Hi there mates, I have a strange "problem" with the manual gearbox ZF 6-speed on my Corvette C4 LT1 year 92.

I sent to reprogrammed a second EPROM in order to let fans jump in a lower temperature value. I took advantage to cancel by software the EGR valve and the system "one to four" that forces you in certain circumstances to change from 1st to 4th.

Well, right after changing the original EPROM for the "new" one, which was reprogrammed, I went to test the car and ... what was a surprise when I found that the car would not let me shift from 4th to 5th, as if the car does not have this route, like a 4-speed car. Tried it again and again founding that only came the 5th if the car is running in the 4th below 65 km/h, if that goes smoothly, but going faster there is no way. I have also realised that works from 3rd to 5th working below about 75km/h.

At first it looked as it might be the synchro of the 5th gear but it was very extrange that happens right after changing the original EPROM by the reprogrammed one, moreover the box had always worked perfectly and the car has had an excellent service since new. I went back to change the EPROM and put back the original one...and new surprise came when I saw that the car did absolutely nothing wrong, it worked perfect as always. So to find out for sure I came back to install again the reprogrammed EPROM and the car returned to have the same problem, so it is clear that there is something in the reprogrammed EPROM that leaves the 5th gear out, subject to certain conditions.

The only component of the gearbox that I know which can be handled by the ECM is "one to four" system, I figure it is like a solenoid which is activated to block the 2nd. A friend of mine experienced in Corvettes told me that in principle should not affect and that has nothing to do with the 5th, and he had never seen that problem in other cars like mine with the ECU reprogrammed similarly, but also told me that my car was a case "sui generis" because it would be of the very few units that still has that system working properly. For this reason I have the hunch that there must be a problem because "one to four" system it is canceled electronically but at the same time the system still with his solenoid connected and maybe working or confused, like a conflit.

I tried disconnecting the solenoid today but as it was raining in torrents and optispark is famous for not get along very well with heavy rain, I had to abort the operation and I have to lift the car with an jack to reach the solenoid.

The next weekend I will try to carry out physical disconnection of the solenoid "one to four" but if any have experienced something similar or you can have some idea of ​​why it can happen I would greatly appreciate it comments.

Sorry for the grammar.

 

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No prob with the Grammer friend! I dont understand why you want the EGR deleted. I beleave it does not effect wide open throttle performance. You might want to use the stock chip with a "skip shift eliminater" from Zip, corvette central, mid america, ecklers ect. Just click on the short cuts located on the left side of this page.

 

Well, the target of changing the stock chip for an other reprogrammed one with better temperature values it's make the fan start sooner (95ºC and 99ºC instead 108ºC and 113ºC) since the climate in my region it's 40ºC during the summer.

As you must know the EGR helps to recirculate gases from de exhaust manifold to the intake manifold, that means rubbish...It's not a performance issue.

 

Unplug it.

If you still have a problem shifting into 5th you have a mechanical issue.

 

I'll do it, but if it was a mechanical issue, why doesn't occur with stock chip?

 

I finally disconnected the CAGS solenoid and the problem disappeared. Everything OK again!!