Solid Illuminated D4 - HELP!

[KeystoneLX]

Hi folks,

First off, I'm new to the forum. Although I've religiously referred to CB7Tuner, I decided I needed to become a member for some real help.

So here's the scoop:

When I bought my 92 LX with 98k original miles, the owner stated it had a blinking D4 issue. After doing some research, I replaced the main fuel relay and that issue never occurred again. Whew. My CB7 now has 136k.

So up until a few months ago, I never had issues with my tranny again, until I noticed D4 being solidly illuminated upon turning the key in the 'on' position, therefore resulting in the infamous 'limp mode' indicative of faulty TCU components. This issue occurred at a more frequent rate as time went on. I even noticed that, at times, while I was driving around that D4 would flicker in no apparent pattern. I tried pulling codes, and the only thing popping up was D4 continuously lit (faulty TCU or '0', if I understand correctly.)

I replaced my lock-up solenoid that was not clicking properly after applying 12v, and that didn't seem to make much of a difference. I finally contacted a fella on a facebook group who repaired my TCU, which previously had a leaky capacitor. After hooking up the repaired TCU - (all 7 capacitors were replaced), bingo! Drove around town a little while to make sure. In my honest opinion, it shifted smoother than the day I bought it about 3.5 years ago. D4 no longer was lit upon turning the key in the 'on' position - until the next morning.

I proceeded to unhook the negative battery terminal and re-connect it, which at that moment, the D4 light did not appear upon turning the key in the 'on' position. I took out the hazard fuse for about 15 mins for the hopes of resetting the ECU. Drove around town a bit, no issues. Pulled codes; none.

Next morning I turn my key into the 'on' position, and lo and behold D4 is illuminated.

What could cause this to occur? I'm at a total loss since:
1. there are no codes, and
2. unhooking and re-connecting the battery/hazard fuse seems to 'reset' the TCU/ECU

Any help would be greatly appreciated. The parts cannon is locked and loaded, unfortunately. Thank you all in advance!

[illinois_erik]

probably caps in the ecu. they talk to each other. id resolve it before it hurts the tcu or something.

[KeystoneLX]

probably caps in the ecu. they talk to each other. id resolve it before it hurts the tcu or something.

I was pondering the idea of it being something ECU, or possibly a ground issue. Thanks for your advice!

[Fleetw00d]

I suspect your TCU still has a problem. A solid D4 as you already know is indicative of a bad TCU. Did the guy on facebook provide any guarantee of his work? I have now had four TCUs repaired by "mandigital" (David Ruberg) on ebay. He repairs them for $75 (you pay shipping to him, he pays return shipping) and has a lifetime guarantee. Two of them were more than a year ago and are working fine. Just got two others back Friday - installed one of those this morning.

To reset the TCU/ECM, all you need to do is remove the 7.5 amp (left rear corner) back up fuse in the under hood fuse box for about 10 seconds.

[KeystoneLX]

I suspect your TCU still has a problem. A solid D4 as you already know is indicative of a bad TCU. Did the guy on facebook provide any guarantee of his work? I have now had four TCUs repaired by "mandigital" (David Ruberg) on ebay. He repairs them for $75 (you pay shipping to him, he pays return shipping) and has a lifetime guarantee. Two of them were more than a year ago and are working fine. Just got two others back Friday - installed one of those this morning.

To reset the TCU/ECM, all you need to do is remove the 7.5 amp (left rear corner) back up fuse in the under hood fuse box for about 10 seconds.

Yes, the fella on FB said he'd guarantee the repair for a year. We've been in contact pretty consistently, and he's willing to send another from a 92 LX like the one previously repaired. I paid about $65 total ..but with the shipping of another it'd be a little more.

I've taken out the 7.5 amp fuse several times with good luck before I determined the problem was occurring more frequently. Weirdly enough, my car's D4 hasn't been solid upon turning key to 'on' for about 4 days straight - not to say it won't happen again soon though.. I may have to get ahold of another repaired identical (besides lot number) TCU and see if this problem persists.

Although I really would like to go AT/MT, it's a lot of work and I don't have the resources to do such myself. Trying to avoid that route as long as long as I may be able to mend this with another TCU. Thanks for the response!