In a nutshell there are 5 filters which can be easily accessed and cleaned out. If your trans isn't shifting at all Refer to page 7 of following link to access the 2 shift control and lockup control valve filters. If it is slamming into gear see page 8 for the pressure control valves and although this link doesn't show it, there are 3 ATF feed pipes with filters on them under the 2 pressure valves. You need to remove the battery tray to access the pressure control set of valves and will take around an hour. The shift control valve filters can be done from the front of the transmission. Should take less than 15 minutes for those.

Any of you with '98s who are displaying delayed or rough engagement after coming to a stop, this may be your problem:

PDF File

Now the narrative:

I've owned my 98 Odyssey since new and it has 165K miles on it. I had never changed the trans fluid, and unless the dealer changed it at the 30K checkup, it had never been done. I'm a pretty good DIY mechanic, but just never had an A/T before, and didn't know that the fluid was any different than a manual as far as service intervals. About 10K ago or so it started having delayed engagement into first after coming to a stop. Engagement into Reverse was also delayed. In addition the engagement into all gears was not as smooth. I wasn't too concerned about it, since it still drove.

Finally these things got worse. The delays increased and the engagement got more jerky. The Check Engine light was coming on and the D4 flashing. Since I don't have an OBDII reader, I just read the flashes after shorting the test clip and got 41, which is "faulty shift control system". The service manual trouble shooting flowchart for that code, says to check the hydraulic pressures and solenoids and if they are ok, then it's the transmission. I don't have the special Honda tool to check the pressures, so didn't do it. All the plugs on the solenoids were on tight but I didn't test further.

I accessed odyclub and found out that I was supposed to change the fluid on a regular basis. So I went and picked up a case of Honda ZF fluid and changed it 4 times in a row, just to try and clear all the old stuff out of there. It helped somewhat for a few weeks, but then the trouble returned, only now getting worse.

Finally thinking the clutch pack may have had gummy deposits or whatever, I tried SeaFoam Engine treatment. Half a can into the Tranny. Ran it for 15 min and drained. Drained and refilled 4 more times with cheap Walmart Dexron III. Didn't want to waste the Honda fluid. This only made it worse. Now it wouldn't even up shift out of 1st. So about 6 months ago, I stopped driving it, thinking I needed a new Trans. I searched Car-Part.com for a low mileage replacement, but the 98 is very rare.

I recalled a message on odyclub, about how if you suddenly change out very old fluid, you can clog the internal filters, but put it in the back of my mind, thinking I would need to disassemble the trans to access these filters. No doubt the SeaFoam cleaned out the gunk and contributed to the clogging of the filters.

This week I ordered a trans rebuild kit for $160. New Clutch packs, steels, pistons and gaskets. I figured I might as well try and rebuild it myself. So yesterday, my 17yr old son and I went out to remove the transmission. but since I'm trying to teach him about trouble shooting, I thought we'd go through the symptoms one at a time. On page 14-108 of the service manual, the Symptom-to-Component chart. Every symptom I had came up with Note D, check for seizure in Shift Control Solenoid Valve. The manual also shows that there is a filter under this valve!

We removed the valve and lo and behold the filter was completely clogged up! Cleaned it out and reassembled it and took it out for a test drive. It now shifts out of 1st. In fact it shifts into all gears! However there is still a 2 second delay and the engine races before it up shifts and the engagement is very shocking. From Park to R or D4 is also 2 second delayed before it slams into gear.

Back to the trouble shooting. "Excessive shock or flares in all shift lever positions". Possible cause 7, " clutch pressure control solenoid A/B valves defective". And the manual shows that under these 2 valves are 3 ATF feed pipes with filters on them. Well wouldn't you know the middle one, the one that feeds the other 2 was almost totally blocked. Blew it out with compressed air and reassembled.

Time for the test. Start it. Shift into R. Smooth as silk. No slamming. Put it in D and take off. After the first slightly jerky 15 seconds to get the air out of the system, it's now like a new car! Truly amazing since I've got Walmart Dexron in it, and after all the abuse I put it to during the last 12 months of slamming into gears and engine racing before up shifting. Unlike the 99 and on Odyssey transmissions, the earlier ones at least the 98 seems to be indestructible.

Due to my deferred neglect, it's possible that there is still debris floating around in the lines waiting to clog up the filters again. So this may turn into a routine maintenance item.

Now I'm left with a 98 tranny rebuild kit. Since I don't ever plan to sell this car, I'll just keep it. I'll post back when I finally have to use it. Hopefully it won't be for a very long time.

Update:

I searched through the entire Transmission section of the Service Manual to see exactly how many filters there are in the 98 tranny.

In addition to the 5 which I had easy access to and cleaned out, there are 3 more.

They are all in the Torque converter housing, rather than the main transmission body.

Under the Regulator Valve Body there is a small round filter.

Below that sets the Main Valve Body under which there is another identical small round filter.

Finally the ATF strainer itself which is under the Accumulator Body.

While these filters are not in the main Transmission Body, they do appear to be a bear to replace necessitating removal of the transmission to access the Torque converter housing. At least that's how the manual describes it.

Prior to doing this, it would be wise to do further tests to differentiate whether the problem is hydraulic fluid restriction due to blocked filters or worn clutch packs, requiring a rebuild. There are pressure tests and stall speed tests in the manual which would accomplish this.

I'm not sure why my 3 internal filters/strainers aren't clogged up as well. Or at least not showing any evidence of it. Perhaps the solenoid filters are first line in the direction of hydraulic flow and pick up all the gunk, but I'd expect the strainer to have that job. I do know that 2 of the 5 filters I cleaned out were spotless. Why they would be clean and the other 3 clogged, would require further study of the flow diagrams.

- Info from theojo at odyclub

For the 95-97 transmissions there are also filters for both the lockup and shift solenoids. Check out items 6 and 8 in the pic below.

- Info from dpkelly at odyclub