Discovery Throttle Problem
From: Stuart Williams (STUARTW@po1.net.cho.ge.com)
Date: Mon, 27 Nov 95 08:11:00 EST
Subject: Disco Throttle Problem - FAQ Candidate!
I recently developed a problem with my '95 Disco (the first real problem in 14K miles) that required dealer intervention. The problem was a brief engine stumble or cutout whenever the throttle was eased after cresting gentle hills or when running on flat terrain at 70 MPH or so. It happened with or without the cruise control operating, and with or without the OD engaged. The engine would immediately resume normal operation, but the vehicle would lurch noticeably as everything caught back up with itself. Duncan Brown tells me that this is normal behavior for his 1960 Series IIa, but I was not thrilled with it happening to my Disco given the upcoming holiday weekend and some planned travels around the mid-Atlantic. At first I thought that it might have been transmission slippage or OD problems but the RPMs would actually visibly drop on the tachometer during the infrequent longer incidents, so I had visions of new Engine Control Computers and other Lucas bits dancing in my head. The dealer's first suspicion was the fuel pump, so they set up a pressure gauge and took the vehicle on the road but could never see any drops in pressure during the fairly easy to duplicate stumbles. Next they attached a diagnostic monitor to the ECC and sent two technicians on the road, one driving and one riding shotgun on the monitor, to see what they could see. Sure enough, the signal from the TPS (Throttle Position Sensor, Roverese for potentiometer) was intermittent under certain conditions and was momentarily sending signals to the ECC that caused the engine to fall back to idle, albeit briefly. They say that this sort of thing should have caused the 'Check Engine' light to come on, and it probably was, but things corrected themselves so fast that the dashboard light was never observed. Maybe a latching circuit would be helpful here, although that might be a double-edged sword given everything that's wired to that indicator! Anyway, they replaced the TPS under warranty and we're back on the road, better than ever. Kudos to Drew, the technician at Land Rover Richmond that found the problem.
For the life of me I can't imagine why the Throttle Position Sensor might have worn out !!!
'95 Disco V8i "Great White"
'85 LandCruiser "Old Paint"