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EMPIRE231

Wiring gremlins!!

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So my car has been pure reliable turn key fun, but this last weekend, a wiring gremlin decided to jump in and do some work.

My car has a key type ignition.  You turn one click and it primes the fuel pump, then turn one more to engage the starter.  The first problem... I would turn the key and get absolutely nothing.  Ended up being the + wire going from fuse block to the lug on the ignition.  This was hard to diagnose, because it would show 12+ volts in the off position, but when turned to the on position, it would go away.  So the wire was damaged enough to still show 12 volt power, but couldn't draw the amps.  I replaced that wire from the fuse panel to the ignition and was back in action for a day and a half.

second problem, now when I turn the key to the first position... I'm getting no fuel pump priming.  If I turn to second position, the  starter turns over.

so My question is....does that wire (the one that causes pump to prime) go to the fuel pump? or does it go to ECU, which then causes the pump to prime?  This is a basic Mefi B computer on a turnkey LS2.

I have checked all relays, the + lug on the ignition is still hot.... looking for any advice before I dive in.

Edited by EMPIRE231

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It’s usually switched 12v that goes to ecm and the ecm will trigger ground on fuel pump relay. Make sure when you turn the key you get 12v to that lug that goes back to ecm. Then check fuel pump fuse as well. 

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I had the first problem last week as well. Ended up being the resetable breaker couldn’t supply the 1-ish amp the relay needed. Was a pain in the Richard to diagnose since it would show voltage when you tripped the switch. 

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20 minutes ago, jtmoney714 said:

It’s usually switched 12v that goes to ecm and the ecm will trigger ground on fuel pump relay. Make sure when you turn the key you get 12v to that lug that goes back to ecm. Then check fuel pump fuse as well. 

Thanks for the feedback.  pretty sure that lug still has 12v, because it is the same lug tied to my shift indicator.... which is still working. But I will double check once I pull the panels off tonight.

my theory is that the main wiring loom is rubbing somehwere, which cause the first wire to go out, and then a day or so later took another wire out.  all fuses and relays have tested good.

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1 hour ago, Rockwood said:

I had the first problem last week as well. Ended up being the resetable breaker couldn’t supply the 1-ish amp the relay needed. Was a pain in the Richard to diagnose since it would show voltage when you tripped the switch. 

I had one of those resetable breakers fail on my one time too.  What a PITA to figure out!

~jw

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also.... I have 6 total relays.  Any idea what they all control? everything I look up says there should be 4 relays total (2 for fans, 1 for fuel pump, and 1 for ECM/ powertrain?)

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Start jumping things out with a 12v hot lead.  I got sand into my fuel pump.  It was just enough to cause friction that the fuel pump would not start.   I jumped it with a hot lead and then it broke free.  Work down stream from there.  Also the actual KEY ignitions are prone to failure from sand getting in them.  Try jumping it from the back side and see if you get it to start.

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Went through this trying to diagnose why my fuel pump wouldn't prime and ended up being a shot Mefi. But to answer your question, the fuel pump signal should go to ECM and then it will control the pump. Should have 4 relays, 2 for fans (on ground side), 1 for pump (on ground side), and 1 for power relay (on hot side). 

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49 minutes ago, acefuture said:

Went through this trying to diagnose why my fuel pump wouldn't prime and ended up being a shot Mefi. But to answer your question, the fuel pump signal should go to ECM and then it will control the pump. Should have 4 relays, 2 for fans (on ground side), 1 for pump (on ground side), and 1 for power relay (on hot side). 

How did you determine it was a bad mefi? 

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1 hour ago, acefuture said:

Went through this trying to diagnose why my fuel pump wouldn't prime and ended up being a shot Mefi. But to answer your question, the fuel pump signal should go to ECM and then it will control the pump. Should have 4 relays, 2 for fans (on ground side), 1 for pump (on ground side), and 1 for power relay (on hot side). 

 

30 minutes ago, EMPIRE231 said:

How did you determine it was a bad mefi? 

Pretty sure he substituted with known good part AFTER troubleshooting all other possible scenarios 

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Okay so I got home and pulled some panels off. Definitely getting 12v at the lug that powers that signal wire. Weird thing I noticed is that when this is switched to the first on position, I’m getting an electrical buzzing, sort of ultrasonic noise coming from my alternator! Does this mean anything?

 

I’m guessing my next step is to test continuity on this wire from the ignition lug to the connector at the ECM, once I find out what pin it leads to. 

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Also, not getting any 12v @ the fuel pump 

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51 minutes ago, EMPIRE231 said:

Also, not getting any 12v @ the fuel pump 

One thing to remember is you will only get 12V at the pump for the first sew seconds. Or if the engine is running.

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12 minutes ago, robseg said:

One thing to remember is you will only get 12V at the pump for the first sew seconds. Or if the engine is running.

Can be testing by either turning the crank with a wrench (unplug the coils to prevent accidental starting) or by removing and manually turning the crank position sensor (assuming it’s self contained and not indexed off of the flywheel or cam gear)

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I had the same concerns with a mefi and after hours of troubleshooting I found the mefi case was not grounded properly. Pulled all the grounds and cleaned, added a ground from mefi case to frame and the mefi kicked the fuel pump on and fired right up. 
take time to check all grounds

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2 hours ago, robseg said:

One thing to remember is you will only get 12V at the pump for the first sew seconds. Or if the engine is running.

Yep I was at the pump w/ multimeter while my wife was flripping the key on/ off

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Non latching relays will hum.  Do you have a signal (ground) coming from the fuel pump relay?  You can try jumping the ground wire that goes to that relay and see if it turns on.

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Take a jumper from fuel pump hot to the battery and make sure it kicks on. 

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1 hour ago, r3meyer said:

Non latching relays will hum.  Do you have a signal (ground) coming from the fuel pump relay?  You can try jumping the ground wire that goes to that relay and see if it turns on.

The hum is definitely coming from the alternator. I used a stethoscope to confirm. 

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23 minutes ago, jtmoney714 said:

Take a jumper from fuel pump hot to the battery and make sure it kicks on. 

I’ll try that. There’s no trickery to these right? If I hook up pos and Neg to the pump directly, it should turn on?

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51 minutes ago, EMPIRE231 said:

I’ll try that. There’s no trickery to these right? If I hook up pos and Neg to the pump directly, it should turn on?

Negative should already be hooked up to battery. Just an alligator jumper to the positive side of the pump from the battery should kick it on. 

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16 hours ago, EMPIRE231 said:

How did you determine it was a bad mefi? 

It was kind of a run around but it went like this:

Fuel pump was not priming in key on position, much like your situation, but everything else was seeing 12V and the car would try to turn over but wouldn't (no fuel). 

So I went straight from the battery to fuel pump to make sure it worked and it did. I then checked for computer ground, pulled the ground off and cleaned it very well, still nothing. Checked for continuity at the switch in both positions, all good. Check the pin in both the harness and computer that controls the fuel pump and it was good to go. Checked the relay for both power and fuel pump relay by switching them with the fans, both good to go. Checked each fuse (fuel pump relay, main power, computer memory, and ignition) all of which looked good, but I replaced them anyways) Then traced every wire from the battery to fuel pump to ensure nothing was cut, grounding, or frayed. Everything looked great. Luckily @L.R.S. had a spare Mefi laying on his bench and offered to let me use it to see if that solved my issues and it did. So I picked up a new one. I sent my old one to a company called MEFIBURN and he opens them up to see if they're fixable. He confirmed mine was toast.

One thing to keep in mind with the MEFIs. Don't try to start the car with jumper cables on it, it'll burn up the computer. You're actually supposed to unhook the computer from power even if you hook it up to a charger. 

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3 hours ago, EMPIRE231 said:

I’ll try that. There’s no trickery to these right? If I hook up pos and Neg to the pump directly, it should turn on?

Yes.  But I was saying to jump out the relay that powers the pump.  I have had 2 sealed relays fail.  They would show voltage, but not give out any amperage to start my big fuel pump.  Jump the fuel pump first.  Then the relay.  Then check the main fuse for that fuel pump.

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43 minutes ago, acefuture said:

It was kind of a run around but it went like this:

Fuel pump was not priming in key on position, much like your situation, but everything else was seeing 12V and the car would try to turn over but wouldn't (no fuel). 

So I went straight from the battery to fuel pump to make sure it worked and it did. I then checked for computer ground, pulled the ground off and cleaned it very well, still nothing. Checked for continuity at the switch in both positions, all good. Check the pin in both the harness and computer that controls the fuel pump and it was good to go. Checked the relay for both power and fuel pump relay by switching them with the fans, both good to go. Checked each fuse (fuel pump relay, main power, computer memory, and ignition) all of which looked good, but I replaced them anyways) Then traced every wire from the battery to fuel pump to ensure nothing was cut, grounding, or frayed. Everything looked great. Luckily @L.R.S. had a spare Mefi laying on his bench and offered to let me use it to see if that solved my issues and it did. So I picked up a new one. I sent my old one to a company called MEFIBURN and he opens them up to see if they're fixable. He confirmed mine was toast.

One thing to keep in mind with the MEFIs. Don't try to start the car with jumper cables on it, it'll burn up the computer. You're actually supposed to unhook the computer from power even if you hook it up to a charger. 

Thanks for the feedback.... this sounds a lot like what I have going on.  relays good, fuses good.  I will be testing continuity from the ignition to the pin that plugs into the mefi tonight.  I will also test the fuel pump tonight by connecting it directly to 12v to see if it buzzes.

did you ever figure out what caused your mefi to burn out?

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6 minutes ago, EMPIRE231 said:

Thanks for the feedback.... this sounds a lot like what I have going on.  relays good, fuses good.  I will be testing continuity from the ignition to the pin that plugs into the mefi tonight.  I will also test the fuel pump tonight by connecting it directly to 12v to see if it buzzes.

did you ever figure out what caused your mefi to burn out?

Never really did. Talking with CBM and Mefiburn they both said sometimes it just happens. Usually not for no reason, but the main one is someone tries to jump the car with the mefi still plugged in. It'll burn it up quick. Second is a ground or strong arc that causes it to burn a pin up and fry the unit. In my research I found a couple guys who just mysteriously had one go out like mine did. 

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