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Discussion Starter · #1 ·
I thought I would post a new thread with a better title to help anyone searching for help on this issue in the future.

Okay, so 185 dollars later (from RN of course!) my problem with throttle is solved. If you look down the page a little, you will see last week I was trying to diagnose a problem with throttle.

It turned out to be the TPS. Should've just gone with it in the first place, but it's always better to diagnose, rather than throw money around.

Just for FYI, noone seemed to know this, including rovers north and everyone i asked on various mailing lists...

The throttle position sensor is not in the idle circuit of teh ECU on at least the 1989 range rover, but I assume at least through 92, if not the whole series. I found this by unplugging the throttle position sensor, and starting the car. Amazingly enough, it still acts normal while stopped, even with NO throttle potentiometer. It will increase rpm as normal sitting still. As soon as you roll, you only get enough throttle to get you home at 5-10mph or so, any more throttle and the throttle will cycle on/off every half second or so. On many cars when the TPS dies, the throttle will simply not function at the "dead spot", so I thought I would bring this to the public's attention!

Once again, the TPS will only cause a problem when the truck is moving, which as far as I know is signaled by the cruise control speed tranducer found on teh speedometer cable, until the post classic range's with teh GEMs ignition.

Enjoy this weeks free info. :)
 

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I'll add a bit here for p38a owners. My problem was the engine coughing at around 2700-2900rpm which was about 70-75mph. I ended up disconnecting the TPS, and the car ran fine, except for shifting being too sensitive. When you disconnect the TPS on a P38a, GEMS (pre-99) supplies default values for throttle position based on input from the MAF sensor. Once I realized that the problem was indeed that the TPS had developed a "dead spot", I bought a new one at Autozone for $80. The Autozone part ended up being the EXACT stock part, made in England by Wells. At the same time I also picked up a Idle Air Control valve for $35 as my idle was a bit erratic. Amazing how much money you can save on parts if you look around!

-Coach
 

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Discussion Starter · #5 ·
OkieRover said:
:confused
I thought I said something about that. Well I'm glad you found it. It was a perplexing problem.
Okie, yeah you said maybe try a new TPS. The TPS cost 175 bucks from Rovers North, and you, and noone else could tell me why it only happens when moving.
Eventually I would've just thrown a TPS at it, but you know how it can get with these rovers if you just start throwing money around at problems. It's always better to diagnose as fully as possible at first, if you don't have the money to replace every part on the truck, hehe.

Thanks for all the help.
 

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I'd love to be in the Rat Patrol
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The only reason I thought of that was a similar problem my buddy had. But it wasn't exactly the same. The fact it only happened when you moved was weird. I'm glad you found this little gem out, someone will have it again I guarantee it.

And I know about throwing money around. You can't just do that. I know I can't. I threw some around when I finished the engine project by buying a NEW radiator instead of getting the old one fixed proper. Got sick of the problems and sick of fussing with it.
 

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RR-Noob (93 RRC LWB)
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Well...well...well...I do believe I'm having this same issue! Idling around a neighborhood today looking for a new house, my hoopty began "tapping" and wouldn't excelerate for crap! I've had this issue before and it went away after changing a couple cracked vacuum hoses. I thought that had fixed the problem but now it's back.

There was no backfiring but there was the smell of sulpher. I can floor the gas when driving but it will not go much beyond 1500 rpms however while in park the rpms rise normally when the gas is pushed.

I cleaned the MAF with cleaner...no change...I checked all vacuum hoses for leaks, no change in idle when sprayed with starter fluid.

When I unplug the TPS the truck used to die...now there is no change when it's unplugged...which makes me wonder if maybe it's stuck in idle position. Not raising or lowering...figure I'll try to clean it and the fire trap...if I break the TPS by trying to clean it so be it...I think I'm going to have to replace it anyway.

Thoughts?

FYI: My engine sounds like a diesel right now...and oil pressure is fine.
 

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Just for FYI, noone seemed to know this, including rovers north and everyone i asked on various mailing lists...

The throttle position sensor is not in the idle circuit of teh ECU on at least the 1989 range rover, but I assume at least through 92, if not the whole series. I found this by unplugging the throttle position sensor, and starting the car.

Once again, the TPS will only cause a problem when the truck is moving, which as far as I know is signaled by the cruise control speed tranducer found on teh speedometer cable, until the post classic range's with teh GEMs ignition.

Enjoy this weeks free info. :)
While I don't doubt your finding and am glad you found the part you needed, I do not believe your premise to be true that the TPS is only involved at above idle throttle positions.

If this were the case, there would be no reason for an idle setting adjustment of the TPS on the earlier trucks.

There is no support to your statement in my paper service manuals, those on CD-ROM and there is no support whatsoever in my LRU text on Engine Management Systems.

Further, TPS problems can and will cause all manner of problems with idling on all Rovers.

Also, you are wrong about VSS only being on GEMS trucks, they are found on Lucas trucks after '95, and Discos after '94.
 

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Discussion Starter · #9 ·
While I don't doubt your finding and am glad you found the part you needed, I do not believe your premise to be true that the TPS is only involved at above idle throttle positions.

If this were the case, there would be no reason for an idle setting adjustment of the TPS on the earlier trucks.

There is no support to your statement in my paper service manuals, those on CD-ROM and there is no support whatsoever in my LRU text on Engine Management Systems.

Further, TPS problems can and will cause all manner of problems with idling on all Rovers.

Also, you are wrong about VSS only being on GEMS trucks, they are found on Lucas trucks after '95, and Discos after '94.
Been a long time since I wrote this (look at the post date) but What was most important from this was that the TPS does not appear to have any effect while not in a driving gear (no load on the engine) as it would rev perfectly fine until in a gear. I don't remember any other details but that
 

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RR-Noob (93 RRC LWB)
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Would it be a fair assumption that if the engine used to die if you unplugged the TPS and now if unplugged there is no change to the engines running that there is an issue?

When in park engine will rev even though it runs rough, however when in drive it bogs down and will only reach about 1500 rpm (max speed @ 35).
 

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RR-Noob (93 RRC LWB)
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Scratch my last question...I'm a jackass...I was mixing up the TPS and the Idle Air Control Valve. If I unplugged the IACV before the engine would stall...now if I unplug it there is no change...forming a new thread about this...
 

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The potentiometer IS in the idle circuit.
When it is there...
I found out the behind-the-scenes behaviour using the ECU Mate instrument which has a
throttle potentiometer screen where the values of the pot and ECU can be
monitored in realtime.

I at first thought it it was wrong that my potentiometer measured an 8% throttle at idle and tried to calibrate it by elongating the mounting holes so I could turn it. As was the procedure for older models. But it turned out that when I turned it to lower than 2% throttle the ECU immediately set the throttle value to 11% and would from that point on ignore any further potentiometer movement. I saw the indication LIMP HOME MODE on the ECU Mate display. So it basically means that if you remove the throttle potentiometer [which would mean the same as 0% throttle value from the pot] that will throw the ECU, 14CUX, into LIMP HOME MODE which means that it will use less than ideal values to control the engine, ignoring several sensor inputs. So you can expect lower performance while you are "limping around" in that mode.

You can have it forget that mode by disconnecting the battery temporarily but as soon as the ECU again detects a voltage that represents less than 2% throttle from the pot then back into LIMP HOME MODE it goes. So you will need to have the potentiometer connected before your do that reset operation.

The potentiometer do not have elongated holes for the 1993 model I have, instead the 14CUX software will adapt dynamically to whatever voltage is found to be the lowest encountered from the potentiometer and will interpret that as zero throttle. As long as that value is safely above 2% throttle you are good to go. When you buy a new pot and screw it into place it will by default end up at around 8% throttle at idle as measured by the ECU and the ECU will adapt from there.

So, you definitely need the potentiometer for normal engine performance.
 

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1995 NAS Discovery -3.9L14CUX Manual Trans
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The potentiometer IS in the idle circuit.
When it is there...

gunnar-rangerover, that is a great description! Thanks for detailing it so well. :))

I found a write up about the 14cux that expands upon your details, with voltages and some additional info. First, he states that the minimum voltage (likely your 2% TPS position) is 0.28 volts.. and that any voltage below that level will be interpreted as NO voltage and thus will remove the TPS from low rpm control. He indicated a normal range of 0.32v to 0.34v (at idle). My TPS is showing ~0.15 or so, at idle.. but it does progress 'smoothly' up to a peak of 4.9 volts at WOT.

Given the ridiculous price of [what is essentially nothing more than] a potentiometer... I will be filing an elongation in the mount holes to attempt to get my base voltage up to that 0.32v value.


These sites seem to have meaningful troubleshooting info:
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Most Useful:
Leave the potentiometer connected. Start the engine and check that the voltage reading is 0.32 – 0.34 Volts.

Incorrect Voltages produce the following symptoms. If it is less than 0.28 Volts, the system will not adjust the idle speed. The idle speed will probably be very low, and the vehicle may tend to stall (note that this is not the only cause of stalling). If the Voltage exceeds 0.38 then the system will not recognize that the throttle is in the idle position, and the idle speed stays at 1100 to 1200 RPM despite setting the Base Idle speed correctly.

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Useful:
"To troubleshoot the Throttle Position Sensor, first disconnect system power and then disconnect the EFI Cable Harness from the ECU. Using an Ohmmeter, verify that resistance between terminals 3 and 25 is between 4000 and 6000 Ohms. Next, reconnect the EFI Cable Harness to the ECU, and turn the ignition key switch "on". Take voltmeter readings from pin 20 to ground. With the sensor in the throttle-closed position, you should read 0.085 to 0.545 volts. With the sensor in the throttle-open position, you should read 4.2 to 4.9 volts. In between these extremes, turning the throttle position sensor should produce a smooth sweep of voltage readings.

The Throttle Position Sensor on our example system is marked "215SA", "84925A", "Lucas", "Made in UK", and "2499". It has a 3-lead pigtail on it that's about 6" long, and the 3-pin connector on the end of the pigtail is marked "Rists". The three cables to the pigtail are color coded "yellow", "red" and "green" respectively."
 
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