1993 Brooklands - CHECK GEARBOX Issues

[cdog534]

Hi all. New member here!

Could anyone help with the gearbox issue I'm having with my 1993 Brooklands 46331?

See info below:

The car keeps saying CHECK GEARBOX and going into limp mode every 30 minutes or so. That means it will only use second gear.

The transmission seems to be fine internally, shifts fine, has good fluid color. And if it goes into limp mode, it works fine again once I turn the ignition off and then back on.

Based on feedback I got from some 4L80E guys, it sounds like limp mode comes on as a result of a problem with the wiring harness or the VSS sensor

I've pulled the fault codes and they weren't conclusive, pointing to the 1-2 shift solenoid.
The codes I retrieved from the car were 68, 71, 87.

Below are the descriptions of each code from the workshop manual:

68 Overdrive ratio incorrect
Symptoms: high line pressure, fourth gear inhibited, TCC inhibited

71 engine speed low

87 Solenoid B, stuck off
Action of solenoid inhibited

However, the symptoms I see coming from the car more closely match these codes:

53 System voltage high
Symptoms: second gear only, maximum line pressure, TCC inhibited

75 System voltage low. Symptoms: maximum line pressure, second gear only, TCC inhibited

81 QDM Fault B Solenoid. Symptoms: maximum line pressure, second gear only, TCC inhibited

We replaced the 1-2 and 3-4 solenoids with no luck.

I took it to Foley Bentley in Northbrook, where they inspected the solenoids, looked at what they could on the tranny, cleaned the wiring connections. After about $2,500 work of looking, they couldn't find any specific issues, but the fault had cleared up, and the car was working fine for about 1,000 miles following their re-assembly. Then it started going into limp mode again.

I can drive the car 25 miles or so and it doesn't have an issue, then it just says CHECK GEARBOX all of a sudden and I'm stuck in 2nd gear limp mode until I cycle the key. It seems to happen when the car has to make a decision about downshifting and gets confused. It's fine on the highway, but then it gets confused when I exit the highway and it needs to downshift from overdrive. Also, keeping the car in ‘3’ instead of ‘D’ seems to help, but not always.

I went through many 4L80E forums and knowledge bases online and found that many people across a lot of platforms had the same issue with their 4L80E's going into limp mode and it seemed to be caused by 3 issues (WHEN the transmissions were not failing mechanically):

1) Bad ignition switch, causing tranny to lose power on occasion. I don't think this is my issue, switch seems solid.

2) Wiring harness with a bad wire, bad connection, or tranny fluid had leaked into the harness, causing a bad connection, and loss of power to tranny. This might be my issue, as that would explain the intermittent nature of the problem, and why it worked for a while once they re-assembled it.

3) Bad or dirty VSS or input/output shaft sensor. It seems like these can get dirty or fail, also causing the issue.

If my tranny IS failing internally, has a bad converter, etc. I'd be happy to rebuild it. But it seems to shift fine and the fluid looks good.

Also, the transmission seems to NOT go into limp mode when the coolant is cold and not in normal operating temperature range. We may be able to resolve the issue if we can disconnect the lead/sensor providing the engine/coolant temperature to the transmission.

Please let me know your thoughts. Any help is appreciated!

Regards,

Chuck C.
1993 Bentley Brooklands LWB #46331

[John Murch]

I expect you have had a trawl through the RROC Aus website.
https://rrtechnical.info/sz/05_sz.html

[John L Elkington]

It may sound silly but my (turbo r) sport gearbox lever knob came off and the spring was collapsed , made replacement but was too soft allowing it to keeping jumping in and out of sport then switched to check GB ,,reset on switch off but then kept coming on , lifted out the knob and spring fault cleared. Worth a check?