Gearbox wont kick down

[greenguzzler]

Hey guys started the car yesterday when leaving work and the AU Fairlane V8 i have idled a bit rough. I thought that it was strange so i headed straight to the local Ford dealership. On the way there i noticed the gearbox wouldnt kickdown and was in 3rd gear but it will manually change to 2nd and 1st but cant get 4th. Anyone ever had this happen? It has the mechanics a bit lost.

[5.0 ED]

Its in limp mode, check tps, speed sensor, etc.

[greenguzzler]

Apparently they ruled out limp mode. Would the diagnostic computer show up limp mode?

[waggaclint]

a scan tool should show some sort of code....have you had any work done on the car recently??

[greenguzzler]

Yeah its been in heaps of times for warranty stuff. Just about out now so i really just want it fixed so i dont have to go back there anymore. I just found out yesterday that it has a LSD i didnt know because it runs like a S/S!!! Is there any mechanics on the Mid North Coast of NSW that actually give a about their customers cars?

[5.0 ED]

Whatever is wrong with it, it should show up on the scanner, and i can not think of anything none electrical that would do what your talking about.

in this case limp mode would probably get discarded pretty easily cause you can manually shift 1st 2nd, normally it just locks up into 3rd, which is why i say you should look at your throttle position sensor, cause it can do this exactly as mentioned and it only needs to go out .1 volt to do it.

Sounds like gear selection switch is working ok cause it shift's 1st and 2nd, which is reads off the switch and then electronically changes it, unless its dirty in one gear spot, but then if it was you would put car in 2nd and it would still drive off and go 1st and change into 2nd itself.

vehicle speed sensor could also cause the issue's, cause the ecu doesn't know how fast it's going to change gears. but i'd doubt it would be your issue, cause your speedo obviously still works, and therefore it would have to be noisy at which it should still work ok sometimes and not others.

Do you have a multimeter or access to one? TPS is very easy to set and replace.

[greenguzzler]

What should the exact voltage be at the TPS?

[5.0 ED]

it should be over 0.8volts, but not over 1 volt, most people try and set it to 0.99v, but i'd stick to more like 0.95v cause of inconsistancy in closing throttle postions after time, and unacurate multimetre's etc. It should be set off the earth wire on the tps not engine or anything else, as this will change the value.

Too high tps voltage won't cause tran's issues like yours but will put engine in part throttle mode instead of idle mode, it will also cause the trans converter etc to stay locked up when you remove throttle while driving.

If the tps voltage drops out at all to below 0.5volt it will trip tps fail switch, and like that will happen if it fails at any point in the tps range, you may not even find a spot in it on the multimetre and yet it could still be faulty.

What you should do is put multimetre on with engine not running and measure it and slowly increase throttle and look for a dropout in voltage, it should slowly increase the voltage to a peak of about 4.5volts, so if in doing it you suddenly get a low voltage, its a definent sign its faulty.

[5.0 ED]

I think the tps wire colours are red/green/black, black is gnd/earth, and green is the output. Red should be 5volts which comes from the ecu.

[greenguzzler]

Thanks for the info mate. I will be ringing the dealer this morning.

[BLACKAUXR8]

Do you manual shift down gears by anychance?

[greenguzzler]

BLACKAUXR8 I havent as i have only had the car for 10 weeks and i dont rate the gearbox as being strong enough to do that. I rung the dealer this arvo and they are replacing the PCM under warranty. Fingers crossed.

[greenguzzler]

Thanks for all your responses guys. It turns out that it was the computer that had shat itself. Replaced under warranty.