Tuesday, 28 August 2012

Toyota Hiace overdrive solenoid bypass mod | Overdrive not working automatic gearbox

Today the van underwent surgery – in fact it turned out to be like a trip to the dentists..
Recently my o/d (overdrive, top gear) stopped working, and I have been stuck with four gears, and a safe top speed of around 45mph without too much over revving of the engine.
The automatic Hiace has a ‘o/d off’ button on the gear stick. You might use this if you wanted to hold 4th gear, for example when towing a heavy load downhill, to take advantage of engine braking, or to stop the engine jumping in and out of top gear on a gentle climb.
The ability for the gearbox to shift into top gear is governed by a solenoid mounted on the rear transmission casing. It can be controlled by this switch, but is also governed by certain other conditions being met in the circuit controlled by an o/d ECU. (for example temp, and speed sensors can play a part in this).
To try and repair my van, initially the transmission fluid had been changed, and the filter cleaned, and the engine temp sensor bypassed neither of which cured the problem. Then I had an auto electrician check the circuits for response – it appeared that it was unlikely the problem lay there.
If it were possible to mechanically (hydraulically) bypass the o/d solenoid that would eliminate all electrics from the system, and mean that the gearbox will always shift into o/d at the appropriate speed. It is possible!

Here is how I did it:
Tools needed:
Spanners, 12mm & 10mm
Philips screwdriver
Large G-clamp
Hi speed drill with grinding bits
Inspection lamp
Oil pan – (clean)
Masking tape
One of the girlfriends stockings
Small funnel
Bench vice with soft grips
1. Go under the van on the passenger side (RHD van) and remove the rear driveshaft protection shield – 3x 12mm bolts.
2. Using inspection lamp to get a clear view, locate the o/d solenoid on the side of the transmission case. Clean the area as well as possible – you don’t want to get any dirt into the transmission. Remove the cable clamps with appropriate spanners and then locate the solenoid plug and unplug it. Its located here: (bottom right of pic with cable attached, you don’t need to remove the gearbox – just an example pic)
3. Remove the rubber cap from the sensor beside the solenoid and remove the earth cable with Philips screw.

Earth cable and solenoid plug
4. Now you can either drain the transmission fluid from the gearbox, or as I did, put an oil pan in position, remove the two 12mm bolts from the solenoid and quickly position a clean g-clamp over the hole when you remove the solenoid (make sure the outer o-ring remains in place to seal the hole, and the clamp can grip onto the trans pan – don’t over tighten) Catch leaking oil, which will spurt out, in the oil pan.
5. Now you have the solenoid in your hand:

Remove the small rubber o-ring, and clean the solenoid well with a clean cloth, removing any loose particles, and as much trans oil as you can from the holes. You can now connect the solenoid to a 12v battery to test it – in the case of mine it did not operate. This means that the fluid never gets a chance to flow freely through the solenoid via these holes and allow the o/d gear to engage. To bypass this we must grind a channel to the depth of the small o-ring groove, between the holes.

Look closely behind the bigger hole, there’s a gap behind it – you don’t want to get any dirt or filings in here when you start grinding – it would not be easy to flush out, so block up the holes with little pieces of cloth and tape it up:

6. Now, clamp the solenoid in a vice with soft grips and use the grinding bits in the drill to grind the channel. Bringing back all your memories of trips to the dentists. You want it to end up looking like this:

I used an air line to blast away any filings before removing the tape and the cloth. Make sure its as clean as possible prior to re-assembly.
7. Now fit the solenoid back on the transmission case using the outer o-ring only this time. If you didn’t drain the transmission and used a g-clamp this will be messy, make sure and catch the oil that spills out.
8. You could just cut the wires off. I connected mine all back up (except the plug) in case I want to replace the solenoid in the future – I will have all the parts in the right places.
9. Filter the oil you caught through a stocking for example, to make sure its clean, and then using a funnel pour it back into the transmission through the transmission dip stick hole. (Or change your transmission oil if you drained the gearbox. Good idea to do this if it hasn’t been done in a while.) I recommend a full drain of the ATF oil to do this job, and a clean refill of oil afterward. Especially if it hasn't been changed in a while.
10. When everything is properly reassembled go for a test drive – you should now have your overdrive back again! (only now you cannot switch it off from the button on the gear stick). Mine changes super smooth at 45 mph – it always feels like it changes at the appropriate point for the driving conditions, revs etc.
11. Check for leaks around the solenoid when you get back, and over the next few days to make sure the seal is intact.
12. Remember to check the transmission fluid level after a day or two. I discovered that mine was a bit too high (possibly some had been trapped in the overdrive and released when the solenoid was freed up) as it had been last done with the solenoid out of action.

Do leave your thoughts and experiences in the comments section and we can build up an additional resource for the future SC Hiace owners..

If this post helped you get your van back on the road, you have saved yourself a lot of money! If you can afford it, please consider making me a reasonable donation (the button is on the right side-bar) - I could certainly use it! Thanks!

Take care, and see you soon! 


  1. Dane Willers says:
    January 19, 2013 at 5:25 am (Edit)
    I have a slightly different problem: the gearbox “hunts” in and out of overdrive at speed: switches in and out of overdrive intermittendly at whatever speed above 82 km/h upwards: even at 140km/h. then sometimes works fine again for 10 or so kilometers, only to start hunting again, even at same pedal position on flat long roads. Oil ok, level clarity etc etc.
    Suggestions before I take a dose of rat poison ??
    1994 3L auto Turbo Diesel super Custom.
    taigenetsudo says:
    January 24, 2013 at 10:32 am (Edit)
    All responses to this question are here:
    Thank you
    taigenetsudo says:
    January 24, 2013 at 10:35 am (Edit)
    I recently had the problem of reverse gear failing, and o/d slipping out once engaged.
    I left the van into Burnside Motors Coleraine N Ireland. Diagnosis was that all the clutches were badly worn. They did a great job, remanufactured the gearbox, all new clutch packs, seals etc – its now shifting perfectly!
    Also they endorsed my o/d soleniod bypass. They sourced a replacement but it was very expensive for this van, so left the solenoid as I had done it.
    (Mine is the early ’93 model with part time 4WD, the old 2.8 non-turbo and transmission.)
    Reconditioning the gearbox is not cheap, but I’m happy to have her back and good for a long time to come. I also had the engine serviced, all filters replaced and everything else checked over while they had her in.
    taigenetsudo says:
    January 24, 2013 at 10:38 am (Edit)
    The later models of Hiace Super Custom may have a different gearbox and solenoid, please leave a comment if you have any more details on this for other readers. Thanks.

  2. hi hi

    man you are a real genius!!!

    You saved
    us with this article quit a bunch of money.

    With been
    to Australia for 3 month and of course we bought us a nearly 30 year old
    campervan. At first everything was fine but after a while we had exactly the
    same problem that you describe above. We went to a mechanic and tried to convince
    him to do exactly the surgery like you explain in your article. First it seemd
    to be impossible, he cound#t belive that this is going to work. He really
    thought that this would ruin us the gearbox because it will start to leak oil
    afterwards. After we ensured him that we would not blame him if something like
    this would happen he did us the favour. In the end we had to pay like 160 aud
    and not the nearl y 450 aud that it would have cost us if we would have changed
    the hole solenoid.

    It was really
    fine afterwards. We had not a problem at all after this. So we can recommend
    this kind of surgery very much.

    We would
    have liked to do it on our own, but without tools in a foreign country it was
    just the only way…

    Thank you
    very very much indeed!

  3. Cheer Man,
    I read your article yesterday after being told by a local auto transmission specialist it would cost me £200 plus to find the extent of the problem but that the cause could be a number of things and the gearbox may need a number of parts. The specialist said, worst case scenario, I could be £2500 worse off before I find out the box is finished.
    I spoke to another garage who got back with a price for a new solenoid (£210 fitted),
    this guy was really helpful but given that a new solenoid offered no guarantee to cure the problem and that the £210 would be greatly needed for our temporarily postponed holiday, I decided to attempt your solenoid mod.
    I finished the job after 2 hours and just got back from a half hour test drive, I now know a new solenoid would not be a waste of money.
    Our holiday is now a reality.
    Thanks lots !
    What a great place the internet can be.
    Best Regards Col.