92AccordLXowner

Hello -- '92 Accord LX, -- 84,000 miles. Had a shifting problem and replaced the transmission control module under the rug with a used module. Seemed to cure the shifting problem, but D4 light continued to flash and using the paperclip jumper, I determined the code was for NC Speed Sensor. Replaced that. Now, on start-up the transmission seems to shift correctly and initially there is no flashing D4 light, but after driving for a while and shifting through the gears the D4 light does again start to flash. Turn off the ignition, and when it starts, the D4 light does not flash. But it does begin to flash again after driving for a bit. I have reset the TCM by pulling the fuse temporarily, but that does not affect the problem.

Problem is, using the paperclip jumper now I do not get any more codes from D4. Rather, I either get nothing from the D4 (no flashes, no light), or I get a brief pause and then continuous short flashing. Any clues?

Frank Allen

when you replaced the TCU did it have a PXO- 722 or 721?? these things all look alike and will plug in but they are not the same, the above numbers are for 92 LX or DX
Frank Allen

92AccordLXowner

It was a PXO-721. I know that now because the number - 28100-PXO-721-M1 matched up perfectly to the one I took out. That # was specified in the EBay listing. The EBay seller also said it had been taken out of a '92 Accord LX. Upon receipt, I opened it up and looked at the resistors and capacitors and they looked fine (as the EBay seller had claimed).

The one I took out had a burnt resistor -- I would have replaced the resistor with one from Radio Shack but I couldn't detect any other problems and I thought there should have been another problem. Maybe I replace the burnt resistor and put the old one back in.

Anyway, what from my description made you think it might have been the wrong unit?

I also drove it today on the highway and got it up to 60 mph. D4 light again started flashing at about 30 mph. Although it shifted, there seemed to be a delay in the shift (higher RPM) from when I would think it should shift. Also, the shifting seemed "harder."

92AccordLXowner

Also -- the original shifting problem which caused me to replace the TCU was one which has been described frequently on the web. That is, a "limp mode" type of problem. Started in a higher gear (perhaps, third), and seemed to stay there. The TCU unit change seemed to have fixed that problem, at least.

92AccordLXowner

Moreover, perhaps I should say again that after replacing the TCU I was able to get the error code for NC sensor (I think I remember nine flashes before pause) by jumping the cables under the dash.

Frank Allen

i had replaced mt tcu with a used unit and had the same problem you descrobe, however it was a slightly different number , i took it back 4 times to the salvage yard before i finally got a good one, i could not vissually see anything wrong with the first one, anyway i thought id pass this along.
Frank Allen

P_Adams

It's surprising how many of these turn up defective. Your salvage yard experience is about correct. Unlike ECM's, Audio Equipment, ABS Modules and such, salvage yards tend to leave the TCM's in the car rather than inventory them. It's usually a lot more straightforward to just buy either a new or reman unit.

P

Good to hear you got it sorted

Frank Allen

found that a pxo 722 is better than a pxo 721, 721 had defective parts from honda either will work in dx or lx 1992, also found that a poor solder joint in the main fuel relay will also cause transmission issues because it supplies a voltage to the tcu, if not present you get the issues had to resolder my pigtail for relay
Frank Allen

92AccordLXowner

Thanks guys. Frank, if the car always starts and doesn't have classic relay problems, it could still be a bad solder joint in the main relay? Simply for transmission issues? Cheap try to fix, about $40 if it may work.

Took it out on the highway and there seemed to be delayed shifting, but the D4 light is still not showing any codes for me. Same behavior as noted above. Car is basically sitting on the driveway.

Odd that the replacement TCU would show code for a NC sensor then fail to show any code at all after sensor replacement. Suggests to me that the TCU was working, at least for a few days? No?

Charles

Frank Allen

Whats unreal i had three of my accords do this, all three were checked by a good mechanic, he said that the transmission,s were slipping a bit in overdrive causing the problem, this was a bad call as i fixed all three with a different tcu, no problems that could be seen with eyeball existed with any of them and no codes existed with anyof them, on my 92 got so bad it would drop to limp mode and back to normal sometimes 7 or 8 times with out stopping, ive drove this car 38000 miles since that time without any problems, i took out the pxo 721 and repalced it with a junk yard pxo 722 which is supposed to be a better unit, i talked to hinda who told me that the 721 had many questionable parts and many failed, 722 is suppose to be improved. i doughbt that your problem is the realy since it always starts, the speed sensor will also cause this kind of problem and the computter dont alwys find the codes, i dont know what part of the country you are in but thes parts should be available cheap most places. i hope you get this worked out, the tcu you bought could still be a bad one, they do some funny thing s as does this relay , i dont know what else to tell you. i have a pxo 721 on my work bench but cannot find a problem with it even though i hace all electrnic test equipment, but it is definately a bad one. Good luck
might be a good idea to carefully check the wiring that goes into and out of the relay to be sure of the solder joints, some 92,s were bad , and in my 93 accord i had that problem as well. since a voltage goes from there to to tcu and has to be present .
Frank Allen

92AccordLXowner

BINGO, Kudos to Frank and P Adams for giving me the clues I needed to solve the problem. Purchased a used pxo 722 TCU and everything is now copacetic. No blinking lights, no funny shifts, no delayed shifts, no stalling, no problem codes. The replacement pxo 721 looked like it was in excellent shape. Perhaps it is, and I misinstalled it in some way. The pxo 722 looked dowdy and battered, but it is doing the job quite nicely. Thanks again guys! - Charles