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Post detail electrical issues

2 May 2013
St Augustine, FL
A weird one. Full detailed yesterday, under hood, engine bay etc. Detailer said a fan ran for a bit but then shut off. Car started fine, no issues, put it away. Drove it this morning, no issues. Go to start it at 4pm, dead battery.....huh?

Then discover that while charging it up, blower motor and/or main cooling fan kick in, but ONLY with ignition OFF. Turn ignition ON, both fans shut down (with climate control off in both cases).

Finally get it charged up, drive it, no more issues so far this evening.

Ideas on where to look or what would cause this, specifically?

In the Amelia Cars and Coffee tomorrow so will be pulling both fuses to be safe when get there, and leaving on charge tonite.

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Please check Eng bay/room fan relay and connector especially if you have removed the fan inside the eng bay.

If the fan was removed but kept the relay, it could be soaked in moisture causing short circuit inside the relay depending on how it was treated after the fan removal.
Happened on several NSX.

If the eng bay fan was already removed, just take the relay out and leave it off the car.
Blow dry the remaining connector on the eng loom and seal it off.
As Honda removed this fan and relay from 95+ MT models (stayed on AT models all the way upto 2005), I normally remove them from any 91 -94 MT models when carrying out major eng service.

This relay is only used for powering the eng bay fan and although the coil side signal has input to the ECU, the same signal is shared with the rad fan resistor bypass relay (rad fan high relay) so the ECU can get the same info from there.
It will only be triggered when the eng gets really hot.

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If the fan in question is the engine compartment cooling fan - then follow Kaz's directions.

You refer to the "blower motor and/or main cooling fan".

If by blower motor you mean the blower in the interior heating / cooling unit, the CCU would pretty much have to power up for the blower motor to run. The blower motor is controlled by the blower power transistor or the blower high relay which get their signals from the CCU. No CCU signal - no blower operation. If the CCU didn't light up, the balance of probabilities says it wasn't your blower motor. You would have to have multiple weird things happening in order to power up the blower with the key in the off position.

If by main cooling fan you mean the radiator fan, then that is in the realm of possibilities. The +12 v supply to the radiator fan is always energized (un switched). The fan is turned on by being grounded through either the fan high or low relays. The controls for the high and low relays come from the fan control unit which is supplied from switched power, so the radiator fan would normally not operate with the key in the off position. This may be the same problem as Kaz describes with the engine compartment fan relay. If moisture some how got into the high or low relay internals it could bridge the relay contacts causing the fan to run. When you cycled the key you may have cycled the relay which resulted in it returning to normal operation. I am not a huge fan of this theory because the fan high and low relays are contained in the A main relay box. In order to get moisture into those relays, your detailer would have had to go super gonzo with the pressure washer - in which case you need a new detailer or learn to live with non pristine under hood environment. However, the fan low relay probably gets operated every time you start up the car and since you live in a rather hot environment, the high relay also probably gets operated regularly. Since your car is a 1991, I am more inclined to believe that the relays may be wearing out (26 years old) and one of them could have stuck in the closed position momentarily. You could pop the two relays out, pry their covers off and check the operation of the relay contacts or just replace the relays on spec, or wait until it happens again and then try pulling the relays. If the fan goes off when you pull the relay, then you likely have the culprit.

It is also possible that the problem is not the relays. If the wiring between the rad fan, the relays and the fan control unit is compromised, you could have an intermittent ground switching the relay on and off - diagnosing intermittent wiring failures is a whole separate world of hurt!

There are other possibilities; but, they all start to get into the low probability or expensive realm.
Last night, thought it had settled down but went to tuck it in for the night and heard the CC fan blowing so left it on charge, and at Amelia Cars and Coffee just disconnected the battery.

So: 1) I don't have the engine bay fan at all, 2) no idea which relays (which ones exactly?), 3) radiator fan was coming on but I think that has cleared up now, 4) detailer didn't use a pressure washer - he has done the car 5 or 6 times with no issues and I saw him this time using a very low pressure jet from his filtered water reservoir.

I am going to leave it all hooked up overnight and see where I am in the morning. Hope fully a day in the sun will have evaporated any residual moisture off

Long say, gotta cave, will review in more detail tomorrow.

Thx guys!
So its the main blower, not the aspirator fan that is coming on? That would kill the battery pretty quick. And the CCU display is not lighting up when the blower is running?

With no key in the ignition, pull the #28 fuse. That is the blower motor fuse and should kill just the blower. If it doesn't, you have some really odd stuff going on. If it does kill the blower, put the fuse back in and if the blower starts up again pull the #4 fuse, that should kill the blower relay and parts of the CCU. If the #4 fuse kills the blower (with no keys in the ignition), I suspect that you have a sneak circuit through the ignition switch that is powering stuff up even when it is in the off position.

You have a 1991. If you have the original ignition switch I suggest that you do the test procedure in the service manual to find out whether it is working correctly.

The Eng bay cooling fan is installed like this and although you can't see it here, the eng bay cooling fan relay is behind the thick eng loom at the bottom center of this photo.


From the factory, the relay is mounted on the cooling fan bracket (yellow marking) and it's protected in big black rubber cover.

I just noticed that in your signature section, your NSX has super charger so it's very likely that the eng bay fan was removed at the time of FI installation but the relay was being kept connected.
And it's very likely that the relay is either held upside down or simply hanging down on the eng loom with the rubber cover opening facing up and this will turn into nice pocket for holding the water.

You need to find this relay.


Inside the rubber cover, you will find brown 4pin 250 connector with custard colour plastic protection sleeve (yellow arrow) but you won't be able to see them from outside as they are hidden inside the rubber cover.

Find the relay with black rubber cover, disconnect it, no need to re-install it, blow dry the brown connector and seal it.


If everything is working as they should, the rad fan, a/c blower motor fan, condenser fans and eng bay fan should not operate without the IG power.

However, depending on what state the Cooling Fan Control (CFC) unit and the Climate Control Unit (CCU) were in at the time when the engine was shut down,
the failure of the eng bay cooling fan relay can activate other relays related to the above fans including the eng bay fan itself, even without the IG key.

You can easily simulate the same situation.

In the above photo, there is no IG key so no voltage reading, no Clock or no lights on the dash.
Yet, if you look at the CCU, you can see it's operational (mine is JDM so the temperature unit is in centigrade and it's set at 25degC) and the blower motor is active.
All you need is to create short circuit at the eng bay cooling fan relay.

As a side note, if the opposite phenomenon happens (all fans start spinning as soon as you turn the IG key into P2 ON position even when the engine is cold),
that's the typical sign of soldering crack inside the CFC unit.
Not so popular as the Main relay issue but happened on many NSX.
I already re-soldered/replaced many CFCs for other owners.

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That is a rather bizarre sneak circuit back through the fan relay. It must have taken a bit of effort to track that down.
So its the main blower, not the aspirator fan that is coming on? That would kill the battery pretty quick. And the CCU display is not lighting up when the blower is running?

Correct on both counts.

Needless to say after all your efforts Old Guy and Kaz, I no longer have the issue so I guess that day in the sun yesterday did the trick to dry it out.

I'll check for that engine bay relay tomorrow.

Thanks both, a lot of great info here for the next time, me or someone else.
Well, after a regular hand wash (not a full engine bay detail, hatch stayed closed), my usual careful self with the hose around rear vent over engine, etc, problem just resurfaced.

I checked and you are correct, no engine bay cooling fan (I am supercharged). But with a flashlight I don't see that relay connector either, and that whole area is bone dry as is the entire engine bay that I can see, and so is under the front hood.....

It's very crowded with wire looms etc in front of the alternator of course, but pretty sure it's not there.

I can't pull the fuses right now, will try and get to that tomorrow. But this is pointing at the ignition switch then? But why is this only after a wash?

Or maybe the relay is there but with snipped wires that are shorting? Tempted to out a clip on fan over that area, but like I said it is bone dry.
Quick update: disconnected battery overnight, reconnected this morning, same issue, drove to work, parked it, same issue, disconnected battery. 2 hours later, reconnected and the fans started up again but kind of intermittent, cutting in and out. Drove for 20 mins, parked, no more problems. So no more issue again....grrrr.