1992 240SX auto.Irratic RPM's in O.D. at 58-62mph..downshifting...ANNOYING

[luna11]

car runs great,shifts smoothly, until about 58 or so, and then if i'm cruising along the rpms seem to drop a bit and the car seems to down shift until i accelerate a bit and it gets to about 63 or so and it runs perfectly smooth...only appears to happen in O.D....(this is an automatic)...don't really like to drive at high speeds for extended time without overdrive, but this is annoying...has new wires and plugs...transmission fluid level is fine...could it have anything to do with fuel delivery/filter or fuel injectors...or is this a sad transmission story???? has not ever been "beat" and has 120,000 miles on it at present....any suggestions to a girl who loves her convertible?????(besides the obvious, switch to manual)

[onosqv]

um, when you say "the car seems to down shift", you mean, down shift rite - your rpms will jump from like 3k->4k. I got confused because you say the "rpms seem to drop" then "car seems to down shift", which are to opposite things.

In any case, what was the last thing you changed on the car before this started happening?

Also, is your speedometer working correctly? I'm asking this because if your rev speed solenoid on the transmission fails, then it goes to the back up, which is the speedo. And if it is in backup mode, and your speedo isn't working properly, it will give you weirdo shifts on the highway - that's what happened to my car when I disabled the rev speed sensor to see if I could fix another problem.

[luna11]

to clarify...the rpm's drop only about 200-300 and it seem to drop to a lower gear..if i give it gas it jumps back up...cruise control stopped working at about the same time if this is a clue???? changed the plugs last......

[luna11]

BTW, someone suggested it might have something to do with the rev speed sensor???? sorry about the mistake with "up" and "downshifting"...seems like if the engine drops rev's that that would be downshifting????? but what does this girl know....i just want it to work....thanks so far for the input...speedometer seems to be working fine which someone else was wondering about......

[onosqv]

Well, no, that's something else.

I'm not sure what your problem is, but it's definately not "downshifting".

Going down in gear at the same speed will make the car rev higher.

i.e.if 4th gear (overdrive) going @ 60 you are @ 2.5kthen 3rd gear going @ 60 you will be at 3k+.

What it could be is possibly your tps (throttle position sensor) and since you normally drive in that range, the sensor could be bad and fluctuates. Iono, just a guess, but maybe test the voltage of the tps - I'm sure you can search for that on here.

Basically, you use a multimeter and probe a couple wires while you move the throttle position back and forth from 0->100% and see if the voltage it spits out is smooth.

What happens if you drive @ 70? or any other speed higher/lower?

[luna11]

thanks for explaining...and you're right,if it were downshifting (like when you downshift in a manual) the rpm's go up....and in this case they only drop a bit...and then i get to about 63, it straightens out and drives perfect...of course then i'm ususally speeding...i will ask my mechanic to check the tps....also when you were talking about the rev speed sensor, i thought that was responsible for shifts from 1st to 2nd???? am i wrong??? read a big long thread on that and it seems that just by unplugging it under the hood ,cleaning and re-plugging straightened alot of others problems out....Do you know the location of the tps under the hood from a shop manual...i think the rev sensor is E13???? any idea????

[onosqv]

it's probably not your rev speed sensor - it's just the first thing that came to mind when I first read your thread.

I don't have my service manual in front of me, but I believe the tps sensor (located by the throttle body has a 3 wire plug coming out). the two outer ones are the ones you want to connect neg & pos too - I'm not sure on this, but from what I remember that should be it.

After the car warms up (maybe not even need to warm up), at idle should read below .5vdc. At 100% throttle, should read 4.5vdc. I don't think your car has to be running when you do this, just in the ON position.

[onosqv]

In addition to that - if both #'s check out, try slowly sweeping from .5 -> 4.5 and see if there are any parts where the sensor is hesitant/jumpy/etc.

[guyaverage]

What it could be is possibly your tps (throttle position sensor) and since you normally drive in that range, the sensor could be bad and fluctuates.

I agree 100%. I was screwing around with mine out of boredom, just to see how I could get my automatic to shift sooner or later. When I advanced it too far (rotated towards the front of the car) the symptoms you (luna11) talk about are exactly what I got.

Its also possible that your tps isnt misadjusted, it may be that it has a "dead spot" at that throttle setting causing the problem. Either way, with this many miles on it, I would replace it. I think I paid about $40 for mine after shopping around. Most places wanted to order it, a few had it. Its easy to adjust. You will need a circuit tester though. The cheapest of cheap ones will do fine.

Also the tps on an automatic car is adjusted by the closed throttle switch on the tps, not by voltage readings (although you should check voltages anyway to make sure it works properly throughout its range). I adjust mine by checking for continuity on the closed switch with the engine off. Loosen the screws, then I just barely tap it until there isnt continuity, then I gently bump it until there is, then I tighten the screws. After they are tight, verify continuity again (to make sure it didnt go off adjustment when you tightened the screws). I have adjusted mine this way every time I mess with it and it shifts and drives perfectly. If I do it any other way, it may run good, but it wont shift right.

[luna11]

well guess what??? my mechanic says a light would come on if it were tps....to check my spark plugs...i told him i had just replaced them with NGK's...he asked if i had gapped them...well they told me they were "pre-gapped"! well when i pulled them there were two at about .038, and two at about .045....i gapped them, through them back in, and presto....no more problems.....still good to know if i have problems in the future about the tps...i priced them online when i thought i was going to have to replace and i found several at about $35-40.....