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This is my brother's vehicle so I have limited details but I'm trying to help him get it fixed. It's a rather lengthy post so I appreciate anyone who takes the time to read it all.

This is a 2003 Hyundai Santa Fe 2WD with the 3.5L motor, auto transmission. It was driving fine until one day coming home it started to die during idle. We got the car parked at home and found that the serpentine (?) belt was off. Further inspection showed that the pulley had worn out in the middle allowing the belt to come off and flap around until what I assumed was the battery dying. We replaced the pulley and the belt but it would not start after this. Checked for spark and there was none. We ran a scanner on the CEL and it was showing a bad crank shaft sensor. After inspection this sensor had indeed been broken (i'm assuming by the flapping belt?) so we replaced it hoping that's all it needed. After replacing it, we had spark again but still wouldn't start so we checked fuel. We weren't getting fuel and couldn't hear the pump so we removed the fuel pump, bench tested it and it works fine. Tested for power at the pump and there is no power at all. We checked for power at the fuel cut off switch and no power there either. All fuses and relays appear fine, even replaced the ECU relay just in case. We checked for spark again just to make sure it was still getting that at least and now there is no spark again. So we hooked up the CEL scanner and now the scanner doesn't even recognize it being plugged in, it doesn't show anything at all. There are 3 of the ECU fuses (#1, #2 and #3) that have no power in the box, the fuses themselves are good. Should these ECU slots have power with the key on, engine off? Does this indicate a bad ECU?

I'm not really sure what else to check for but to sum the issue up, we have no spark OR fuel and no power at 3 ECU fuses but the ECU relay has power (new relay) and the ECU B+ fuse has power (fuse tested good) as well as all other fuses not related to the ECU. I don't understand what would have messed up the ECU or caused any of these issues when it was running fine before the belt came off, now there's power loss for fuel and spark??
 

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This is my brother's vehicle so I have limited details but I'm trying to help him get it fixed. It's a rather lengthy post so I appreciate anyone who takes the time to read it all.

This is a 2003 Hyundai Santa Fe 2WD with the 3.5L motor, auto transmission. It was driving fine until one day coming home it started to die during idle. We got the car parked at home and found that the serpentine (?) belt was off. Further inspection showed that the pulley had worn out in the middle allowing the belt to come off and flap around until what I assumed was the battery dying. We replaced the pulley and the belt but it would not start after this. Checked for spark and there was none. We ran a scanner on the CEL and it was showing a bad crank shaft sensor. After inspection this sensor had indeed been broken (i'm assuming by the flapping belt?) so we replaced it hoping that's all it needed. After replacing it, we had spark again but still wouldn't start so we checked fuel. We weren't getting fuel and couldn't hear the pump so we removed the fuel pump, bench tested it and it works fine. Tested for power at the pump and there is no power at all. We checked for power at the fuel cut off switch and no power there either. All fuses and relays appear fine, even replaced the ECU relay just in case. We checked for spark again just to make sure it was still getting that at least and now there is no spark again. So we hooked up the CEL scanner and now the scanner doesn't even recognize it being plugged in, it doesn't show anything at all. There are 3 of the ECU fuses (#1, #2 and #3) that have no power in the box, the fuses themselves are good. Should these ECU slots have power with the key on, engine off? Does this indicate a bad ECU?

I'm not really sure what else to check for but to sum the issue up, we have no spark OR fuel and no power at 3 ECU fuses but the ECU relay has power (new relay) and the ECU B+ fuse has power (fuse tested good) as well as all other fuses not related to the ECU. I don't understand what would have messed up the ECU or caused any of these issues when it was running fine before the belt came off, now there's power loss for fuel and spark??
Check : Fusible Link(ECU) 40A, provides power to ECU,
ECU(B+) Fuse 15A, provides power to Fuel Pump,
ECU connectors for proper connections.
 

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Should these ECU slots have power with the key on, engine off?
Yes, they should.

And well done for checking that they were being supplied with power. Most folk just check that the fuses aren't blown and never think to check that they are actually being supplied with power :clap:

bluelightnin6 said:
Does this indicate a bad ECU?
It might, but there are a few other possibilities worth investigating before jumping to that conclusion.

bluelightnin6 said:
I'm not really sure what else to check
It sounds like the ECU relay isn't being switched on. The ECU relay is controlled by the ECU itself, in response to you switching the ignition on. So the first thing to check is the fuse that supplies that initial ignition input to the PCM.

Looking at the diagram posted by wongpfh (that is for the wrong engine, incidentally), it shows that ignition input coming from fuse 21 on the dashboard fusebox. But on the diagram for your 3.5 engine, it's shown as coming from fuse 25 (just goes to shows the importance of following the correct diagram). Regardless, my advise when dealing with ANY electrical fault is to ALWAYS check ALL the fuses.

BTW, fuse 25 on the dash fusebox also supplies power to the ignition coils on the 3.5 engine. Could you have maybe shorted out the power supply to the coils when you checked for spark the first time?

Regards.
Scott.

BLUE : Fuse 21.....RED : Fuse 25

 

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Discussion Starter #4
Thanks for the replies! I checked the fuse 25 and the fuse is fine and there is power at the plug for it when the key is in. All fuses themselves are good , checked them all many many times with my meter. Anywhere else to look or is diagnosis a bad ecu?
 

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Ok so i jumpered where the ecu relay plugs in and when I do this the other ECU fuse slots mentioned earlier (#1, 2 and 3) regain power , but car still wouldn’t start. So I believe what you said is true, the ecu relay is not activating and if the ecu itself is the only thing controlling that relay then it leads me right back to thinking it’s a bad 1,200$ computer for a $2,000 vehicle ?
 

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the ecu relay is not activating and if the ecu itself is the only thing controlling that relay then it leads me right back to thinking it’s a bad 1,200$ computer
No, you can't say the computer is at fault yet. I said the fuse was the first thing you need to check, not the only thing.

I just read through your original post again and you said after replacing the crank sensor you had spark but no fuel. Then you replaced the ECU relay, then you discovered you had no spark again. And it was after you replaced the relay that you discovered there was no power supply to the ECU fuses and the scan tool wouldn't communicate with the engine ECU. All that got me wondering if the replacement relay might be part of the problem. Have you tried refitting the original ECU relay?

The other thing I'd check before going any further is the ignition power supply to the ignition coils. If there isn't an ignition supply to the coils there won't be an ignition input to the ECU to make it switch the relay on.
 

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Discussion Starter #7
I actually did think of that as well so I placed the old relay back in and there’s no change.

I did not check any ignition power supplies other than the fuse #25 you mentioned because I don’t really know what else runs that ignition power. Any places I should start checking?
 

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More info I found from diagrams is that the ECU # 1 fuse (one that has no power at slot because relay isn't activating) actually is for the Ignition Failure Sensor. I haven't been able to find what this sensor does exactly but I'm assuming if it isn't working (no power to it because ECU fuse 1 has no power to it) then that itself would prevent power to the ignition coils??

This whole issue is very confusing to me. I don't understand why the CEL was on and showing a code before we replaced the crankshaft sensor, then after that not only is the scanner not connecting to / recognizing the vehicle at all but I can't even get the CEL to illuminate on the dash at all. It was driving just fine, started dying on the way home from work. Saw the belt was off, ok obviously the battery is just dying due to no alternator running so jumped it off and it started back for a minute or 2 until we got it pulled all the way into the driveway and then it died again. Crankshaft pulley was worn out in the middle so replaced it, replaced belt. Now it won't start at all(was starting prior to even replacing the belt by jumping the battery off?) I'm far from a technician or mechanic but I know first things to check, is it getting fuel / spark / compression? No spark, some fuel dripped out of the fuel line but not with any real pressure (couldn't find a schrader valve) but I just focused on the spark issue for now. Is the CEL on? yes, pulled the codes. only code was for crankshaft sensor. Ok, that will definitely prevent spark. Checked the sensor, it was indeed busted so replaced it with a brand new OEM one. maybe it will fire up now. Nope. Checked spark again, yep there's spark. Checked fuel again, it drips from the line same as before but still seems like there's no real pressure so now I look into this deeper. Listen for fuel pump, I don't hear it. Pulled pump, bench tested fine. Checked for power at it, no power. Ok, must be a fuse or relay. All fuses test good, swapped out fuel pump relay with another identical relay and nothing. Replaced ECU relay with a brand new OEM relay. Still no fuel pump. Hmm, ok I wonder if we are still getting spark at least. Check spark again, nope now there's no spark again and still no fuel. Tested for power going to the crankshaft sensor, no power there now. Haven't fooled with checking compression, I don't believe if it had somehow got out of time that would prevent these ECU fuses from powering on?

I'm pretty sure I've pinned it down to this ECU relay not activating for whatever reason, it's just I don't know what activates it. The ignition coils can't have power since ECU fuse #1 has no power and this fuse has no power because the ECU relay isn't activating. If I jumper the ECU relay plug ins then the ECU #1 fuse regains power. I am thoroughly lost... I'm just trying to help my brother out and it's giving me a whole lot of stress that I don't need. I really appreciate all your help if you have any other advice.
 

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More info I found from diagrams is that the ECU # 1 fuse actually is for the Ignition Failure Sensor.
Actually, it's not. Well, not on your V6 engine anyway.

The Ignition Failure Sensor gets it's voltage from the aforementioned fuse 25 on the dash fusebox.

bluelightnin6 said:
I really appreciate all your help if you have any other advice.
Check you have a good 12V ignition supply to the ignition coils or the ignition failure sensor, whichever is easier to access. If there's voltage at the coils you'll next need to check that the voltage is reaching the ECU. And if the voltage is good at the ECU you'd want to ground the relay control terminal on the ECU connector to see if that switches the ECU relay on.
 

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Discussion Starter #10
Ok I checked for power and am receiving power all the way to the ECU.

I’m not really sure what you mean by ground the ECU relay terminal at the ECU?
 

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The ECU controls the relay by applying a ground to the relay control wire. The easiest way to test the relay control wiring is to simulate the ECU relay control signal by linking the relay control terminal to ground.

You could use a paper clip to link terminals 8 & 9 on the connector as shown in the diagram below. With the link fitted, check for voltage on terminal 1 of the connector. You should have 12V. Remove the link and the voltage on terminal 1 should switch off. The voltage on terminal 1 comes from one of the ECU fuses.

 

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The ECU controls the relay by applying a ground to the relay control wire. The easiest way to test the relay control wiring is to simulate the ECU relay control signal by linking the relay control terminal to ground.
So you want to ground the ECU relay control terminal???
If you do that then is there any danger of blowing up the ECU???
 

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Discussion Starter #13
Thanks for dumbing that down for me lol. Ok so grounding those 2 pins does indeed activate the ecu relay , I hear and feel it clicking. Just to make sure , when it did this I also checked the 3 ecu fuses that were not getting power and voila they do have power. There is 12v at pin 1 with those 2 relay pins grounded, instantly no power once I disconnect the 2 pins. Where do I go from here?
 

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Oh, I think I get it now.
The key signals the ECU to ground that relay which then connects power to the other ECU fuses.
Guess the control signal from the ECU is what needs to be looked at.
Why isn't it grounding the pin?
Check at ECU and wiring to relay from ECU
Sounds simple enough??
Be easier with a schematic

Could the issue be inside the fuse box??
 

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Where do I go from here?
I hate to say it but it's beginning to look like your initial guess was right and this is a ECU failure :surprise:.
The question is, if it failed what caused it to fail? You really want to try and figure that out before fitting another one or the replacement could end up facing the same fate.

Anyway, I'd advise taking the time to check the ECU power supplies and grounds before spending $$$ on a replacement. There aren't too many power supplies to check with this fault anyway since the ECU relay isn't switching on. You don't need to check the supplies that come from the relay since they obviously aren't required to switch the relay on. I've attached the PCM pinouts below showing the power supply terminals in red and the grounds in green. Test with the ignition switched on. With a bit of luck you'll find one of them missing.

PCM Harness Connectors - Viewed From Terminal Side.
 

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Ok thanks, I will check these power supplies tomorrow.

If it turns out needing a replacement, I did find one on eBay for 100$. It has identical part numbers on it to mine. It's for the 3.5l motor 2WD Automatic. But the listing says it has to be reprogrammed to the new vehicle for it to work. I checked pricing on that and it's not something I'm going to be able to have done.

So I guess my first question is, does it really need to be reprogrammed if it's the exact same part number? How does the computer know what chassis it is on as long as all of the features of both vehicles are the same? Because if it does then the car is just going to the junkyard lol

Second question, if it doesn't need reprogrammed and I do get it like you said I do need to know why it messed up to begin with. Would the busted open crank sensor have shorted the ECU somehow? I don't believe it was a coincidence that the belt came off and broke the sensor then the ECU messed up at the same time. I'm pretty certain it is all related.

Thank you for all your help.
 

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Thinking more on this (all I can do now, it's late / dark lol), if the crankshaft sensor being busted shorted out wouldn't it have just blown a fuse that goes to the ECU? I would think the ECU is pretty protected from any shorts like that. There has to be something I'm missing but I swear I've tested every fuse 10 times over. So if even one of those power supplies is missing, the ECU will not operator correctly?
 

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So I guess my first question is, does it really need to be reprogrammed if it's the exact same part number?
To be honest, I don't know the answer to that. I've never done any work on that particular engine because it was never fitted to the Santa Fe over here in Europe. My "guess" is it shouldn't need to be programmed because, as you say, it doesn't know what car it's actually fitted to.

bluelightnin6 said:
Second question, if it doesn't need reprogrammed and I do get it like you said I do need to know why it messed up to begin with. Would the busted open crank sensor have shorted the ECU somehow?
I doubt a short on the crank sensor wiring would damage the ECU. Your right, the sensor inputs are pretty well protected, plus you had spark after replacing the sensor which means the ECU relay had to be switched on at that point.

Where ECUs are more vulnerable is the outputs and you seemed to loose the spark after you replaced the ECU relay. Was it a genuine Hyundai relay you fitted? I'm just wondering if there is maybe a flyback diode across the coil of the replacement relay, or if the relay coil was shorted, and that damaged the relay driver inside the PCM.
 

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Discussion Starter #19
Welp turned out the ECU was indeed at fault , sort of. I replaced it and now am getting fuel and spark, engine light shows no codes but does connect. Unfortunately it somehow jumped time during this whole process of serpentine belt / crank pulley failing because it still will not start but it’s cranking and backfiring w/ low compression.

Does anyone have a diagram or instructions for timing this 3.5l motor? I have a diagram showing the timing marks but not sure Is it like older motors where #1 has to be up or is there more to this foreign motor? Looks like it’s going to be a pain in the arse.
 

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Just took the manifold off and I’m thoroughly confused at the way these plug wires were ran. Isn’t wire 1 supposed to run from cylinder 1 to 2? Wire 3 to 4, wire 5 to 6? Or maybe my diagram is wrong but I’m seeing 1,3,5 in the rear and 2,4,6 in the front .

Right now 1 is running to 4. 5 is running to 2. 3 to 6??? Not sure how this was running to begin with, that doesn’t seem correct to me but to be honest I’ve never seen only 3 coils on a 6 cylinder , everything I’ve worked on has had a separate coil for each cylinder .
 
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