Sovereign usually shifts into overdrive on the highway at around 55 mph..... Just noticed yesterday that its not happening now. I do have the "lightning bolt" warning on that there's a fuse out. I did check all fuses by the driver, passenger and center console. All fuses are good, which may have nothing to do with overdrive not kicking in, but any advice?

BTW, trans fluid level is good.... BUT the trannny linkage cable is definitely stretched and needs to be replaced. I've adjusted the shift cable multiple times where it worked for a while until it stretched even more each time. Now the adjustment under the console ski slope is at it's max. So the car will not start in Park. I must put the car in neutral to start the engine which is fine for now. Sometimes it has an issue finding reverse as well, but a little jiggle of the shifter has been working. I figured the shift cable has little or nothing to do with shifting into OD at highway speeds, though. Is this the right assumption?

 

Hi Jagbro,

I assume when you describe OD you mean 4th gear, which is the overdrive ratio (maybe 1:0.73)?

I would think a likely explanation is that your stretched cable is causing the transmission to think you have selected 3rd gear on the J-gate instead of Drive. Have you checked the end at the transmission to be sure it is secure? It's possible that you could adjust your rotary switch to get things back in range again.

Another possibility that comes to mind is that your kickdown switch could be shorted, causing the TCM to keep the transmission out of 4th, but I think the cable adjustment is a more likely suspect given your history with it.

Cheers,

Don

 

I think "overdrive" is also known as the torque converter "lock-up" and yes that happens in 4th at around 55mph when all is copacetic.
Not sure it would have anything to do with the selector?

Larry

 

In our '93 I didn't realize our torque converter was not transferring torque properly until I changed from Dexron VI to Redline Oil D4 ATF, which is a synthetic Dexron III equivalent. Dexron VI has a significantly lower viscosity than the original Dexron II(E) and Dexron III specified for the ZF 4HP24.

 

the factory Dex II fill sheared way below Dex VI’s minimum when the car was still under warranty. it’s not a viscosity related problem

 

Hi xalty,

It's true that Dex VI was designed to shear to a lesser degree than Dex III (both temporary and permanent shear). But I believe the real issue for the ZF 4HP24 is that the viscosity of Dex VI is significantly lower than Dex III, and a lower viscosity fluid develops less friction under pressure, leading to reduced torque transfer in the converter.

Obviously, newer transmissions have torque converters designed to function properly with lower-viscosity fluids. But the ZF 4HP24 is still in production, and to this day, ZF still recommends Dexron III as the correct fluid, not Dex VI, despite the claims of GM and many other fluid developers and marketers that their fluids are backward compatible. It is telling that for the 4HP24, ZF does not recommend Dex VI or any of its own newer ZF-branded fluids such as Lifeguard 5, 6 or 8.

Typical viscosities for Dex III were 35 - 37 mm2/s at 40°C and 7.5 - 8.2 mm2/s at 100°C, with a viscosity index of around 198.

Typical viscosities for Dex VI are 29.8 - 30.2 mm2/s at 40°C and 5.7 - 6.4 mm2/s at 100°C with a viscosity index of around 154.

Dex VI shears to a lesser degree than Dex III and suffers far less permanent shear, but its viscosity is always significantly lower than Dex III. I believe this is the reason our torque converter never felt as connected on Dex VI and only behaved properly after I switched to Redline Oil D4 ATF (synthetic Dex III equivalent, viscosity index 198).

BTW, Dexron III's greater susceptibility to permanent molecular shear is a good reason to change your transmission fluid regularly.

Cheers,

Don