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Unread 07-27-2009, 11:43 AM   #1
RiverMCK
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RPM's going crazy!

Greetings all,
I'm a long time fan of The Forum and as of today a new member.

Here's my issue: Whether I'm driving 55 or 75 on the highway, my engine's rpms will fluctuate randomly from 2000 to 2500., going back and forth, up and down. I can definately hear the engine working hard when this fluctuation occurs. It usually happens when the engine is warm, but like I said, it is random. Could this be a crankshaft position sensor or a throttle position sensor issue? I'm at a complete loss. Below are some quick specs on the rig:

97 ZJ, 4.0L, cold air intake, new water pump and thermostat, "new" rebuilt tranny, replaced all plugs ~ 2.5 yrs ago.

Thanks for help!

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Unread 07-27-2009, 11:49 AM   #2
ZeJokee
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I had a similiar problem but mine was the torque converter locking up and unlocking. I replaced my brake pedal switch and it fixed it. I was running around 2200 rpm then would drop to 1800 rpm. Maybe yours is doing something similar but in the opposite way. But it could be something else.
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Unread 07-27-2009, 12:58 PM   #3
RiverMCK
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Just read an old thread on the similar subject. Could it be an Output Speed Sensor? Where is that located and does anyone know how to test it?
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Unread 07-27-2009, 01:48 PM   #4
cmonson
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sounds to me like the output speed sensor.

It could also be the throttle position sensor. I had the same issue, and I repaced the TPS b/c of the code it threw. It solved the issue for good.
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Unread 07-27-2009, 07:06 PM   #5
Pbux690
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Mine did the same thing and its the TPS sensor. Throttle Position Sensor. You can get it at advanced for like thirty bucks and its an easy fix
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Unread 07-27-2009, 08:08 PM   #6
crazydude2500
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im gonna but in here what would the brake pedal switch have to do with it ?? i just had some problems with the lights staying on and im pretty sure i need to clean out my tb because my rpms do the same thing
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Unread 07-27-2009, 08:29 PM   #7
ZeeJay1997
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I think the output speed sensor would cause the speedo to cut up.

The torque converter clutch disengages when the brake pedal is applied. Try applying light pressure on the brake pedal at 70 mph and you should see an RPM increase of 200-250 RPMs (at least on my 100K tranny).

If you can't nail it down to the brake pedal, I suggest taking it back to the shop that did your tranny. 500 rpms is a bit much.

There's an outside chance the CPS could be changing your timing. Possibly damaged in the tranny swap?
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Unread 07-27-2009, 09:25 PM   #8
RiverMCK
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Okay, 500 rpms might be a slight exaggeration, but it's definately around 250-300. On a side note since the brake pedal was brought up, occasionally my brake lights do stay on after I shut off the ignition. I then have to reach up and manually press the brake switch behind the pedal so that it returns to the full "out" position. Then the brake lights go off. Regarding the tranny, I think the problem was occurring before I had the rebuild done almost three and a half years ago. But that was a long time ago...I'll see what the RPMs do the next time I'm at highway speed and tap the brakes.
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Unread 07-27-2009, 09:34 PM   #9
ZeJokee
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Quote:
Originally Posted by RiverMCK View Post
Okay, 500 rpms might be a slight exaggeration, but it's definately around 250-300. On a side note since the brake pedal was brought up, occasionally my brake lights do stay on after I shut off the ignition. I then have to reach up and manually press the brake switch behind the pedal so that it returns to the full "out" position. Then the brake lights go off. Regarding the tranny, I think the problem was occurring before I had the rebuild done almost three and a half years ago. But that was a long time ago...I'll see what the RPMs do the next time I'm at highway speed and tap the brakes.
I would tap the brake pedal and pull up on it with your foot to see if your rpms change. I didn't have any problems with my breaks or anything else. The switch was the last thing I replaced. I still would check everything else though too.
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93 ZJ Ltd, Blacked out, 5.2L, Reman 46RH, Reman 242 Swap, 3.5'' IRO + 2" BB, 33x12.50r15 Mud Claw MTX radials, Bushwacker Flares, 216,XXX miles (Still parked in NC)
04 F-350 SD Crew Cab FX4 Lariat, 6.0 PSD SRW, Head studs, AFE Stage 2 SI Pro Guard 7 intake, Blue Spring mod, 150,XXX Miles

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Unread 07-27-2009, 10:09 PM   #10
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Does it when you have the cruise set? if it does try not setting your cruise at all when you get on the hiway for a few times. mine did the same thing and it was a vaccum issue with the cruise cont. I never fixed it I just dont use the cruise that much so it wasnt worth it to me. Took me two trips out before it stopped, but since I havent used the cruise I havent had an issue.
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Unread 07-27-2009, 11:40 PM   #11
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X3 - Bad TPS
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Unread 07-28-2009, 12:10 AM   #12
xJoshxx
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Quote:
Originally Posted by ddruker View Post
X3 - Bad TPS
x2the4 TPS
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Unread 07-28-2009, 03:46 AM   #13
ZeeJay1997
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Quote:
Originally Posted by RiverMCK View Post
occasionally my brake lights do stay on after I shut off the ignition. I then have to reach up and manually press the brake switch behind the pedal so that it returns to the full "out" position.
There's your sign
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Unread 07-28-2009, 04:21 AM   #14
ratmonkey
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Much, much more likely to be the brake switch. You would also have cruise control issues with a sticky brake switch. You also have brake light issues so that points you there.
Tps is second most likely culprit, go dealer only on that. The cheap parts store ones are often out of spec pn initial and final voltages.
3rd would be the tcc solenoid assembly in the trans.
Output speed sensors have nothing to do with the speedometer and would more likely cause it to not go into od at all.
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Unread 07-29-2009, 12:23 PM   #15
RiverMCK
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Thanks to everyone for the replies. I'll definately look into replacing the TPS and brake switch. How difficult is swapping out the brake switch? The po installed a "valet" alarm switch and there's some sort of shock sensor mounted to the underside of my steering column. With all the wires down there it looks like a bowl of spagetti!
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