unusual gearbox shift

[nicstrike]

had a quick pooch through the archives but couldn't find anything quite like our bongo (4wd td) occasionally throws up.when cruising in top (45mph upwards) anything more than the weight of a toe nail pressed on the throttle and the 'box seems to momentarily blip into 3rd before settling back into 4th again.rather annoying if you only want to jog on a little faster and not set the tyres on fire!other than this all seems well with it and no faults appear to be present (hold light) .seemingly most gear box "issues" seem to come back to the oil and filter.does anyone think this may be a factor?.i have checked the oil and it seems clean and at the right level.if a change and filter clean is the way to go i'll get my sleeves rolled up and get grovelling underneath!!

[westonwarrior]

hi and welcome to the forum
Sorry not sure about your problem though.

[francophile1947]

My ATF also looked good, but a change made a world of difference - I didn't have your problem though

[nicstrike]

thanks guys.i was thinking an oil change was probably the way to go but i'm open to any other suggestions.bit of an odd one though!

[Steve L]

It may not be a gearbox issue. I had a similar problem which I cured with a change of fuel filter and cleaning out the banjo filter. What I think happens is that if the filters are blocked it restricts the amount of fuel through (obviously) but the position of the throttle makes the gearbox think you are in 'kickdown' mode so it changes down. I hope this makes sense. I had been running mine on biodiesel for a while (poor quality) which I think blocked what was a fairly new fuel filter. This caused symptoms similar to yours. I'd look at the fuel filters first.

[dandywarhol]

Sounds like a TPS problem to me - try interrogating the ECU for a fault code

[nicstrike]

cheers guys.i'll sort the "nuts and bolts" bits , see how things go and then delve into voodoo (electronics) if the glitch persists.i prefer a spanner to a voltmeter!!

[nicstrike]

i havn't had a chance to get stuck into anything yet (trying to get on with a rear disc conversion on my suzuki samurai) but having driven the bus on some different roads i have a slight amendment to make to the actual symtoms.the blip down into 3rd before immediately dropping back into top seems to happen when re-appyling the throttle after you have lifted right off (while coasting/freewheeling etc.)as opposed to just increasing throttle as i thought earlier.i wonder wether this would point to the tps more than anything else? i'll still change the atf and filters as a matter of course though.

[apole]

Hi,

I'd second the throttle position sensor post above. Mine didn't post any fault codes when it went faulty, but worth checking them nonetheless.

Changing the fluid and cleaning the strainer etc may well improve the way the gearbox changes etc, but from what you describe that probably won't fix your issue. Worth doing anyway though.

Remember to use the correct spec oil and check when engine hot, car on level ground and the wheels chocked and car running in D.

Good luck and keep us posted.

Andy

[patnben]

I think you are experiencing the change from "Lockup" back into top gear
which happens at lower speeds when you release the accelerator and
then re-apply it. With a very light accelerator position it will change
into "Lockup" much earlier than with a heavy accelerator position,
when you re-apply the accelerator it drops back into top gear again
for a few seconds.

This happens with both my Bongos although one is more pronounced
than the other one. Both my Bongos have regular oil changes and work
perfectly well otherwise, and I assumed this was normal behaviour. It
would be interesting to find out if this is not the case. A small adjustment
to the TP sensor may alter it's characteristics but it does seem to be
a logical thing to happen to avoid labouring the engine at lower speeds.

[nicstrike]

mmmm...dont think the "low speed labouring" is an issue.the 'box seems to be doing this at anything up fo 60mph (not had the opportunity to try it out at any higher speed).

[nicstrike]

i thought all was well of late with the "lock-up" but following the our first decent long run involving a bit of speed , i'm not so sure! i think i correct in saying that the lock-up should operate up to about 60mph but on our recent trip (at 70....or there abouts...ish , if you know what i mean! ) the box was behaving as it would at low speed with the lock-up kicking in and out with throttle on/off. the box performs beautifully other than this but it just didn't feel right to be doing that at higher speeds? any ideas?

[Northern Bongolow]

the tps would be the first thing i would adjust, not by much, just a tad both ways then test. as the lock up info is temp related/controlled via the ecu i would look at the fan switch at the rear drivers side of the head. a few of the copy part ones ive seen chuck up weird readings like this. replace it with a gen mazda one, (needs bleeding to fit).
edit.
other things ive seen affect the box are fuel filters,and dodgey alternator,may be worth checking the alternator at different rev speeds for consistancy.

[westonwarrior]

sounds like my old one used to be
take your foot off at speed and the revs dropped a bit put your foot back on and it seamed to "chase" the gear for a second with a little rev then go again.

I took it as thats how it was supposed to work as I had the gearbox done, it never gave a moments problem after sorting but did do what you describe.

I always took it as normal.

[francophile1947]

sounds like my old one used to be
take your foot off at speed and the revs dropped a bit put your foot back on and it seamed to "chase" the gear for a second with a little rev then go again.

I took it as thats how it was supposed to work as I had the gearbox done, it never gave a moments problem after sorting but did do what you describe.

I always took it as normal.

Exactly the same with mine - I've never thought of it as a problem