a one-off unit that has been matched to your car.

I opened it and the wire checks out ok. I tried re soldering the joints for good measure. But nothing. I think I'll start ems shopping. :/

Alright, so I was thinking about it and even though I didn't hear a the fuel pump on the other car, I realized I may not have been listening closely enough since it was a stock pump on that car vs. my Aeromotive.

So...I borrowed another AEM EMS tonight and hooked it up to my car.....still nothing. So...it is not the EMS. I did also try hooking up a jumper wire to the Green/black wire pin from the inside of the AEM with it all hooked up when I had the cover off and it made no change...so it must be further up the line. I guess I'm back to the drawing board a bit here. Maybe it is the ignition switch? Maybe a fuse or connection I've somehow missed? I'll have to keep digging. It has to be something simple that I'm just not seeing.

You have changed the main relay a while ago, but, have you swapped it again for this issue. Don't assume it is good just because it was changed recently. Also, check your grounds. I have seen ECU faults due to bad grounds. Sorry if I missed it, but have you jumpered out the fuel pump resistor and fed power directly to the fuel pump as a troubleshooting step?

I did change the main relay a year ago. I also still have the old one which still functioned fine that I have swapped in. I also then tried resoldering the old main relay. The main relay isn't getting a ground signal from the EMS.

I did jump the fuel pump relay and the pump comes on with the igntion. Only change is that it doesn't just prime and shut off with the ignition switch...it just stays on.
Where is the ground(s?) that feeds the ECU?

With the ignition in the run position, using a back-probe, connect the green/ black wire at the main FI relay to ground. You should hear the main relay click. If it does not click and the #2 15 A fuse is good, then you have a faulty relay or there is a wiring problem up stream from the relay. If it does click, then go to the EMS termination and ground the black / green wire at the EMS termination using a back probe. If the relay clicks, then you have a problem internal to the EMS or its termination plug. If the relay does not click, then there is a problem in the wiring between the EMS and the main relay.

PS I really do not think it would be the #2 fuse as there would be other problems if it was blown

I had an issue with my AEM Series 1 not outputting the proper signal for the fuel pump, and I just used another wire for the fuel pump output. I'll try to follow up with a more thorough explanation when I get home later. May not be your problem, but if not others may benefit from the information.

E-Nough Logic said:

I had an issue with my AEM Series 1 not outputting the proper signal for the fuel pump, and I just used another wire for the fuel pump output. I'll try to follow up with a more thorough explanation when I get home later. May not be your problem, but if not others may benefit from the information.

Click to expand...

I'm all ears at this point! But it is odd that the other borrowed ems I tried last night wasn't properly working either. So I think my problem is elsewhere.

I'm not sure if you have a Series 1 30-1002 box or 30-1042 box but the same instructions apply, it's just the wire outputs have different assignments in the ECU.

If you go into configure outputs you can define which output you want to use for the fuel pump. The nsx has 2, the main relay, and 1 extra for the high flow relay. You can find an extra output and set it's function as fuel pump, and try to use that output.

I always recommend an extension harness as it makes some of this wiring easy and you don't have to dig into the stock harness at all. Conversion back to stock is easy.

Thanks! I'll try this if all else fails this weekend.

So, I just went out to move the car forward in the garage where I can work on it easier. Fired it up with the jumper wire in place at the fuel pump relay. I noticed something that I noticed the last time I moved the car and forgot to mention. The anti-lock brake light comes on on startup and stays on for probably at least 30 seconds. Then I hear a relay click on the firewall finally and the light goes out. I know the light comes on normally when you start the car, but never stays on that long. Can that somehow be tied into my fuel pump power issue?

Additionally...which wire(s) does the ECU get its ground connections from? Since I tired a different EMS with the same result...it must be upstream from the ems.

Just did some more testing. I noticed that if I hooked up my diode/continuity tester to the green/black wire at the main relay and to ground I would see a the reading go up when I switched the ignition on. I tried the same thing at the black/yellow wire from the main relay and didn't see any change when switching the ignition on. I removed the main relay cover...when i switch on the ignition...I only feel the relay on the passenger side clicking on and off...not the drivers side.

Then with the cover off I used a jumper wire on the inside of the relay to connect the green/black terminal to the black/yellow terminal....and the fuel pump started right up. So.....maybe I've been wrong and somehow both of my main relays that I have miraculously have gone bad? It just has never seems likely to me, but I guess it's possible? The new one is only a year old and the old one still was working when I replaced it. I'm thinking I'll order up a new one and see...I'll be redfaced if it was it the whole time.

"I noticed that if I hooked up my diode/continuity tester to the green/black wire at the main relay and to ground I would see a the reading go up when I switched the ignition on." - what does 'up' mean? When you switch the ignition on and if the ground signal from the ECU is not working you will get +12 v on the green black terminal of the main FI relay. Look at the internal wiring diagram for the main relay. You will not measure any voltage on the black yellow terminal of the main FI relay when you switch the ignition on thing

"I tried the same thing at the black/yellow wire from the main relay and didn't see any change when switching the ignition on." - Look at the internal wiring diagram for the main relay. You will not measure any voltage on the black yellow terminal of the main FI relay when you switch the ignition on unless the main FI relay gets a ground signal on the green / black wire from the ECU. This ground signal causes the fuel pump circuit of the main relay to energize applying 12v to the black / yellow terminal of the main relay.

When you jumpered the green / black terminal to the black / yellow terminal of the relay that connects to the fuel pump relay you caused current to flow through the main FI relay isolation diodes and main FI relay fuel pump circuit relay coil to the fuel pump. This would be a high resistance path and the pump would not have been running very fast.

Have you tried jumpering the green black terminal of the main FI relay to ground with the ignition on? If so, did the main FI relay operate - the internal relay in the main FI relay that supplies the fuel pump circuit should energize when the green black wire is connected to ground. This would then apply 12v to the black / yellow wire which supplies the fuel pump relay.

As a cautionary note, the main FI relay has three black / yellow wires connected to it. When you talk about the black / yellow wire, I assume you are talking about the black / yellow wire that supplies the fuel pump relay.