Technical Throttle Potentiometer Fault

[neoseal]

Hi,

I bought a Brava 1.2 last week and have noticed that throttle potentiometer is not in its place but is tagged with a string on the side plate. I asked the previous owner and he said that its always been like that since he bought the car. As of now the car runs ok but I have noticed that mpg is low. I dont know anything about it so if someone can throw some light upon this issue and tell me how to fix a faulty throttle potentiometer. Is it programmed with the engine management code?

neoseal

 

[jug]

throttle position sensor sends back a signal to ecu which has an impact on fueling. it works by varying resistance. by "tagged with a string on the side plate" i guess that means it isnt fitted where it should be, so surely the obvious solution is to fit it correctly. if the TPS isnt working it could mean the car is overfueling if the default parameter from the ecu is in use, in which case you should fail an emissions test. refit the sensor and see if it makes any difference, it may be worth checking the resistance once its fitted to make sure it is responding to an opening throttle.

 

[neoseal]

Thanks Jug, I will fix it and let u know what happens.

 

[neoseal]

Jug, refitted the sensor and the revs started going up and in about 4 minutes touched 3000 rpm, it doenst stop to increase. Does this means it is faulty. Couldnt check with multimeter. A local mechanic suggested disconnecting the battery and reconnecting, will this help?

 

[neoseal]

Today, I disconnected the battery to reset the ecu. when I started the car the revs were 1500 and then it started going up and touched 2900 in about 10 minutes and stopped increasing irrespective of time.
I went for a test drive for about 20 minutes in normal conditions. Here is the gear, revs and speed table I am getting without pressing the accelarator.

Gear Revs Speed
1 2500 15 miles
2 2300 22 miles
3 2000 30 miles
4 2200 35 to 40 miles
5 2500 50 miles

Can someone try to make sense of this and tell me what's happening.
In fact if i want to drive at 30mph in 3rd gear then I can do that whithout pressing the accelerator. In short my car runs without the need of accelarator. When I press clutch, the revs touches 3000 irrespective of gear.

Does this mean that my ecu has not reset? Fortunately I didnt radio code when I reconnected battery, so I am not sure if I have reset the ecu or not. The car is driving in the same way as it used to before.
More when I do some more research on my car.

 

[jug]

the best thing to do is replace the TPS with one from a scrap yard. expect to pay £5-10.

i think the TPS is telling the ecu that your throttle is open, so it cant get the revs down to idle. i'd try disconnecting it until you get a replacement.

 

[neoseal]

Jug, I replaced it yesterday evening with the one I got from scrapyard. Disconnected the battery to reset the ecu. But its the same thing. The revs go upto 3000 rpm and stays there. I went for a test drive, the car drives well, but when I press the clutch the revs shoot up to 3000. Does this mean that ecu has set the idle to 3000 rpm? I think the fault is somewhere else?

 

[jug]

does the problem stop if you disconnect the TPS?

 

[neoseal]

Yes, the idle is 800 rpm, if I disconnect the TPS. and car runs normal.

 

[jug]

tricky problem. i'll have a word with a few people i know and get back to you.

 

[jug]

i'm starting to think the wiring may be at fault. try wiggling the wires or pushing on them with the car running and see if it affects the idle. it shouldnt, if it does you should inspect the connections for any corrosion.

to keep the car at a high idle the voltage returning from the tps must be higher than it should be. we need to find out what makes the voltage higher than it should be. i'd usually say a faulty tps, but you've tried changing the tps so we have to think about other possiblities. bad wiring is the only thing i can think of at the minute. generaly a bad connection caused by corroded metal would reduce the voltage so i doubt its that, but its worth checking.

 

[neoseal]

Ok Jug, Its raining heavily right now, as soon as it stops, I will wiggle the wires and play around with it and see what happens.

 

[jug]

i just phoned a very experienced (old ) mechanic and he said its common for the tps connector to get damaged or corroded on many cars. sometimes the angle at which the wires are bent can also cause internal damage over time. i though that would make idle low but he assures me rusty, frayed or broken wires and dirty or loose connectors easily cause an idle of 2500rpm or more.

i told him your idle drops to normal with the tps disconnected and he said that means you dont need to worry about the stepper motor or throttle body which would usually be next on the list of things to check.

good news i guess, it looks as though you definately have a wire problem. i'd try cleaning the conector pins and bending them slightly to get a better connection. if that doesnt work let me know and i'll give you instructions for testing the voltage output of the tps.

 

[neoseal]

Thanks, Jug, will do it as soon as the rain stops.

 

[neoseal]

Jug, I wiggle - fiddle all the wires I can, but there is no change in the revs. I cleaned the contacts with alcohol. It idles at 800 when I disconnect the TPS. So now what could be the problem? What will happen if I run the car with TPS disconnected?

 

[jug]

it wont hurt to drive without the tps connected. the car will overfuel and wont accelerate as well but it wont cause any real damage.

when i get home i'll have a look at my wiring diargams and tell you how to test the wiring, it may have been damaged somewhere so an end to end test of each wire will be the best way to check. you will also need to test the voltage output of the tps, or alternatively the resistance through it, as you move the tps though its whole range. this will tell you if the tps is working ok.

 

[neoseal]

Thanks Jug.

 

[neoseal]

Jug, waiting for the test instructions.

 

[jug]

sorry i've taken so long i've had trouble finding the 1.2 wiring diagram. i can only find 2 diagrams for the tps sensor so i'll tell you both, hopefully one matches yours.

if you look at the wiring plug on the tps you need to see how many wires there are and what colour they are. you should have 3 or 4 wires. each wire will go to a contact pin on the multiplug for the main ecu. i'll list the wires and ecu pin numbers-

Weber IAW MPI ignition and fuel injection system
3 wire colours: (violet type)
white & violet 23
light blue & violet 16
black & violet 53

Bosch Mono Motronic ignition and fuel injection system
4 wire colours: (white type)
Grey & White 10
Green & White 9
Orange & White 24
White & Black 11

right so lets assume you've checked your wires and found you have either the violet type or white type. next you need to get a multimeter capable of doing a continuity check. most multimeters will have this setting if you check the instuctions. the idea is to test each wire to see if a signal can pass from the tps connector to the ecu connnector. you need to stick one of the multimeter probes onto a wire's terminal in the tps connector and then touch the other multimeter probe on the appropriate pin on the ecu connector. it doesnt matter which colour probe you use at either end. if the multimeter indicates continutiy you can assume the tps signal is getting to the ecu ok or vice versa.

right so the next obvious question is "how do you know which pin is which on the ecu connnector?" thats easier than you'd think. there should be numbers at each corner of the connector. you can start at number one and count your way along to the correct number. the pins are arranged in rows, when youn count to the end of a row you then continue onto the next row. before you assume a wire fault has been found check carefully that you are testing on the correct pin, its easy to miscount.

right, check the wires, let me know what happens, if we can rule out a wiring faults its makes finding the problem easier. if you havent got a multimeter you could try using a bulb, battery and 3 lengths of wire, if the bulb lights when you pass current down a test wire then there is continuiyy, i dont like this method becuase the current from a battery is more like to cause damage than the lower current used by a multimeter. if you must use the bulb method try it with a 3v aaa battery and a torch bulb, its less likey to cause damage.

if your wires are different to the 2 types i described then we need to get a wiring diargram for the ignition and fuel injection system.

 

[neoseal]

Jug, the wires I have in the TPS connector (7 pin) are

1 - No Wire
2 - Black
3 - White/Red
4 - Violet
5 - Orange
6 - Green/Red
7 - Grey/Red

They are not matching colours but I assumed Red instead of White and checked the pin numbers accordingly but no connectivity is found. I even checked for different combination of it but no connectivity. The TPS is Bosch (0132008650), hope this is monotronic TPS listed in Haynes.

Moreover, there is some sticky substance in the holes of ecu connector, I think that is "moisture displacer" but am not sure, do you know about it? May be it is acting as an insulator and hindering the connectivity.