'94 Convertible ** S13 auto trans prob = NOT auto trans problem

[honeybus]

QUOTE=guyaverage]

Now if you can find an extra 1991-1994 ATCU.........

Your plan to test the tps hot and cold is a good plan. You might also want to check the temperature sensor hot and cold readings too (2500 ohms cold, 300 ohms hot). The plug for it is behind the battery, check out page AT39 in the factory manual. Other things that you should really also check are the speed signal circuit and revolution sensor circuit (pages AT-32 and AT-33).

Barry says: Thanks for this tip.

Since you have the same symptoms on 2 different transmissions I still think either the ATCU is faulty or there is a problem with the wiring between the sensors on the transmission and the plug on the ATCU.

[/QUOTE]

So I'm back again, with an update!!

I have a TPS, ECU, and ATCU from a '92 240SX, and a TPS from a 5-speed 240SX. So I have some experience at the junk yard in pulling these TSPs off. ((( I am reading and researching and studying on the TSP, but I am relying on GuyAverage to give me the right scoop since he knows I am working on an AUTOMATIC!! )))

Last night I checked the two TPS (auto and 5-speed) on my workbench. I used two different digital millimeters for my readings so I could be confident that I was getting the right measurements. Of course, the millimeters gave different readouts, but I attributed that to the connections at the probes. (( I had three alligator clipped wires running from 1-2-3 on the TPS so I only had to connect - reconnect to the ends of the alligator clipped wires, and I would not disturb the TPS body. ))

I got a decent set of figures that told me that when I turned the inside of the TPS, as it would be turned with the throttle arm, I got a range of from 2k - 10k or there about when the TPS was advanced.

Then I went to the TPS on my '94 240SX. After taking a bunch of stuff off, i was able to get my alligator clipped wires to the TPS while it was on the car. Wow!! I was getting irrational readings, like 25K ohms and all strange stuff. I tried with the pedal up - half way - down / depressed, and I did get some variances, but nowhere like I saw on the two TPS on my workbench.

I swapped the TPS from the '92 into my '94. I took the '94 TPS to my workbench to check the ranges of resistance. Darn!! This '94 TPS is better than the one I installed!!

I test drove the 240SX last night, and I did not run it enough to get the original problem, but I noticed that I was getting a jerk in going from 1st to 2nd (( like I experienced with my other trans )), and after it was warmed, my idle was bordering on stalling, less than 500rpm. I did not do the test GuyAverage said to do on the TPS because I was under pressure to get the 240SX out of the garage. I will definitely get that right next time around. (( Hey, should I swap the '94 TPS back in ??? ))

I am waiting for another ECU and ATCU, these are from a '94, so they may be a better match than the '92 ECU and ATCU I have in hand. I expect them in this week, and I will be back on the '94 this Saturday.

Thanks for watching my frustrations!!

[honeybus]

If you've been following my plight (trans down shifts on its own at highway speeds; seems to shift to neutral at cruise; jerks going up the gears) the past month or so, may I tell you that the TPS did not seem to be the culprit, even though I was showing a code 43 on my engine light, and recognition flicker 3 on my AT-29 check.

After replacing the TPS, I got NO encouraging results. A test drive showed the transmission still down shifting at highway speeds, and the codes kept coming (32; 34; 43 + #3 flicker recognition).

I swapped out the ECU with another 1994 ECU, and I swapped out the ATCU with another ATCU. I took it for a test drive, and the results were phenomenally good!!

Now only did my unsolicited downshifting disappear, my shifting from 1st to 2nd which had been a jarring shift had smoothed out completely!!

I still have a check engine light, but I am going to ignore that because I got the ECU all buttoned up again, and I am heading out for a 100 mile round trip test run right after I get home from work today.

Thanks to all, and special thanks to GuyAverage for sticking with me throughout.

[masterteque]

I wish I saw this thread sooner...I would of told you to go after the Auto trans unit first....I have a ecu and trans unit that I could of sold you....Most Trans units are affected by internal heat inside the unit. YOu can let it cool down and it works again....Glad to hear its fixed....

[guyaverage]

Awesome, glad it worked out!

Ironically, now that its getting colder out my transmission is really causing problems. Shifts great 1-2-3, then fourth.. then BOOM! when fourth locks up. Its like having a shift kit but only for fourth gear lockup. ATCU isnt throwing any codes, all the electrical checks described in the FSM are good, fluid is clear, clean and smells normal.. I'm at wits end. Swapped out the atcu today with a different one I tracked down on Ebay, no change. I'm going to have to drop the pan, pull the valve body, check the 3-4 accumulator, etc. What a mess, at least I can do all this with the transmission still in the car. Like I've got time for this.