Hello Everybody,
I am new to this forum and have read many of the posts and their answers and I find it quite fascinating. I am no mechanic, but willing to get greasy.
Background
I acquired a 1998 Disco I V8 Automatic Transmission. By the state of the brakes and swivel pins, it is fair to assume that Disco was poorly maintained. I changed the brakes, redid the seals on the steering box, redid the seal on the swivel pins and replace wheel bearings… etc. I also wanted to change the ATF and the filter. Of course the Job is a real pain in the neck due to the crossmember and exhaust pipes… I did not complete this job and end up putting the old pan back in. I changed the ATF as well all the fluids in the differentials and transfer case.
The problem: The Disco is behaving perfectly (as far as I can tell) it switches gears smoothly, although I have limited experience with AT. However, at highway speed around 70 mph, the Disco slows down as does the engine. If I give less gas (to go around 60mph) then the Disco does just fine.
I do not understand, and my lack of experience with AT prevents me from using the proper terminology to describe my problem.
I hope someone understands in spite of my murky description.
Thanks for your help.

 

There is a vehicle speed sensor (VSS) that is mounted on left side of transfer case. See attached pix. If it is bad, it can signal engine to slow down because it thinks you are exceeding the top speed (like 115 mph or so). If unplugged, I guess it would shut down speedometer and cruise control, but that might be a diagnostic to try. Many GPS units will give you "speed over ground", and the old 51 seconds between mile posts is a good indicator, if speedometer is out.

In "Gulf States" of the Middle East, it is also used to sound a warning when over 70 mph I believe. At that point you switch to flying carpet to outdistance the government police.

 

Thank you very much Sir.
I suspected that could be the problem, but was not convinced.
I read a lot about transmission slipping, but I have no clues on how it feels when driving.
I am used to manual transmissions, and I lack knowledge and experience when it comes to AT.
I looked at the RAVE manual, where a series of transmission road tests is described,
Has anybody tried them? Is there a risk in performing them, especially if the VSS is malfunctioning?
Thanks again.
This forum is simply great!

 

I agree with BUZZ on this issue. Might also look at my major service list posted in the sticky at the top of this section as a guide to what else should be done.

 

Have you done anything with plugs, wires, cleaning of air intake plenum and MAF?

New Air filter? The little hoses on the valve covers to the plenum?

Lots of little things need to be brought up to date and it will run much better in long run and stay away from low octane cheap gas, burn the best you can find, makes a huge difference!

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