rev limiter? or problem?

[justin240]

in 4th and 5th gear at about 4k rpms my car just stops accelerating like i let off the gas without having done so it cuts back in after it drops under about 3k rpms or if i put in the cluch and let it idle for a sec, i dont know if its the rev limiter or a fuel cut but i want it to stop, any answers?it hasnt done it in any other gears like a few people i have talked to but that doesnt mean it wont eventually

by the way its a 89 240sx ka24de

[Vegascorbin]

This NOT the rev limiter. Rev Limiter should be at about 6000 to 6500 RPM. This is the same problem that we have been trying to figure out with our race car for over a year.

Most people will tell you to check the MAF or the ECU. Do check these things. Best way is by replaceing with known good one from a friends car etc.

Did this problem just start? If so what did you do to the car before it started? I am particualy interested in any electrical changes you made before this started or that you know were done to the car prior to you getting it. Our testing and research lesd us to belive that the problem is electrical in nature as opposed to mechanical.

Please keep me posted on the progress you make on this problem. BTW our car is a 90 240SX KA24E fresh rebuilt motor running in a Pure Stock class on a 1/3 mile dirt track.

[justin240]

i know the MAF is good but i dont know about the ECU, i have only changed the spark plugs and wires, the cap and rotar, and put on a cone air filter, i have also added a few electrical things but i always run all of my accessories directly off of my battery

i never had a problem with it untill i buried my tach when i hit 115 and buried the tach it did the cut out and i figured it was a speed limiter or rev limiter so i didnt investigate it, i did alot of in town driving after that so it didnt happen again until i took it on the freeway and it did it at 85, i was kinda lost at this point, but recently i have started to play with the "limiter" and it always happens at 85 in 5th like its set, so i took it to 85 and as soon as it cut out i put in the cluch, let it drop bellow 3k RPMS and let the cluch out and layed on the gas, i made it to about 100 before it cut out again, so its definatly not a set control but i cant think of what kind of short, or lelectrical problem could possibly cause it. i can keep it from cutting out if i am very gentle on the gas and gradually accelerate past it but i still dont make it past 90, also it doesnt like hills, i cant do more than 75 on hills before it cuts i think it has to be related more to speed than RPMs becasue i recently took my car out for a little fun in the snow and it will rev past 4k and 5k RPMs in 5th over and over again with no problem, im pretty much stumped.

[Eddiemac]

I had the same problem with my 89 coupe. A previous owner cut the green/yellow wire at the ecu and grounded it to the body, I assume in an attempt to bypass the limiter. I reconnected it and swapped in an auto ecu. I can now go faster than 70 without getting shut down. I also replaced the speed sensor at the same time, but im pretty sure that was a different issue. Hope that helps.

[gingerbredman]

i have a 91 with ka-e that did this exact same thing the other night on the interstate. it cut off exactly at 4k rpm in 5th gear, and a bit higher in 4th. i thought it was the speed sensor, because since i first got this one the speedometer hasn't functioned. i did find a plug that was ripped off on the pass. side of the transmission bell housing, i crimped on some connectors and connected them to the prongs of the sensor, but nothing happened.

i figured it was the speed sensor that was the root of this problem but i dunno.. hell out of my 4 hatches one ecu should be good, i'll swap em out when i get some time i reckon, see if that fixes things.

[Vegascorbin]

Thanks for the information. It is interesting that it appears to have started when you hit top speed.

We curently have nothing wired to our transmission. The drawings we have show the speed sensior output only going to the speeddometer. It must feed back someware to the ECU.

For my problem I am going to re-check trans to ecu wiring. I am also going to try running without a tach.

We have no dash in our race car so things are a little different.

[gingerbredman]

hmm since mine cuts off roughly around 80mph i may have a different problem.. we'll see i reckon

[del82]

Fuel delivery? That one's too broad to get anything like a definite answer, but if you've got the tools, hook up a fuel pressure gauge to the test port on the rail, some way that you can see while you're driving down the road. See if your fuel pressure is dropping when you're nearing those 'cut' speeds. Alternatively, you could splice in an O2 sensor gauge. You should expect it to run rich at WOT unless the ECU's been modified, if it starts to lean out, you're losing fuel pressure.

This is a common problem for a few other makes of cars with engines that, back in the early 90's, people didn't expect anyone to modify. A friend of mine had a fairly serious fuel delivery issue with his stock fuel pump after some head work on the 3.1 V6 in his cavalier. His was so bad, after the ECU had adjusted the fuel table he'd lean out and max out 4900 RPM even in first gear. After modifying the gas tank (added a second factory pump) it was back to normal, at least until he threw a rod, heh heh. Man, that engine was a monster. He went through 7 brand new motor mounts in a few months of launches at the dragstrip. It was actually trying to claw out of the engine bay and eat the competition, I think.

[Vegascorbin]

Not to down play your advice but I doubt it is fuel delivery, at least in my case. We had to swap the standard fuel tank for a race fuel cell and a Walbro 255 in-line. We are running about 50psi constant but not able to test while driving down the road (track). Our car is running stock injectors etc. should not want for fuel.

Also the KA24E doen not have a test port on the fuel rail.

Thanks for the input though.

Does anybody understand how the speed sensior on the trans feeds back to the ECU?

[del82]

Ah well, can't really guess anything that hasn't already been said from here.

If the speed sensor has a two prong connector and looks like a cylinder, it works via a magnet and stator, like a hall-effect sensor would in a distributor. As the stator's teeth pass over the magnet, it generates a current that the ECU reads. Changing how this one reads usually involves changing the little gear in the trans.

If its a big square sensor, usually with multiple bolts and a plastic/metal gear that can be pulled off, the actual sensor part works similar, but it allows for interchangeable gears to correct for tire sizes/gear ratios.

For mechanical speedos, the sensor is a geared cable that leads up to the dash where it has a magnet on that end of the cable, which drives the speedo needle as it spins faster. I think I've heard of a system that uses another gear on the speedo side to drive a small generator, and the faster it spins the more voltage it applies to the speedo needle driver.

[gingerbredman]

hmm.. i just wonder how the sensor works sometimes, and sometimes doesnt. if it's a mechanical sensor then it'd pretty have to be the ecu, unless gears inside the sensor are broken.

an electrical sensor of some sort would have me thinking a bad connection. ugh.. well if i dont work tomorrow i'll be working on my 2 main 240s. my single cammer just recently got this strong gas odor from the engine bay, not dripping anywhere but i probably should check it out as well.

well Happy New Year guys~! im out

[Ladreik]

Isnt the rev limiter in 5th gear and 5000 rpm and also at 113mph (or around 113) ???

[Ladreik]

Well, it is.... i dont know why I asked that.

[Vegascorbin]

Actualy I thought it was about 6300 in all gears with a 105 to 115 MPH speed liniter.

[justin240]

i started playin with the "limiter" again, when it cuts and the gas isnt workin if i let the pedal out it sounds like im engine braking but when i start pushin the gas it sounds like everything is running at the same speed(not accelerating, not slowing, just holdin pace) i have found a few other posts that suggest that its a problem with the 5th gear sensor or other gear sensors, and i think they are right, it seems like the cut is in the transmission rather than the fuel system or anything else

[partyax]

hey if u get this let me know by calling me 863 669 7652 i want to know how you fixed your passenger window problem...

[liquid_cool]

Not to down play your advice but I doubt it is fuel delivery, at least in my case. We had to swap the standard fuel tank for a race fuel cell and a Walbro 255 in-line. We are running about 50psi constant but not able to test while driving down the road (track). Our car is running stock injectors etc. should not want for fuel.

Also the KA24E doen not have a test port on the fuel rail.

Thanks for the input though.

Does anybody understand how the speed sensior on the trans feeds back to the ECU?

long time no talk too !..lol...did you swap a auto ecu in?...couse manuals have the 4th and 5th gearm limiter..but auto's donot!..this solved my issue....get back to me on this man.

[nofatchicks0073]

im really surprized at how long this went on for without any answers if you swap out an auto ecu that should work from what ive read but if you do a search you will find the answers you need i had this problem with my first 240 like six or so years ago you have three limiters in your 240 one being a speed governor at what i belive is 118 and the other two being rev limiters in forth and fifth gear i solved this problem by un pluging the two sensors on the side of the transmission and cutting a wire at the ecu what this does is keeps the ecu from knowing what gear your in or atleast keeps it from knowing when your in forth and fifth and by cutting the wire at the ecu you are stopping your speedometer from telling your ecu what speed your traviling i found this info i belive on this site but i could be wrong i may be new to this forum but i have been reading it for years and i have to say to whoever said they were dealing with this problem for years in a race car im a little disappointed all you have to do is search a little bit and i guess it helps to know what exactly it is your looking for by the way im not trying to come off a** a d!ck or anything hope this info helps ill look around a little and if i find the thread that helped me ill post a link to it

[liquid_cool]

Vegascorbin is a homie man who works on his race car with his dad..verry cool kid..and a vegas local....allthou this is a race car technicly...its just for dirt..and from the last conversation i had with him and his pop's...it must remain stock in engine and suspention for his class...so not all searches are straitforwards as you may think...so please...refrain from implyed rude comments and ask!...we are allways willing to answer here with kind words and helpfull advice:)

BTW..welcome to nicco....hope to see some more helpfull and constructive post from you in the forums.

[nofatchicks0073]

if i came off as an a** im sorry that wasnt my intention anyway i just got home from work and hit up the google machine and found this for ya i needed to find it again anyway cause im fixin to pick up another 240 soon and i couldnt remember witch wire it was but its the yellow wire with a green stripe on your ecu (this is for a 91 and up s13) the 8th one over from the right on your harness this is the vehicle speed sensor signal. Cut that wire. That is it. If you have a California car I think it will trigger a code, but for the rest of us, it is an easy trick. Your speedometer will still work because the signal comes from the transmission, goes to the gauges, THEN feeds the computer. dont forget to pull those two sensors out of the transmission to though or else you will hit the rev limiters still well hope this helps you out if you have any other questions about this let me know its not that hard but i have done it on three of my 240s so i can walk you through it more if needed

[compactfean]

Im not running 4th or 5th gear sensor and don't have any problems. What's the purpose?

[nofatchicks0073]

the forth and fifth gear sensors are the so the computer knows when your in those gears and it can activate the rev limiters for those gears

[compactfean]

Any downfall to not running them?

[nofatchicks0073]

none what so ever

[shadowhaven]

Sometimes mine acts like it hits rev limiter at just over 5k rpms. In 1-4? Any ideas? Doesn't do it all the time, mainly just when engine is cold.