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Main Relay issue?

Joined
14 November 2009
Messages
98
Location
Salem, Oregon
It's been quite a long time since my last post. My '04 NSX has been in garage for past 2 months, decided to drive it into work this morning. I've had it on a battery tender - no problems with starting and charging system was fine according to gauges during the trip this morning. My trip to work is about 25 miles - mostly highway/freeway. At about mile 20, while on freeway doing 75 mph (speed limit 70 folks!), dash lit up with CEL, red battery indicator, and one other red light that I didn't catch. The engine did NOT shutoff, but I also did not have power (thrust) when hitting the gas pedal. I coasted over to the right breakdown lane and came to a stop. This whole time, the engine is running. I tried to get the car to move forward, but no luck. I shut the engine off. I moved the passenger seat full forward, slapped the panel behind that seat where the main relay is located, waited a few seconds, tried to star the car again. Success. Put her in first gear, let out on the clutch and away I went, producing good power like nothing had happened. It's parked now at work.

I've looked through the forums and it seems as though this is a classic case of main relay failure (partial). I'm going to pick one up, but I'm perplexed with the symptom of the engine still running, but not producing power to the wheels. I'll pull the CEL code when I get home, but does this sound like the main relay? No guages on the dash appeared as though there was a problem before/during/after this problem and it certainly did not look like a battery/charging problem. Thoughts?
 
Until you know what codes came up it's really hard to tell what the problem is, although slapping the back panel is a good indication that the relay might be the culprit. What ever set off the CEL apparently resulted in the ECU limiting power.
 
Until you know what codes came up it's really hard to tell what the problem is, although slapping the back panel is a good indication that the relay might be the culprit. What ever set off the CEL apparently resulted in the ECU limiting power.

So I hooked-up the OBD reader when I got home (BTW - no issues when driving home) and it did not register any pending or active codes. I've ordered a new main relay, but this is a bit of a mystery. Recalling what happened when I had the problem, I realized that gas pedal inputs did not produce any RPM rise - almost as if the throttle body input had been (electronically?) disconnected until I shutoff the car. I'll try to read-up on the ECU and see if any posts on this forum have experienced similar symptoms.
 
I don't think its the main FI relay. Failure of the main FI relay usually results in loss of electrical power to the ECU or fuel pump and subsequent engine shut down. Your problem sounds more like a drive by wire issue.
 
I don't think its the main FI relay. Failure of the main FI relay usually results in loss of electrical power to the ECU or fuel pump and subsequent engine shut down. Your problem sounds more like a drive by wire issue.

My son let me borrow his Autel DS708 Diag tool (very nice!). Hooked it up and still found no issues. All applicable NSX tests (including Throttle test) came back fine. But, that's to be expected since problem is not showing itself right now! I, too, thought that a main relay temp failure would've caused engine shutdown, but I've also seen where MR issues can cause all sorts of different reactions. I'm going to pull the MR and see if anything looks obvious with the solder traces or cracking.
 
On reflection, I suppose that it is possible that if the ECU momentarily lost power because of a main relay 'momentary' failure, the positioning signal from either the accelerator sensor or the feedback from the throttle servo motor positioning could have got screwed up. On momentary loss of power, the ECU goes through a reboot and if it read that the accelerator sensor was open (foot still pressing down on the gas); but, that the throttle servo was closed, it might interpret that as a problem and put the throttle control into the fail-safe mode. When you turned the engine off and restarted the car, the ECU rebooted and probably read matching positions on the sensors and perhaps determined that everything was OK (kind of the equivalent of the IT support guys saying reboot your computer and see if the problem goes away). That is a guess at best as my 2000 service manual is in the trunk of my car which is now in winter storage so is inaccessible.

Replacing the main relay will not hurt and will allow you to inspect the old relay to see if it looked suspect. The next candidate might be to check the condition of the wiring on the DBW system if the problem ever re emerges.

The Autel looks like a primo code reader - certainly beyond my normal cheapo price sensitivity range. I expect that it has the extended Acura / Honda error code set; but, it would be good to confirm that. The extended codes applicable to the DBW are 1241 - 1248.
 
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In an 04 the relay is not where you smacked the panel:). It is located on the passenger side interior panel area, by the air scoop to the right of the passenger seat. So although the hand smack appeared to fix it, it was a coincidence.
 
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So although the hand smack appeared to fix it, it was a coincidence.

Yes, coincidence perhaps; but, probably very satisfying.

The fact that the engine was operating when the car coasted to the breakdown lane is an indicator that at the time of the hand smack, the main relay was not in failure mode otherwise the engine would have stopped. Intermittent problems are the absolute PITA to diagnose.
 
Yes, coincidence perhaps; but, probably very satisfying.

The fact that the engine was operating when the car coasted to the breakdown lane is an indicator that at the time of the hand smack, the main relay was not in failure mode otherwise the engine would have stopped. Intermittent problems are the absolute PITA to diagnose.

Agreed;)
 
In an 04 the relay is not where you smacked the panel:). It is located on the passenger side interior panel area, by the air scoop to the right of the passenger seat. So although the hand smack appeared to fix it, it was a coincidence.

So I replaced the main relay yesterday. Took a few photos of '04 location (right where Larry said it would be). Examining the old relay, all of the solder joints appear to be just fine. Quite a bit of flux here and there, but no cold or collapsed joints and no evidence of lifting traces. At least I have a good "hot spare"!

Obviously, I still do not know what caused the initial problem. Time will tell if it was a one-time issue.

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I suspect the top center and top right solder joints might have a hairline crack between the solder mound and the relay terminal. I would reflow the solder on all those joints, even if you are just keeping it as a spare.

I repaired my NSX main relay and a relay board in my wife's MB that were intermittent, and in both cases they were difficult to see without 20x magnification and light from a certain angle.
 
I am looking at the photo of the relay board and just realized that it looks like that 3W resistor is sitting fairly high up off the surface of the circuit board. Normally I would expect a largish component like that to be right on the board surface. Since the board is mounted vertically in the car, vibration in a vertical plain can cause the resistor to flex on its leads potentially leading to fatigue failure. To reinforce Frank's point, I would certainly check for fatigue cracks in the mounting points for that resistor.
 
I am thinking the resistor is that way on purpose, to allow better cooling when it heats up during high current flow, especially since the board is vertical.

My $.02:)

Regards,
LarryB
 
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