Seat Switch Circuit Diagram

Please can anyone find me a circuit diagram for the power seats for a 92 ? The manual I have is NAS which doesnt have the memory ecu etc

If you have memory seats that don't work chances are the mirrors don't work either. The problem will be the internal battery in the ECU has leaked and corroded a track and/or components inside. Forget a new ECU you just can't get them but if you're really lucky you might be able to get your old one repaired but don't hold your breath. Second hand ECU's can be had but the price is steep. AFAIK there is no circuit diagram that shows how the seat ECU works but my limited understanding is that the seat switch works on 5v from the ECU and activates relays inside the ECU which then switch the 12v supplies to the various motors. There are then separate circuits for the mirrors and the position switches for the seat and mirrors. HTH.

Hi again, thanks for the info. I am getting 5v at most of the switch connections and as the drivers side is completely dead along with the mirrors it seems to be the ecu rather than the switch at fault. I was hoping to bypass the ecu and forgo the memory function even if it means making my own switch panel and thought there may be some wiring diagram out there.

Ahh...glad I joined the forum, I have the same problem on my 93 not bothered about the memory seats just the mirrors..

I have taken the trim off around the seat and can see the ECU ( the greyish box at top of pic ) but getting it out is going to be difficult. Ive loosened all the harnesses but I cant disconnect anything on the switch side as the seat base is stuck roughly in the middle. Even if I get get the seat fully forward I may be able to take it out from the rear

Does this help - PDF attached

big andy said:

Ahh...glad I joined the forum, I have the same problem on my 93 not bothered about the memory seats just the mirrors..

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I'm pretty sure you have to have a working drivers seat to get an MOT these days so would suggest you look in to it. BTW you can't run a memory seat switch the same as a non-memory seat switch because of the difference in operating voltage/current. It might be possible to use a non-memory switch and re-wire the seat control but then not sure what you'd do with the mirrors?

I am quite happy to lose the memory function but as you say its a MOT requirement that the drivers seat can be moved back & forth so my next step is to get this ruddy ecu out, open it up & see it its repairable. If not & I am not feeling like spending hundreds of pounds on a replacement I will try and manually wire the seat & mirrors. I am going to study the diagrams when I get a break over the weekend.

shandonman said:

I am quite happy to lose the memory function but as you say its a MOT requirement that the drivers seat can be moved back & forth so my next step is to get this ruddy ecu out, open it up & see it its repairable. If not & I am not feeling like spending hundreds of pounds on a replacement I will try and manually wire the seat & mirrors. I am going to study the diagrams when I get a break over the weekend.

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Dunno if these will help - You can see where the battery is and when it leaks it damages the tracks and also a couple of the chips on the board. The same ECU is found in Rolls Royce/Bentley cars of circa 1989/90 vintage or therabouts. If you trawl the net there are is some information available not I didn't find much to help with making repairs or by-passing the damaged section of the board, after all all you need to to get a switching voltage to the right relay to make things work - I thought "how hard can it be?" - impossible (for me!) appeared to be the answer.

classic kev said:

I'm pretty sure you have to have a working drivers seat to get an MOT these days so would suggest you look in to it. BTW you can't run a memory seat switch the same as a non-memory seat switch because of the difference in operating voltage/current. It might be possible to use a non-memory switch and re-wire the seat control but then not sure what you'd do with the mirrors?

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The seat does go backwards and forwards but has no backrest adjustment, I found two ECU's in the boot so this has been an ongoing problem. Is it possible to fit a manual seat ?

classic kev said:

Dunno if these will help - You can see where the battery is and when it leaks it damages the tracks and also a couple of the chips on the board. The same ECU is found in Rolls Royce/Bentley cars of circa 1989/90 vintage or therabouts. If you trawl the net there are is some information available not I didn't find much to help with making repairs or by-passing the damaged section of the board, after all all you need to to get a switching voltage to the right relay to make things work - I thought "how hard can it be?" - impossible (for me!) appeared to be the answer.

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I see what you mean, I can count over 50 wires in that loom . Although I have not had time to go over the the wiring diagram yet I have read that a connector that fits to that loom has 8 pins, 4 pairs of wires for each motor. If the polarity is reversed the motor runs the opposite way so it should be viable to build a little switch box directly supplying 12v to the motors.

Andy - just a thought but it could be you now have a working ecu but a part faulty switch now needing a clean

shandonman said:

If the polarity is reversed the motor runs the opposite way so it should be viable to build a little switch box directly supplying 12v to the motors.

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I think that is viable, I did some continuity testing on that ecu I posted pics of, the relays are the main control for the seat motors, the rest of the board is as far as I can tell given over to a microprocessor driver and memory chips for the mirror and seat memories but I could be wrong, I'm no micro electronics wizz.

If you do a search I have a vague recollection that somebody did make their own switch box.

I am still struggling to get the ecu out and ran out of time again. I have found the 8 pin connector which supplies the motors with 12v ( connected to the black connector bottom of pic 1 ) and have applied power to move the seat back so that my nose is no longer pressed against the windscreen. I have to get the seat removed now to get the ecu out so will do the same to access the 4 bolts. I am hoping to repair the ecu as although I can switch the seat motors, the mirrors look actually more difficult to work without the ecu.

classic kev said:

I think that is viable, I did some continuity testing on that ecu I posted pics of, the relays are the main control for the seat motors, the rest of the board is as far as I can tell given over to a microprocessor driver and memory chips for the mirror and seat memories but I could be wrong, I'm no micro electronics wizz.

If you do a search I have a vague recollection that somebody did make their own switch box.

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You are right

Datatek said:

You are right

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Thanks Datatek, nice to know. I don't suppose you would know of a way to hotwire the ECU do you? The reason I ask is I have a damaged ECU which I think I might have made worse by an inexpert attempt to repair a damaged track, the relays are all good but I have no way of knowing if I can get it to work by by-passing a part of the board. I suspect it would be well beyond my ability anyway but always worth asking the question. Cheers.

classic kev said:

Thanks Datatek, nice to know. I don't suppose you would know of a way to hotwire the ECU do you? The reason I ask is I have a damaged ECU which I think I might have made worse by an inexpert attempt to repair a damaged track, the relays are all good but I have no way of knowing if I can get it to work by by-passing a part of the board. I suspect it would be well beyond my ability anyway but always worth asking the question. Cheers.

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With the car and the ECU in front of me, it would be perfectly possible, but as I have no info on the wiring etc it cannot be done from here. It would not be a five minute job though.

Datatek said:

With the car and the ECU in front of me, it would be perfectly possible, but as I have no info on the wiring etc it cannot be done from here. It would not be a five minute job though.

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Oh well, never mind, perhaps one day somebody will unravel the secrets of the seat ECU and put the info out there so that apes like me can fix 'em. Thanks for your input anyway.

Whilst I need to find some time to tackle the removal of the ECU, my attention has moved to the passenger seat. All functions work except rearward base so the seat is stuck fully forward. Studying the relatively simple wiring diagram I see the switch uses no ecu, no relays and has a full 12v being applied to the motor through the seat switch.

shandonman said:

Whilst I need to find some time to tackle the removal of the ECU, my attention has moved to the passenger seat. All functions work except rearward base so the seat is stuck fully forward. Studying the relatively simple wiring diagram I see the switch uses no ecu, no relays and has a full 12v being applied to the motor through the seat switch.

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Different switch current rating I would guess. To bypass the ECU, all you need is to ID the sets of switch wires, should be 3 per switch and the relevant motor wires, 2 per motor, you need 2 relays per motor to give forward and reverse.

Got the passenger side working today by cleaning the switch internals as described on this forum. I am still going to remove the ecu & hopefully repair but if not I am very tempted by the ad I saw for new non memory ecu. Just plug it in & all working lovely instead of hassle buying switches, relays, getting the time & still having to sort out the mirrors ( a cop out I know )