1996 Pathfinder SE/ jumping tachometer

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jkc
Posts: 3
Joined: Sun Jan 08, 2012 12:08 pm
Car: 1996 Nissan Pathfinder SE

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When cruising (55+ mph), my tachometer jumps rapidly from 2100 rpm up to 25-2600 rpm, stays there for several seconds, then returns to 2100 rpm. This occurs repeatedly every few minutes. On occasion is will jump up to 3200 rpm and then return to the normal 2100. The engine revs when this occurs but there is no change in speed and no apparent shifting.

Thanks in advance for any ideas on what might be causing this and potential fixes.


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Towncivilian
Posts: 4995
Joined: Sun May 23, 2010 10:21 am
Car: 2001.5 Pathfinder SE 3.5L 2WD A/T
2007 Altima 2.5L CVT
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Sounds like the torque converter locking and then unlocking, but I wouldn't expect that to cause such large changes in RPMs.

Inspect the vacuum hose for the ASCD pump. Replace it if it looks damaged at all.

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jkc
Posts: 3
Joined: Sun Jan 08, 2012 12:08 pm
Car: 1996 Nissan Pathfinder SE

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I did not mean to imply the cruise control was on. This occurs during open highway driving at constant speed. I thought the ASCD pump was specific to the cruise control system. Sorry for the confusion.

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Towncivilian
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Joined: Sun May 23, 2010 10:21 am
Car: 2001.5 Pathfinder SE 3.5L 2WD A/T
2007 Altima 2.5L CVT
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2012 G37 Sedan 7A/T
Location: Florida, USA
Contact:

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Ah, I thought you meant with cruise activated. My apologies.

What's the condition and level of the transmission fluid?

1. Drive around for 10 minutes in the city after the transmission is at operating temperature.
2. Park on a level surface and apply the parking brake.
3. With P selected, shift through every gear and return to P.
4. Pull the transmission dipstick with the engine idling, wipe it clean, reinsert it completely, and read the level. It must be within the notched areas labeled "HOT". Check for fluid contamination after wiping on a paper towel.

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The fluid should be red, and not smell burnt. If it is, do a drain & fill or a cooler line exchange.

jkc
Posts: 3
Joined: Sun Jan 08, 2012 12:08 pm
Car: 1996 Nissan Pathfinder SE

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I do a drain and fill on the transmission every 2 years using amsoil synthetic. The fluid level is within the specified range. I have had a check engine light on for some time (several months). I pulled the PCM but there was no diagnostic switch. I also went by autozone to use their scan tool, but apparently mine is ODB-I not ODB-II (it was actually built in '95 and '96 was the switch year). When I pulled the PCM, I unplugged it and the check engine light went out and has yet to come back on. I was not able to get the code. Not sure if this has anything to do with it or not. I have had transmissions that 'slipped' before (prior to the transmission going bad) and this is different. It seems more electrical than mechanical. Something with the sensor package or the overdrive?

Thanks for your help.

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Towncivilian
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Joined: Sun May 23, 2010 10:21 am
Car: 2001.5 Pathfinder SE 3.5L 2WD A/T
2007 Altima 2.5L CVT
2012 Sentra 2.0L CVT Special
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Location: Florida, USA
Contact:

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Hmm. Since you have kept up with maintenance and are using an excellent fluid, you are probably correct in that it would be more likely to be a solenoid issue if there is an issue with the transmission.

What's the manufacturing date of your Pathfinder? I thought all vehicles sold as "1996" model years had to be ODBII compliant. The 1996 Pathfinder has two data port connections - perhaps the AutoZone employee did not see it? See page EC-54 of the FSM.

See page AT-42 of the FSM for a diagnostic procedure - see if you can coax an error code out of the transmission. As for the service engine soon light, you'll just have to keep driving and wait for it to potentially reappear I think.

ARKQX33V6
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Joined: Fri Aug 06, 2010 3:35 pm

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You need to check your engine transmission as this revving takes place. You state that the engine revs but transmission remains in gear.

Does the engine rev? Use a separate individual tach so you can see. You cannot trust the built in tach in this situation.

Your ears are a good tester, but you can be fooled.

At top gear, at highway speeds the motor and transmission are locked together, the torque converter if it lets loose will allow the engine and transmission to rev differently and the trans may shift depending on road speed, amounts of slip and torque...maybe you can feel it maybe not.

Most people feel what is expected it is the unexpected that causes concern.

An oil pressure gauge is needed on the output of the transmission to see if pressures are changing, but I don't expect that from you, but if you can borrow a tach that can be placed on a spark plug you can see what takes place as the engine revs up.

The 96 does it have an electric tach or cable driven? If cable it may need to be cleaned and lubed. An electric may be failing, the pulse counters are scrambled or condensers failing or PWM system is crapping out.

You must be certain as to what is taking place. Failed or failing components to read RPM may be causing this and without further investigations you along with me are shooting in the dark.


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