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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / April 2006Idle RPM sensitivity to signalling?
I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when the signal light is lit and cycles back up when it turns off and continues during the periodicity of the signalling. My dash lights also slightly dim with the same trend. My question is, is this normal? Should the engine idle and interior accessory lights really be this sensitive to such a small electrical draw? I have checked my battery at idle and it's charging at 13.8V, and I haven't had any other electrical problems. However, I do have some other erratic idling when not signalling, so if this could all be attributed to the same electrical problem I'd love to figure it out. Thanks. 407 > I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my > foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when [quoted text clipped - 9 lines] > > 407 Do you mean that the directional signals cause the idle to drop, or that the brake lights cause the idle to drop? if the latter I would actually suspect a vacuum leak in the brake booster. nate The car is an automatic, and if I'm sitting stopped with my foot on the break, in gear, signalling at a stop light for example, the RPM fluctuates with the periodicity of the signal light. If I merely shut my signalling off the erratic idling stops. Does that clarify anything or did I basically just repeat what I just said? Hopefully it helps... 407 So you have all the electrical accessories on. Do you have an after market radio with hi wattage amp? I think you are just seeing the load that a fully loaded alternator can put on an engine. Stan > I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my > foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when [quoted text clipped - 9 lines] > > 407 I do indeed have an aftermaket stereo with fairly decent capability of power drain (~600Wrms), but this problem arises whether the stereo is on or not. Last night for example, I had this problem and the stereo hadn't been on for at least 20 minutes of driving, so I would think that the battery should have been recharged fully by this point. But please keep firing away with possible causes, I really appreciate any help. 407 Does this same problem happen during the day when the headlights are not on? I would guess not. At night when this happens at a stop, put the trans. into neutral and see if problem goes away. Stan > I do indeed have an aftermaket stereo with fairly decent capability of > power drain (~600Wrms), but this problem arises whether the stereo is [quoted text clipped - 5 lines] > > 407 I just assumed that since the car has daytime running lights that it wouldn't be much different at night unless I used the highbeams. Anyway, I have noticed that when I put it into neutral the problem does go away as the engine is under less load. Out of curiousity, at night without the highbeams and A/C off, approximately how much power should all the accessories (including the daytime running lights and aftermarket stereo) consume? I guess small electrical problems like this could possibly be attributed to this draw and the cheap Canadian Tire battery I have. Sorry, I meant -excluding- the aftermarket stereo. I have seen that when the body loses it's ground. The GM's usually have a mesh strap from the bell housing to the body with a small black wire from the battery post to the body. When the mesh strap goes, the small wire will be severely overloaded and will usually start to melt the insulation before it burns up and the vehicle dies. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my > foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when [quoted text clipped - 9 lines] > > 407 Thanks, I'll have to check that out! I'm liking this explanation, based on the OP's clarification of the problem. I would also check all connections between the alternator and battery. nate > I have seen that when the body loses it's ground. > [quoted text clipped - 24 lines] > > > > 407 Either your idle speed is too low or the alternator is not capable of meeting the electrical demand at idle. This could be due to defective internal alternartor parts such as the diodes or stator neutral connection. It could also be due to excessive resistance due to bad connections somewhere in the charging circuit. The proper diagnosis would begin with an accurate battery condition test such as a load test and capacitance test. If the battey proves to be acceptable then the charging system could be tested for excessive voltage drops and a graphing multimeter or oscilloscope could be used to observe the voltage ripple in order to determine if the diodes are in good shape. Beacuse there are so many possible causes for the symptoms you described, I would recomend diagnosis by a competant technician. Otherwise you may wind up replacing lots of expensive components for no good reason. Kevin > I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my > foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when [quoted text clipped - 9 lines] > > 407 YOur battery is shot. Buy a new one. > I have a 95 Grand Prix GTP, and if I'm signalling at idle (in gear, my > foot on the brakeat a dead stop) my RPM decreases by about 50 RPM when [quoted text clipped - 9 lines] > > 407 Is the actual engine RPM as indicated by a tachometer independent of the cars electrical system dropping, or is it just the electronic tachometer that is fluctuating? If it's just the electronic tach, it could be a high resistance somewhere in the electrical system like a ground strap or battery terminal. Pete C. The tachometer fluctuating corresponds to the dimming of the rest of the system, but the actual engine RPM variance is so small (~25-50RPM) that it's too hard to tell if it is actually changing just by sound (I might have to check with an OBD-1 scanner). I've just been assuming that the tachometer wouldn't be susceptable to electrical loading in the rest of the system and was quite accurate, but I may be wrong. Also, I checked my ground, it's definitely solid. Since everything is still running ok, I'm not too overly concerned, but would the battery's ability to hold charge and the voltage regulation be the next things to verify? 407 Mine also can get swingy when my fan belt gets polished up. I test that by taking a cold off engine and seeing if I can hand slip the alternator pulley. If I can it will slip under load because the alternator takes a few HP to spin up under load. I would look at the battery cable ends next and clean them just because it helps. When I get corroded cable contacts my volt gauge gets swingy at idle and the battery doesn't get a deep charge. Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 http://www.imagestation.com/album/pictures.html?id=2115147590 (More Off Road album links at bottom of the view page) > The tachometer fluctuating corresponds to the dimming of the rest of > the system, but the actual engine RPM variance is so small (~25-50RPM) [quoted text clipped - 8 lines] > > 407 > The tachometer fluctuating corresponds to the dimming of the rest of > the system, but the actual engine RPM variance is so small (~25-50RPM) [quoted text clipped - 8 lines] > > 407 Weren't you were talking about low rpm - idle speed??? An alternator puts out few amps at idle. Enough to run the car but perhaps not enough to run the car, accessories, lights, etc. At low rpm there may be a draw on the battery. Rev the engine to 1000-2000 rpm and see if the dimming still occurs. If not, the battery is shot. If it does, then both the alternator and battery are shot. Or a bad connection. |
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