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'99 APPS P0121 problem, Can't find the Cause

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Hey all,

I'm posting this for a friend. He has a '99 24v with an auto. About a month or at least within 6 months after buying it, it started having APPS codes coming up. It started giving him throttle problems and what not. Replacing the APPS helped, for a while.

He did drive around for months with a bad VP44. He has added an Edge 'chip'. Since then he has had some work done on the tranny (New torque converter and some valve body and what not), and another APPS or two.

At one point in there, taking the cover off of the APPS when it was hot out 'cured' the problems for about 6 months. Now it doesn't seem to matter whether he is towing or not or if it is hot out or not. He has been going through all the grounds and is now checking fuses and all the connections he can find. He just told me that two wires to the APPS have 4.5v on one and 5v on the other (He unplugged the connector to check this). I don't know if that would be enough voltage drop to cause this or not.

Currently it is causing the engine to flare about 2 or 300 rpm while going down the freeway. I thought he told me that shifting out of overdrive would cure it (Too many different trucks with too many different problems running through my head right now. Doesn't help that they have started to throw these kind of problems at us in school right now too....). In the past it has caused weak engine and even dead pedal with nothing above idle. It is weird to me that it seems like changing the APPS will cure it for a while.

If any of you have any ideas, please let me know! I think I will make sure he gets on here so he can straighten out my story telling or to answer any questions any of you may have on this. It would be better than great to be able to point him in the right direction to get this problem cured!

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I had a similar problem, (on an 2003 CR though) would replace the sensor and it would go bad a again a few months later, sometimes it would go dead pedal alot and other times i had no issues, i noticed a trend that it would occur after the motor had been running hot for awhile (like on a trip) or going slow for a long length of time(cruising a dirt road), i eventually found that the ECM had gone bad on that particular electrical driver and it affected it. i also got a new harness as the old one was hammered from a transplant and found too many scotch locks on the orginal one.not sure if that helps, but he could have the ECM checked to make sure all the drivers are good and go over the wiring and make sure their are no shorts..

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have ya checked the voltage on the apps to make sure it is in spec. sometimes the converter locking and unlocking spurratically is casued by the sheilding on the wires breaking down and catching stray voltage ..i had the same problem with mine

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Sounds like the torque converter is locking and unlocking.

What shape are his batteries? Lumpdog was telling me that so far on 2 trucks it was the batteries causing this issue.

Mine was #3 actually. One battery had a dead cell.

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Thanks guys for all the ideas. I printed this out and gave it to him. He has new batteries and the problem didn't go away. He tested them anyway and they are supposedly good.

He isn't sure if the torque is unlocking or not. I kind of think it is as he was level and unloaded on the freeway. He is going to pay more attention as to how it is acting.

I am trying to get him on here so he can correct my story and answer any questions that may come up. He should just about be through all the wiring and connections by now. So far he hasn't found any corrosion........

I'm starting to lean toward an ECM or maybe the electronics on the injection pump (If the wiring and voltages check out.....)

What say you? Any more ideas?

THANKS so much for all your input so far. I really hate to see him so mad at it. I'm afraid he will just send it down the road. Everytime he gets to messing with it the problem will quit for a while and then come right back......

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Wow, this is an old thread! This has been going on way too long!! I have kept after him, trying to get him to keep this truck. He has been ready to get rid of it many times because of this on going problem. Well, he may have finally gotten the gremlins out of it!!

His '99 had multiple ground problems here and there. And after replacing most everything sensor and controller wise it turns out that the alternator was 'fading' out after a while! It would start off fine and after working for a while the voltage would just fall off gradually far enough to start messing with the computer! He says to not rule anything out when trying to fix a strange electrical problem like this! Stay in there and really see what all is happening.

He also says not to ever go to Sundance in Boise! On one trip they supposedly found more ground cable problems and fixed them. They left off one of the main grounds from one of his batteries. On another trip they cut his chip off the truck! That tech hates them so he just cut it off at the plug. . . . The manager is no good either: he stated that 'We have trouble with him like that sometimes' but 'We won't replace your chip'. He isn't the biggest guy but he is big enough to put some hurt on them! I kind of wish he had!! That whole deal was nothing but crap!!

Anyway, they didn't find this 'fading' alternator. They wanted to replace the PCM, again. Fortunately he found the alternator problem on his own. Now with all good grounds and a working alternator (And a whole shelf of 'extra' good parts), he has a great running truck again! In fact that 'smile' is back! He told me his wife doesn't drive it much. She backed out of the drive and then 'nailed' it like she would her ride and spun out. It kind of spooked her so she backed up and asked what was wrong with it. He just smiled and told her to ease up a bit. :rockin:

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I wanted to make sure I pointed out that this alternator 'fading' was not regular or quick to occur. He might go for weeks with no problem. When it did happen it wasn't right after he started the truck. It might be most of the way into a 600 mile trip. It had no rhyme or reason or pattern. He has had this truck for over a year and most likely the previous owner got rid of it because of it. Many people and different shops have had a crack at it and didn't find it.

I only wanted to point that out so if you ever see this you might remember to keep an open mind. Like I said; back up and look at the whole picture. Don't rule anything out. Always go back to the basics. That is something that keeps getting me into trouble. It is too easy to get into a mind set that you know what is wrong or what is going on and you miss something as simple as the voltage is falling off when the problem occurs.

With my wonderful computer controlled vehicle it makes me wonder if I shouldn't install a separate, highly accurate volt meter as one of my add on gauges. . . . .

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