Can’t figure out a Gearbox problem

[DeltaCT]

Can anyone help me please? My SV6 has an intermittent problem that’s driving me nuts.

Checked with a mechanic - no fault codes.

Started with the cruise control disconnecting and wouldn’t reengage. Wait a while and then it’s fine.

Now it’s developed to dropping down a gear or even 2 while driving steady on the motorway. Can be quite startling at 100k/h when it drops to 3rd and revs go to about 3,500. Feels like hitting the brakes. When it dropped down yesterday I went to manual and selected 5th which it showed on the dash but it didn’t change gear. I was shifting between 4th and 5th on the dash but no rpm change so I don’t think it was changing gear.

Also sometimes get the stability control off warning then the gear changes but not always.

Sometimes it happens once or twice other times up to 10 times. Can drive for an hour just fine other times starts happening pretty quickly. Can go for a few days no problems other times can happen all the time. But definitely the trend is getting worse.

 

[greenacc]

Better start with the basics. Which transmission have you got and how many km has it done? When was the last transmission service done?
Sounds like you have a 5 speed.
It could be telling you it is dieing

 

[DeltaCT]

Thanks for the reply!
It’s a 5sp auto. Car has done 250km. Gearbox has been serviced twice in its life (I think). Car has been serviced on schedule its whole life. I bought it at 20km. Pretty much all the kms on the clock were me driving between cities on the motorway, so just ticking along at 100. Pretty gentle life

 

[greenacc]

Sounds ominous. It could be worn out and needing a rebuild. Either way it needs to be checked and fixed by a proper automatic transmission specialist.
That is different to a regular mechanic. If you're very lucky it might just need a trans service and new fluid.
If you just keep on driving it the next time it drops down from fifth to third could be the end of it. Then you need to get the whole transmission rebuilt or replaced.
Tell the transmission guy about the sudden downshifting.
Those 5 speeds are expensive.
If it needs a rebuild be prepared to be told it would cost 4-5 grand.

 

[J_D 2.0]

Those 5 speeds are expensive.
If it needs a rebuild be prepared to be told it would cost 4-5 grand.

The rarest Commodores on the road in the future will be anything with the 5 speed box in it! I got a repair quote of high fives on my old SV6 so I drove it into the ground seeing as the car was only worth maybe a couple of grand more than that at the time!

 

[DeltaCT]

Because it’s intermittent, and started with the cruise control, and because it often kicks off the traction control I was thinking it might be electrical? Was hoping someone could point out a sensor or something like that that could be giving dud info

 

[greenacc]

That's possible but you will be playing the lottery just replacing parts to see what happens.
1 such Lotto ticket you could try is the throttle body. That could explain dropping down gears but not really cruise control.
I would definitely be checking the colour of the transmission fluid at absolute minimum. If it's even close to black it is roasted and needs a service now!

 

[Dayvo]

How old is your battery these commodores do strange things when the batteries get old and can't hold charge.

 

[Mickyinks]

cruise control, stabilty control faults are likely caused by one of your wheel speed sensors or faulty wheel speed sensor harness.. Id start there

 

[Fu Manchu]

One: Hope like hell the shitty 5L40e is not playing up. The previous posts are well experienced in the opinions of these gearboxes in VE’s

Two: load test the battery. It’s free at Battery world. You ideally need the charge circuit tested as well. Also have battery health checked as well. (It’s capacity to hold charge compared to when it was new)

Three: start looking for a transmission specialist that is the best in town.

Why do you need to do the load tests etc? Because when the VE is low on available voltage, the ECM will ration it in order to keep more critical systems working. If there is an available voltage issue, it’s expected that system bugs or running issues will appear.