I got to troubleshooting step 5 quickly, but had no information for the yes/no question.

So my likely wrong answer is the fan relay is broken. And I'm guessing that because (a) everything works individually (b) It has to be something that would break on its own.

 

I swapped the relays and that made no change. I skipped step 5 and moved on, and assumed the wiring is good between the PCM and the pressure switch connector.

 

Oh yes, I see you did.

There are two diodes in the path, one for each fan. Is that a possibility?

 

"Jump the SCS line with the HDS. If anyone knows what this is doing I'd love to know. I'm guessing it's keeping the PCM powered somehow so as not to cause any sudden power issues but that's just a guess on my part. I obviously don't have the Honda Diagnostic Scanner that can do this. Is there another way to do it and what is the "SCS line?" This is mentioned in may other sections of the book. That's a side note question I'd love to have an answer to.

So, I've been doing some Googling and apparently this means to jump pin 4 to pin 9 on the DLC in order to put the computer in a test mode. I figure you could backprobe the wires and do this or get cheap OBDII connector from eBay and make a jumper you can plug in. The question I would have left is does the key have to be on when you jump this line? The manual doesn't tell you. It just says to jump the SCS line with the HDS. The HDS (or probably the Snap On Verus) would do this while connected but I don't know if you have to be key on engine off in order to jump it.

 

OBDII is active even when the key is off, although obvious, not all functions are available at that point.

 

Yes, the power line to the DLC is hot all the time. Just not sure about the SCS line and the wiring diagram isn't clear, at least not to me. I would think that if the computer still runs for 15-20 minutes after the car is shut off that it would know if you jump the line, even with the key off, but just not sure.

I assume if you do it with the HDS you have to have the key on engine off to get the HDS to connect. However, it may be a feature of the HDS without actually communicating with the computer. The service manual never says, "Turn the key to On(III) then jump the SCS line with the HDS." In the above troubleshooting procedure the key is On (III) for the previous step and it hasn't been turned off before they tell you to jump the SCS line with the HDS. in other troubleshooting procedures the key is off in the prior steps.

Anyway, I'm looking forward to someone jumping in who has good wiring know-how to see if they come up with the same diagnosis on the high speed radiator fan.

 

Well, I've found the answer to the SCS question. It's in the general troubleshooting section. Here's what it says to do when unplugging the PCM:

Note: The PCM overwrites data and monitors the EVAP system for up to 30 minutes after the ignition switch is turned to ACC or Lock. Jumping the SCS line after turning the ignition switch to Lock cancels this function. Disconnecting the PCM during this function, without jumping the SCS line first, can damage the PCM.

1. Make sure the ignition switch is turned to Lock, then jump the SCS line with the HDS (why can't they write that in the troubleshooting procedures?!)
2. Remove the PS reservoir from the body.
3. Remove the cover.
4. Disconnect PCM connectors A, B, and C.
5. Whe diagnosis/troubleshooting is done at the PCM connector, use the terminal test port above the terminal you need to check. (they really don't want you screwing up a terminal by front probing it!)

So, once again, RTFM would have answered the questions before I posted this side-note question. Hopefully, someone else will find it helpful. I'm just going to buy a cheap OBDII connector on eBay and put a jumper wire in it on only the 4 & 9 terminals. Then I can just plug it in when I want to jump the SCS. Seems way too simple.

 

Nobody wants to take a stab at it? According to the diagnostic procedure, Honda says my PCM is bad. Is that right?

 

So, since nobody wanted to give their theory I'll go ahead and post the solution.

I went through the troubleshooting procedure Honda gives for this. It's posted above in my pics. In step 4 I initially tried the jumper procedure and got no fan action. I don't know if it's because the plug wasn't connected to the switch (the procedure doesn't actually say to plug it back in before jumping) or if my probe wasn't making connection but the fans didn't turn on. I tried it several times, making sure I was getting contact but who knows. I continued the procedure until I got to where it said to unplug the connector from the PCM and check the wire between the PCM and the pressure switch. Since I didn't have a way to jump the SCS line (I do now) I skipped that and assumed the wiring was good as there was no reason for a wire to be broken, though I did wiggle the plug a bit on the PCM just to see if it changed anything.

At that point, the procedure ends and says to substitute a known good PCM and if the problem goes away to replace the PCM. So, essentially it said I needed a new PCM! Holy crap!

I decided to swap fan relays and got the same results, meaning the relays were good. I could hear them both click in both locations so I knew the coils were being powered. I went to the low speed circuit check and did that check and found no faults.

After some headscratching for a while, I decided to go back and backprobe the pressure switch again and jump those wires again. This time the plug was connected as I had plugged it back in when I decided the switch was good. When I did that the condenser fan went to high and the radiator fan stayed off. The procedure in that step says to jump the wire and "Do the radiator fan and condenser fans run at high speed?" Well, no, only one does so that leads back to bad PCM still, right? Well.....

I decided it was time to break out the wiring diagram. In looking at the wiring diagram the pressure switch on the high pressure hose feeds the high pressure signal into the PCM. Based on that, when the switch senses 221psi, the PCM commands the fans to run at high speed. It appears on the wiring diagram that there is only one low speed signal out of the PCM and one high speed signal out. So, if the condenser fan was being commanded to high speed that meant the PCM was receiving that signal and sending it out the ONE high speed fan signal wire. It just wasn't getting to both fans.

This is where it gets interesting. Tracing the diagram the PCM sends the low speed signal through ONLY the condenser fan power and relay so both fans run on low via the condenser fan fuse and relay. The PCM sends the high speed signal through each fan's associated power and relay. So, if that's the case then the wiring diagram indicates that the ground side of the radiator fan (the PCM side) was getting the signal so there must not be power coming in to the fan. Since I tested the relay I knew they both worked so that left only..........wait for it.....







The @#$%%$## fuse!!! Sure enough, pulled #11 30A fuse and found it blown! WTH?? The troubleshooting guide in the manual mentions NOTHING of checking that fuse. I didn't think of it initially because both fans ran fine on low and didn't realize that the radiator fan runs on low via the condenser fan relay. Why wouldn't the troubleshooting procedure for high speed mention anything about that? You could have a bad relay or a blown fuse and have this same fault. The procedure mentions nothing about what if only one fan runs on high, nor does it mention to even look at the relay or fuse.

Lesson here? The wiring diagram is the most important item when troubleshooting any electrical problem. The troubleshooting procedures can give some helpful information but I can't help but wonder if I took this vehicle to the dealer if they would have replaced my PCM. I hear about it happening all the time and now I can see why.

So, once the fan was fixed the pressures came down to normal and the AC works fine. As high as the pressures went, though, it could be damaging to the system to have that going on long term. I don't know how long that fuse has been blown, nor do I know why it blew. It's a 30A fuse so it's something to wonder about. It hasn't blown again and is working fine 3 days later. Will keep an eye on it. Moral of this story? Check your fans closely once in a while. Don't go by just the noise of the fans to make sure all is working as it should.

 

So, to make a long story short, if you have a 2007-2009 Factory Service Manual I suggest adding the box Honda left out of the Symptom Troubleshooting Index on page 21-14:

Symptom
Both fans run on low, the condenser fan runs on high, but the radiator fan does not run at high speed.

Diagnostic Procedure
NA

Also Check For:
Blown fuse No. 11, 30A in underhood fusebox
Radiator Fan Relay stuck open


They have a Symptom Troubleshooting Index line for just the opposite... i.e.Both fans run on low, the radiator fan runs on high, condenser fan does not run on high and they have you look at fuse #9 and #30.

 

Amazing. What's really weird with this car (and all new cars) is that when something doesn't work in one circumstance, it often does in another. Like the symptom I had with the remote lock not working, and lights not working with ignition off, but worked with ignition on.

There can be multiple modes, and just because something runs in one situation doesn't mean you don't have a blown fuse.

Congratulations on the detective work. I was hoping you'd find what it was, since it was quite a puzzler.

 

Wow, thanks for sharing this. I've just replaced my pass. side fan and verified that both fans run on low speed when the A/C is turned on but did not take the time to check if they run on high speed too. I'll take the time and check if high speed is ok too.

 

Yep. Gone are the days when a temp sensor simply trips a relay and turns on the fan. Now we have high and low speed, controlled by the PCM. The other system that's a real headscratcher is the charging system. Pray that never breaks on you. There have been a couple threads on it recently.

 

You'd do what I did with my BMW when the cooling system had an issue. Just replace every part with a new one in the cooling system

 

Nice. Missed this one till today. I was wondering about the fan operations with the Muzzler II installation. It seems that from reading this, I should not be too worried about the fans with the Muzzler installed. I hope I find this when my day comes. I have a 2007 with a mere 61500 miles on it.

Thank you for posting so many details.

 

nice detective work, I did not even realize the fans have two different running modes.
Will have to check that fuse tonight!

 

That's what threw me. Both fans ran on low so I didn't even suspect a fuse and the troubleshooting steps didn't mention one either. I should have just checked them for good measure but it's easy to say that now. Once I figured out the diagram and what was happening I knew the fuse was blown before I removed it and looked at it.

 

I would like to thank again, my 30 A fuse was blown and the radiator fan was not running when high speed mode was turned on. I checked the 05-06 manual and it does say to check the fuse.

 

Thanks for posting that. Is that the entire procedure or is it continued on the next page? If it's continued, can you post the rest of it? I wonder if, for some reason, they purposely left that whole section out of the 2007-2009 manual or if it was an accident. I don't even have that section. I have the section I posted above that covers "both" the radiator fan and the condenser fan high speed circuit.

I am still trying to figure out what caused the fuse to blow. Is the motor failing and just drawing too much current intermittently? I couldn't find any wiring problems anywhere. I don't have an amp clamp to check the current draw to that motor. I might be able to use my meter to read the amp draw across the load side relay terminals to check current draw on the motor. A wearing motor will draw higher than normal current. Or maybe there is an intermittent fault in the underhood fuse box.

I also wonder how many other vehicles have this problem and people don't even realize it. There is no real symptom that is very noticeable. I just happened to notice on a couple of occasions where I actually drove the van (my wife drives it most of the time) that the AC got a bit warmer than it should at idle, but my wife never noticed and never said anything. You had the same problem and didn't notice a problem either. When I put AC gauges on the car (because I actually thought I had a refrigerant charge issue at first) the high pressure side was going up to 350psi. The fans are supposed to kick on high at 221psi and bring the pressure down. 350psi is way too high and can damage the system if it isn't addressed. I can only wonder how many compressors are failing due to this issue, especially when combined with the very common stuck relay problem.

 

My condenser fan was making a horrible noise and replaced it recently, I think the blown fuse it related to the failing motor. I'll keep an eye on it.

This is the last page, there are 2 other pages before this one.

 

Wow. That's a completely different Symptom Troubleshooting chart than in my manual. The same problem in the 2007-2009 manual says to go to the Radiator AND Condenser fan high speed circuit troubleshooter. Very strange.

Just to make sure we're on the same page on this, the radiator fan is on the driver side and the condenser fan is on the passenger side. If you look at the wiring diagram, the condenser fan is never powered by the radiator fan fuse (#11.) So, that means a failing condenser fan will not affect the radiator fan fuse #11. On the other hand, BOTH fans are run in low speed mode by the condenser fan fuse #9 so a failing radiator fan could blow the condenser fan fuse #9 and/or fuse #11. If your condenser fan was shredding itself it might blow it's own fuse #9 but wouldn't blow fuse #11.

The only thing that can cause fuse #11 to blow is a short to ground between the radiator fan and fuse #11. The fan control relay (a 5 pin relay down under the power steering pump reservoir) is tied into that side of the circuit too. But, even if the fan control relay were to stick and not move into the high speed position when the radiator fan did, if the condenser fan were shorting out internally it would only short power from the radiator fan to power on the condenser fan. There would be no short to ground to blow the fuse. Now, if the fan control relay was failed and somehow making contact in both of it's positions at the same time you could have a short to ground in that fan control relay. I don't think the condenser fan failure itself caused your fuse #11 to blow. You could possibly also have a problem in the fan control relay. I could see the fan control relay possibly being damaged if the condenser fan were drawing high amps or shorting out internally. The condenser fan could take out the fan control relay. It could also take out the condenser fan relay, too.

 

The condenser fan is the passenger side one per the manual. You are correct the bad condenser fan could not have blown fuse 11, so I have no idea what and for how long ago it's been blown. I've just replaced both fan relays about a month ago (just to be safe, the old ones tested fine) I should have checked the fuse 9 & 11 too, shame on me.

 

I also changed both fan relays about 3-4 months ago but not for any reason other than they were the same relay that often fails for the AC clutch. I figured they were old and I bought 5 of them on eBay for cheap so I just replaced all three of them. I have no idea if the fans were working properly then or not.

There is also a 5 pin fan control relay down under the power steering reservoir and that's the one that could possibly take out the radiator fan fuse if it somehow were to short internally. It's a different relay than the others so I haven't replaced mine either.

 

Update:

I was looking this circuit over again and just realized that for the troubleshooting procedures I was in the AC chapter. The AC chapter covers radiator and condenser fan issues but not radiator fan issues alone. For that I needed to be in the Engine Cooling chapter where the proper procedure (as posted above) is located. Had I found that initially it would have taken me to the #11 fuse right off the bat and I wouldn't have spent two hours chasing the whole thing down and suspecting my PCM for a while.

In my defense, though, the only time I had a problem with the circuit was when the AC was on. The condenser fan ran on high but the radiator fan shut off. Under normal engine cooling operations both my fans were working normally on low so it was an AC problem. It's definitely poorly organized in the service manual.

 

You have nothing to apologize for. It was an excellent piece of detective work.

 

A big THANK YOU to John for the investigative work.

 

It's funny you bumped this today. I just had a 2007 Odyssey in my shop today for a blend door motor issue and check engine light. I noticed it had an inop radiator fan when fans were on high. Went right to the 30A fuse and found it blown. 30 seconds later I had it fixed. I asked the owner if they noticed warm AC at stop lights or idle and he said they had. He was pretty happy! Probably gained a good repeat customer.

 

DP Ooops

 

Great thread John, I was noticing my ac was not so great today in my 08. Actually it has been that way for a while. It also seemed that high speed fan operation was excessive. I checked the AC pressures and they seemed ok. Then I did notice my that only the passenger side fan was running on high and driver side was off. On start up they both ran at low speed.

In the morning i'll be checking fusses. I think i have the same issue.

Great tip Thanks