Possible relay issue

Hi,

I've got a concern about my overdrive (2002 2.4l petrol RZH101 - 160k km). The transmission was working fine but on very infrequent occasions (once or twice per long drive) it behaves as if the overdrive has been switched off for about 4-5 seconds). I'm positive that it's not changing down on purpose because of demand. It seems to be happily going along at about 50-60 mph at just over 2000rpm, then it jumps to about 3500rpm for a few seconds before the revs drop and you feel the gear change back up.

If I drive in the same conditions and switch the overdrive off and on, the experience is exactly the same, so I'm convinced this is what's happening. I've got it booked in for a fluid change in a few weeks (first time so going to get a pro to do it for me). The fluid colour is clear brown but not dark, so I'm hoping it not wear that's causing this - though I've no idea how long it's been since the fluid was last changed.

I'm wondering if it's an issue with the electrical side of things? Of note, when this happens, the O/D OFF light does NOT come on. The last time it happened, I immediately released the O/D switch and then very quickly turned the O/D back on - and the overdrive gear kicked back in. But I can't tell if that was because of the switch press - or just circumstance, and that was when the overdrive would have come back anyway.

With the light not displaying on the dashboard, I assume it's not related to a dodgy gear leaver switch. So I'm wondering if it could be related to the solenoid, the relay or the wiring? Could the solenoid behave that way if it was on the way out, or do you think this sounds more like the relay and/or wiring?

Thanks,
Steve

Just to add some complication, In the wiring diagram, it says RHD gasoline SCMs have an OD relay (O5), whereas diesels and all LHDs have the transmission computer (O4) 89530A. It also shows the location of the relay as to the right of the fusebox, , probably up behind the steering column.

If I try to open the EPC page for the computer, it says my vehicle does not have it: https://toyota.epc-data.com/regius_ace/rzh101g/142904/electric/8410/89530A/ I guess this is born out by having the A45DL linkage transmission and not the A45DE electronic one.

All good so far,

But can I find the relay in the EPC, can I ****. So no idea what the Toyota code is.