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Fixing Electric Seats

Making the whirrs happen again

by Julian Edgar, pics by Julian Edgar and Georgina Cobbin

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If you buy an older luxury car there are two things near certain: the first is that it will have electric seats, and the second is that at least one of the seat functions won't work! So how hard is it to fix a defective leccy seat? Obviously it depends a lot on what the actual problem is and the car in question, but as a guide let's have a look at fixing the seats in an E23 1985 BMW 735i. The seat architecture in other cars will be different, but if you don't have any idea where you'd even start to fix such a problem, this story is sure to be useful to you.

The front seats in the BMW are amongst the most complex that you'll find in any older car. They have electric adjustment for front/back travel, front of the seat up/down, rear of the seat up/down, head restraint up/down and backrest rake forwards/backwards. However, they don't have electric lumbar adjust and they don't have airbags. (If the seats that you are working on have airbags, you must read the factory workshop manual to ascertain the safe procedure for working on the seats.)

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The seat functions are all controlled by this complex switchgear, which is duplicated on the passenger side of the car. As can be seen here, the driver's seat also has three position memories. Incidentally, the rear seat is also electric, with an individual reclining function for each side! But in this car, the back seat was working just fine.

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The driver's seat had three problems.

  • The button which moved the seat rearwards didn't work. However, the seat could be moved backwards with one of the memory keys.
  • The front of the seat couldn't be raised.
  • The head restraint wouldn't move up or down, although in this case the motor could be heard whirring uselessly whenever the right buttons were pressed.

Getting the Seat Out

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The first step was to remove the seat from the car so that access to all the bits could be gained. The seat was electrically moved forward and then the two rear floor-mounting bolts undone.

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But how was access going to be gained to the front mounting bolts? Pressing the adjustment button didn't cause the seat to move backwards, and by this stage the memory button had stopped allowing that action as well!  The answer was to manually apply power to the seat to activate the motor. All the connecting plugs were undone and those plugs containing the heaviest cables inspected. (There will be wiring for seat position transducers and things like that in the loom, but the motors will be powered by noticeably heavier cables.)

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Using a heavy duty, over-current protected, 12V power supply (this one was made very cheaply - see DIY Budget 12-volt Bench Supply), power was applied to pairs of terminals connecting to the thick wires until the right connections were found. The seat was then powered backwards until the front mounting bolts could be accessed. These were removed and then the seat moved forward until it sat in the middle of its tracks, making it easier to get out of the car.

Fixing the Head Restraint

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This is what the BMW seat looks like underneath. Four electric motors can be seen, plus there's a fifth inside the backrest. Each electric motor connects to a sheathed, flexible drive cable that in turn connects to a reduction gearbox. As I later discovered, inside each gearbox is a worm that drives a plastic gearwheel, which in turn drives a pinion operating on a rack. At this stage, though, a simple test could be made of each motor by connecting power to its wiring plug and making sure that the function worked as it should. Every function but the head restraint up/down worked, so the problems other than the head restraint showed that they must be in the switches, not the motors or associated drive systems. But how to fix the head restraint up/down movement?

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The rear trim panel of the seat came off by the simple undoing of four screws. As with the other seat motors, the mechanism consisted of a brush-type DC motor driving a flexible cable that went to the adjust mechanism. The motor worked fine with power connected, but the head restraint didn't move. Feeling the outside of the drive cable sheath indicated that the drive cable inside was turning, so the problem must lie in the mechanism closest to the head restraint itself.

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The adjustment mechanism was held in place with one screw, which was accessible with the leather upholstery disengaged from small metal spikes that held it in place. The legs of the head restraint clipped into plastic cups on the mechanism (one is arrowed here) and these were able to be popped out with the careful use of a screwdriver.

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The whole upper part of the adjustment mechanism was then able to be lifted out of the seat back and placed next to the seat. Note that the electric motor stayed in place - it didn't need to be removed as well.

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To see what was going on inside the unit, it needed to be pulled apart. It was obviously never designed to be repairable, and so the first disassembly step involved drilling out the rivets which held the plastic sliders in place on their track. With these out, the action of the pinion (a small gear) on the rack (a toothed metal strip) could be assessed. Neither looked particularly worn and applying power to the motor showed that in fact the pinion wasn't turning. So that meant that the problem was inside the gearbox itself.

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The gearbox was held together with four screws, each with an oddly-shaped internal socket head for which I don't have a tool. However, knowing that I could always find replacement small bolts, I used a pair of Vicegrips to undo them - that is, it didn't matter if they got a bit mutilated in the process of disassembly.

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Inside the gearbox the worm drive and its associated plastic gear could be seen. Initially I thought that the plastic cog must have stripped, but inspection showed that this wasn't the case. So why wasn't drive getting out of the gearbox? Again I applied power to the motor and watched what happened. What I found was although the cable could be heard rotating inside its sheath, that drive wasn't getting to the worm. Pulling the worm gear out and inspecting the square-section drive cable showed that the end of the cable was a little worn and it was slipping back out of the drive hole of the worm. (The slippage was occurring inside the area marked by the arrow.)

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The fix was dead-easy - simply pull the drive cable out of the sheath a little, crimp a spring steel washer on it (backed by a plain washer that here is out of sight - it's fallen back into the mouth of the sheath) and then push the drive cable back down in its sleeve. With the crimped washer preventing the worn part of the cable from sliding back out of the square drive recess in the worm, drive was restored to the gearbox.

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The mechanism could then be reassembled. New screws were used to replace the Vicegripped ones, while the drilled-out rivets were also replaced with new screws and nuts (arrowed). The gearbox was re-greased before assembly and a smear of grease was placed on the tracks that the nylon sleeves run on. Back in the seat, the mechanism was checked by applying power - and worked fine.

So in this case the fix cost nearly nothing, except some time.

Lubrication

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Since all the motors had now been proved to be in working order, fixing the electric rearwards travel and front up/down motion could only be achieved with the seat back in the car - it looked as if it had to be a wiring loom or switchgear problem. But while the seat was out, it made sense to wipe over all the tracks and exposed cogs and re-grease them.

Fixing the Rest

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Under the driver's seat is a control module containing both relays and the seat memory facility. Close inspection of the plugs and sockets on both the unit and the associated loom showed that some corrosion had occurred. (Perhaps at some stage a drink had been spilled on it.) The corrosion showed itself as a green deposit on the pins and some tedious but careful scraping with a small flat-bladed screwdriver removed it. Once that was done, the associated plug was inserted and pulled out 20-30 times to scrape off the deposit inside the pins of the plug, which were otherwise impossible to access to clean.

And after that was done, all the seat functions again worked!

Conclusion

At commercial rates, fixing the seat would have cost hundreds of dollars - both in labour time and in a complete replacement head restraint up/down mechanism. No one would have bothered repairing the gearbox drive - they'd have just replaced the whole thing. The corroded pins? That would have been cheaper, but the total bill would have still been prohibitive.

So if you have a car with an electric seat problem, it's certainly worth taking a long hard look yourself - it might be something impossible to easily remedy (eg a stripped gear) but it might also be a problem as easy to fix as this one proved to be....

Restoring the Bounce

While the seat was out of the car, I tried something which I thought might just work - and it was so successful it's worth covering here.

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Compared with the front passenger seat, the driver's seat had noticeably sagged - the base was flatter and less supportive. Inspection showed that the seat used a simple series of coil springs, rather than the foam rubber more usually employed in modern cars. Replacing these springs would have been an immense job, but adding some helper springiness was dead-easy.

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The rubber that was used to add some support was this high density pipe foam insulation - it's usually found slid over the pipes coming from domestic hot water heaters. It's available from hardware stores.

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The rubber cylinder was manoeuvred into the seat base and then slid between the adjacent coils of a row of springs, being held in place by the springiness of the rubber and the coils. A second cylinder was then placed in the other row of springs that supports the seat base.

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The added support is quite noticeable, without making the seat uncomfortable. In fact, the driver's seat now feels very much like the passenger seat - which was the aim.

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