QuestionQUESTION: I have a 1999 Jeep Grand Cherokee 4.7L and when I floor the accelerator all of my gauges go up, the low fuel light chimes, headlights and instrument lights get very bright. What could this be? Some kind of electrical or voltage regulator problem? Perhaps the computer?
Thanks!
Ryan
ANSWER: Ryan,
Strange, but I seem to recall this symptom from 7 years ago, but I don't remember the fix, unfortunately. The '99 WJ was a major change in the GC line, and since it was first year, *lots* of things went wrong with 'em. I checked the prodigious TSB list, and while a few had to do with instrument clusters, none of them addressed this issue directly. That doesn't mean a controller flash wouldn't solve this situation -- an anomaly in the controller software (PCM or BCM in this case) could trigger this response, and not be documented by Jeep.
Has the car been serviced in this area before? It's possible that a ground wire has loosened up, and causing something like this. Most erratic electrical conditions can be attributed to poor ground and improper connections -- *not* failed components. Sometimes, reseating things like the ASD relay (in the PDC) can resolve these problems. The PCM regulates voltage in the system, not the alternator (technically a generator), so it *is* possible that the PCM is a suspect, although I'd be more concerned with connections and grounds. The PCM has a number of grounds, some meant to be redundant, but they cause issues nonetheless. My biggest concern here is the condition of the fault -- is it *only* under wide open throttle, or any time the vehicle is accelerated? Does it happen any other time? Did anything, like a repair or car stereo installation, cause this to start happening?
While you may be able to track this down, the first thing I would do it hook up a DRB3 (scantool) and go for a ride. The vehicle computers have the ability to store codes, so that could lead a tech in the general area. If no codes, at least you can see a realtime shot of what's happening, which is necessary when tackling these problems. However, it can turn costly very quickly, and some people just live with it, especially if it's *only* under WOT. Whatever you decide, good luck to you!
---------- FOLLOW-UP ----------
QUESTION: Yes I know that the 99 Grand Cherokee are known for their many problems because of its redesign, unfortunately. The car has recently been serviced to replace the fuel sending unit. I never noticed it before this fix but then again I usually try to stay off the accelerator unless I need to (uses too much gas!! haha). The only time it happens is when the accelerator is fully down, and even then if I accelerate for a second it wont do it, but if it revs out through a gear or two it will. I thought maybe some kind of regulatory problem only because it is not under normal conditions but when the engine is revving high. So you think it is likely a grounding issue, possible as a result of the fuel sending unit? Thanks for your help!!!
Ryan
AnswerNo, component grounding would only cause a component issue -- like low fuel pump output. I was thinking more along the lines of instrument cluster or PCM grounds. However, the fact that it only happens under WOT in extremely high revs, leads me to believe that it could be an anomaly. Almost as if the generator is at full field, and the PCM's over-voltage regulation shuts down for a split second. Are you past the redline on the tach (if you have one) for that brief time before the tranny shifts?
Strange. I *know* I've seen this before, but I might consider this a 'feature' of the vehicle that hasn't been tested -- most people don't hit WOT, and if they do, it's only for a snap. I'm leaning away from a ground/connection problem. Regardless, for an issue like this, you really need a tech with a scantool.
