V-10 Idle problems
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New User
Joined: Apr 2016
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From: McCleary, Wash
V-10 Idle problems
I have asked this question twice and no one seems to have any good ideas what to do. I took my truck into the shop the other day and they hooked up their tester and went through all of the tests related to the idle circuit. The IAC is working correctly except when the A/C compressor is engaged. The idle will increase 300 to 600 rpms when the compressor comes on and will hold the idle up until the compressor shuts off. To me it sounds like the ECM is getting some kind of erroneous signal and thinks that there is a heaver load and raises the idle too high, that is just my thought. Mind you this was not done at a Ford shop. The mechanic said that I might need to go to a Ford shop and have my ECM updated or reflashed. Will Ford do that? I dont want to spend 2 or 3 hundred dollars to find out that I need to buy a new ECM at what ever it will cost. Does anybdy have any ideas?
Fleet Mechanic
Joined: Jan 2012
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You don't tell us the year or model of the truck that is the first step in asking for help guessing you have a 2 valve v10 since you mention iac. I don't see how having the computer reflashed is going to fix this. I think it would be much more likely that the compressor is going bad than it being a computer issue. Ford will re flash the computer but I don't see it helping you and will likely charge you an hour of labor to do it do you have anything to read data from the obd port you can monitor load % on the engine so you could compare with the compressor engaged and not. You honestly may be better if going to ford and having them diag it. Other than that I don't have any ideas and am far from an hvac expert but I do agree that is too much of a change in rpm for the compressor engaging.
Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
v10 idle
Have you tried disconnecting the PCM connector to reset and erase all bad learned data? Have you replaced the idle air bypass solenoid? Have you cleaned your throttle body? Does this happen only when you use your A/C or does it happen when you are are using the defrost also? Basically, does it happen anytime the A/C compressor cycles, max, A/C, defrost, and possibly on floor and vent? The compressor cycles on all of those settings and possibly more. Is it possible that there may be a vacuum leak? There are some places that you could have a leak at the PCV hose or in the evap system. Do you have a Motorcraft PCV valve? Some after market valves are not compatible on Ford products. Start with looking for vacuum leaks and the PCV valve, if ok then clean the throttle body and remove the air bypass valve and clean it with carb cleaner or replace it. After you do those things , disconnect the PCM connector to reset. It is below the under hood fusebox, 10mm socket, DO NOT use power tools. If your idle problems are only when you select A/C or max, I would be questioning the PCM then and only then. If it acts up on all settings, your PCM may not be receiving a signal at all and that could be why the idle is going so high. A good scan tool should show a signal that the compressor has been demanded.
Good luck, keep me posted, Koolkat
Good luck, keep me posted, Koolkat
Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
v10 idle
The dealer can only do a reflash IF there is an update for your truck, otherwise they can only read your trucks PIDs (data at that time as the trucks PCM is reading the info at that time). Your best bet before taking your truck to the dealer is start with the info I gave you. I am a retired Ford dealer driveability technician 25 years. What I outlined is what they would do if you took your truck to them and they will if you do it yourself or not, they won't take your word on what you have tried. They have to start at the beginning just so they are thorough on their diagnosis.
Good luck, Koolkat
Good luck, Koolkat
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Joined: Apr 2016
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From: McCleary, Wash
Thanks to both of you for your info. I guess I should have put a little more info. This is a 1999 F250SD 2v V10 4X4. I have removed the IAC valve and cleaned it out I have cleaned the throttle body checked all the vac hoses. Problem still there. This problem occurs when the A/C compressor cycles. It does it in all positions except vent mod. So I will check what the percent load is when the compressor cycles. I have replaced the IAC valve and that did not change anything. There are no other idle concerns or engine miss or rough running or power power problems. Just when the A/C cycles.
Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
Idle
Its good that you have all of that info. Looking at the percentage of load won't help you as much as monitoring the PIDs for a A/C demand signal. When the PCM sees that the compressor is on, it sends a signal saying so. When it does see a signal all the PCM is suppose to do is maintain idle, if there is no signal it will over compensate trying to maintain control. The Idle is higher than normal because the PCM doesn't know what is going on. In my experience, I believe that you either have an open circuit in the signal wire or the PCM has an issue and trying to do a reflash will do nothing for you. In the meantime you may want to disconnect the A/C compressor clutch connector to maintain your driveability, since it is winter and all. Good luck, please keep me informed. Other than what I have mentioned up until now I don't have anytihing else to offer you at this time. Just one more thing, I would if it comes down to replacing the PCM, try to find a good used unit before a brand new one because once you buy it you own it, no returns.
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New User
Joined: Apr 2016
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From: McCleary, Wash
How about the A/C cycle switch, it looks like a 3 wire switch could there be an open in that??? How about the ambient temp switch? Without the A/C the defrost is worthless
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Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
Idle
Power coming to the function switch is white/violet pin#1 coming out of the switch is pin #2 violet goes to the a/c clutch cycle switch. From there it goes to the high pressure switch then comes out red/yellow. Then goes to the PCM in a tan/light green wire pin #41.
The signal comes from the PCM from pin #69 in a pink/yellow wire to the A/C clutch relay.
From what I can gather using a '02 Excursion schematic, the 2 pins that connect to the PCM is 41 and 69. Since your A/C compressor is cycling, it shouldn't if there wasn't a signal to pin 41 from the cycle switch there wouldn't be a ground from the PCM to the A/C clutch relay to cycle the A/C clutch.
I at this point would try to find a used PCM to try. Barring any other out side issues that could cause a head ache. You didn't say if you have a Motorcraft PCV valve. If you have a aftermarket, ditch it for a Motorcraft. You couldn't believe how many problems an aftermarket valve can cause.
The signal comes from the PCM from pin #69 in a pink/yellow wire to the A/C clutch relay.
From what I can gather using a '02 Excursion schematic, the 2 pins that connect to the PCM is 41 and 69. Since your A/C compressor is cycling, it shouldn't if there wasn't a signal to pin 41 from the cycle switch there wouldn't be a ground from the PCM to the A/C clutch relay to cycle the A/C clutch.
I at this point would try to find a used PCM to try. Barring any other out side issues that could cause a head ache. You didn't say if you have a Motorcraft PCV valve. If you have a aftermarket, ditch it for a Motorcraft. You couldn't believe how many problems an aftermarket valve can cause.
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Mountain Pass
Joined: Mar 2010
Posts: 176
Likes: 21
From: GR, MI
Idle
Goggle 1999 F250SD A/C wiring and check out the wiring schematics. You should be able to see how the circuit works and maybe come to your own conclusion. If you have never looked at a schematic its not to terribly hard, the easiest way to read one is from bottom up from the grounds to power.
Good luck!
Good luck!
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