By Antony K on Tuesday, April 03, 2001 - 08:22 am: Edit

My 1997 V8 3.9i Auto has an intermittent fault that is driving me insane. Every now and then I'll be driving along the motorway (UK) on Cruise control and I'll lose power. The Rev Counter appears to stay at 2500rpm for instance and the speedo starts to wobble and drop down to zero and then back up to 70MPH, then down to say 50MPH then back up to 70MPH. I don't know if this is an automatic transmission fault or an engine fault. With the Rev counter staying still I suspect it's possibly ECU or transmission. Until this happened yesterday I had previously suspected engine. The reason for my original thoughts are that sometimes when braking from speed to a halt at traffic lights or to make a turn at a junction, the Disco stalls before I've come to a halt. That's not too bad if you intended stopping but if you need to negotiate a corner without powered steering it's no fun.

It's done 43000 miles. I've had it for 1000 miles and 1 month.

Any help would be greatly appreciated.

Antony

By Antony K on Tuesday, April 03, 2001 - 08:36 am: Edit

Forgot to mention that there are NO warning lights showing on the dash when all this happens.

By Jay R. on Tuesday, April 03, 2001 - 08:49 am: Edit

I have had similar problems with my 97 Disco SE (NAS). Vehicle Speed Sensor was replaced which solved the problem with the engine cutting out at hwy speeds. I also had the "stalling just before stopping" which was solved by checking the throttle position sensor & adjusting the base idle.

Jay R.

By Jon on Tuesday, April 03, 2001 - 08:55 am: Edit

Well, the most likely cause of your stalling as you stop is failure of the torque converter clutch to unlock. It's a common problem on General Motors transmissions, but I guess is possible with any make. Essentially, you choke the engine off, just like it would if it was a manual transmission and you neglected to push the clutch in as you stopped. The converter clutch lock-up solenoid is the cause, and can be expensive to replace. The other problem I can't quite understand. In saying "driving along...and lose power," do you mean the engine stalls? Or do you mean you lose electrical power momentarily? If the engine stalls, the rev counter will continue to indicate the engine speed and the speedometer will indicate vehicle speed, as long as the ignition is in the run position. Supposing your converter clutch remains locked (which it shouldn't), the engine would continue to turn in proportion to vehicle speed until it restarted. I guess it's possible, but you would lose speed quickly when the engine stalled and your rev counter wouldn't remain at 2500 rpm throughout the ordeal. Otherwise, it sounds like you just have a quirky speedometer, (which doesn't surprise me a whole lot) and you might consider having the speedo drive cable and electronic drive motor checked at a shop. I've seen speedometers behave like you said yours does, and it usually means the speedo drive motor or drive magnets are bad.

By Antony K on Tuesday, April 03, 2001 - 09:15 am: Edit

Loss of power was engine power not electrical.

It is as if the engine stalls at 2500rpm but it doesn't affect the rev counter.

I don't think it's a faulty speedo because I can feel the car die and slow down in accordance with the movement of the speedo.

In respect of the stalling when slowing down for the corner, the surprising thing is that the engine actually started up of it's own accord once I was through the bend a few seconds later. Could these two problems be related. It's only restarted itself on one occasion so far.

Thanks for your replies so far. You are all being helpful.

Antony.

By Antony K on Friday, April 06, 2001 - 09:00 am: Edit

I looked around the Plenum last night but couldn't find the throttle position sensor, even when looking at a workshop manual....

Could anyone tell me where it it please?

By Antony K on Wednesday, April 18, 2001 - 01:28 am: Edit

Found it and replaced it.

It's just on the front of the plenum chamber. Fixed by two screws. There are three wires connecting via a simple black wiring connector sitting on top of the inlet manifold under the plenum chamber.

It's not cut out on me on the motorway/highway again since but there's plenty of time yet....

It still stalls when starting up and sometimes when slowing down to a stop, before I've actually stopped. Will replace the plugs and leads next. It's due for it's major 48000 mile service soon so a change of tranny fluid may help. (It might also stop the sound of ringing bells when I'm accelerating in 3rd (Automatic) too. Any comments on this very welcome.

Antony.
Disco V8i Auto 1997

By G on Friday, April 20, 2001 - 04:30 pm: Edit

Antony,

Have the Speed Sensor checked. I had to have mine replaced.

G
Disco V8i 3.9L, Auto 1995