hey guys, new to this forum but very down with 80's/90's japanese cars. my friend's '92 accord has the classic TCU symptoms (D4 light lit, won't shift out of P once started, but can be started in N and then manually shifted through gears) so i pulled the TCU, expecting some charred black stuff on it like pictured here, just down the page a bit:

Honda Electronic Transmission Problem- Troubleshoot, Inspection, Repair, Replace.

when i got it apart though, it all looked fine from a distance. when i looked closer though, one of the white with a red top things had a small brown dot on it. it's # was D__, i can't remember exactly but it was in the area highlighted in red in the above link.

so here are my questions:

1) the C- prefix stands for capacitor, the R- stands for resistor, so does the D- prefix stand for diode?

2) from all the articles i've read about this issue, it's always been the capacitors and/or resistors that cause this transmission problem. has anyone encountered it from simply a bad diode?

3) if a diode is burnt, will it necessarily look like the burnt little fella in this pic

http://techautorepairs.com/images/tcubeforeafter.jpg

or can it just appear as a brown dot on the body of the diode? of all the D- things on the circuit board, it's the only one with a brown dot on it.

thanks in advance.

 

Does you friends brake lights work?

It seems you are going to the hardest thing first instead of the simplest.

1) do the brake lights work? if not fix them
2) if they work then look at the shift interlock solenoid

 

D means diode. More pics:

"http://people.consolidated.net/etrider/accordtcufix.html"

The red/white component is a diode; the diode symbol is silk screened underneath it.

In the link above, it looks like there are five more of the same diode on the left side of the PCB. Do any of those have brown dots?

Test R41 with a DVM while you have the TCU out. Resistors don't have to be burned looking to be bad. They can open up internally and look fine from the outside. Electrolytic caps can open up without any noticeable leaking. Often the top of the can will bulge out, but it can be very subtle.

 

crispin: i will double check but i'm pretty sure the brake lights work fine. how come? i haven't heard of any connection between failed brake lights and this auto non-shifting problem. i will do a search on shift interlock solenoid.

roader: cool, i'm pleased my guess was right. in the 6th pic down of the link you posted, the diode in question is actually on the lower right, next to the burnt resistor that the red arrow is pointing to. how do you test the diodes, capacitors, and resistors? do you check them for resistance?

which reminds me, is there any easy way to release the three plastic clips that hold the circuit board down? i aborted my attempt for fear of snapping the board.

 

well when I read the problem the first time I thought your only problem was failure to shift out of park once started.
After rereading I see that the car does not want to shift automatically.

Have you checked Ebay? I have seen the TCU's going for around $20

Seems like a worthwhile expense as you will know know even if you replace a burnt out diode if you did it correctly.

 

Dbl, I was suggesting scrutinizing the other seemingly identical diodes for similar brown dots to rule out those dots being manufacturer's component markings.

Resistors are easy to check with a DVM. Unsolder and lift one leg of the resistor to get an accurate reading.

Capacitors, not so easy, especially small ones like that 220μF shown in your link. Slight bulging of the can is common sign of a bad cap. Leaking below the cap is an obvious sign of failure.

Diodes are easy. Lift one leg and switch you DVM to diode check. Put the read lead on the arrow side and the black lead on the bar side. If the diode is good the meter will read 0.7V or so. Reverse the leads and the meter should read OL (overload.)

I have a later model Accord without a separate TCU so no help on taking the PCB out.

 

Repairing ECU's is difficult. The traces are very thin and easy to damage w/ excess solder heat. There is a clear varnish (urethane?) over everthing which makes soldering difficult, and diagnostics difficult and prone to error. You can spend a lot of time on a repair to get nowhere. A used ECU is much faster and probably more reliable. At $20-$30, this is better use of time usually.

Just yesterday I opened a 94 EX ECU that has a CYP error fault. It's probably a simple fault, but I was able to trace that fault must lie on a small attached card w/ huge number of surface mount items. I've had some experience w/ surface mount components; they are prone to crack and fall off. When you attempt to resolder one end, the other may come loose. I decided to throw in towel and "close"; this patient is a goner.

good luck

 

Doing prototype PCB assembly/rework with mostly surface mount components, I've found that with the right equipment and materials I'll take an SM repair over a through hole any day. A cheap Chinese rework station from ebay:
...along with solder paste in a syringe and a 16 gauge dispensing tip makes repairs very easy and faster than through hole. I keep the paste in the freezer and it works fine years past the expiration date.

 

thanks for the info guys. i appreciate it.

i have looked on ebay for the specific part # (as well as local wreckers and car-part.com) and the cheapest one i found was for $40 on ebay but from a seller with a zero rating and the and the ad had little info so i passed on it. all others are about $80. i'll keep checking though.

roader, no. only the one diode had a brown dot on it. all other components appeared uniform.

 

Lift one leg and test it. Or post a pic.