Speedo calibration unit

[Andav469]

I'm getting a high idle when coming to a stop (2000rpm then settles after 10 secs) I suspect it's a faulty speedo calibration unit on a BTR diff, anyone got one that is surplus to requirements?

[jojackson4]

Andav469 said:

I'm getting a high idle when coming to a stop (2000rpm then settles after 10 secs) I suspect it's a faulty speedo calibration unit on a BTR diff, anyone got one that is surplus to requirements?

1200. For me

[Andav469]

1200 isn't good but 2000 is a real PITA, especially on the appoach to a roundabout or junction, there is no engine braking and wants to accelerate and not slow down!

[jojackson4]

Have you done the clean and set on the pick up?

[Andav469]

jojackson4 said:

Have you done the clean and set on the pick up?

Yes, though it made no difference.

My understanding is that the speedo will fail or read sporadically if the pick up is set wrong

[ianwayne]

Does it do it when the engine has warmed up? Mine sometimes does this when cold but stops when the engine is warm so I eliminated that as a cause.

[Andav469]

ianwayne said:

Does it do it when the engine has warmed up? Mine sometimes does this when cold but stops when the engine is warm so I eliminated that as a cause.

It does it all the time, ok when started, only does it when the car has been moving

[Alexdaredevils]

Well this is awkward

[phazed]

Alexdaredevils said:

Well this is awkward

No, not at all.

Awkward is being found standing in the middle of the garden with your trousers around your ankles.

[Alexdaredevils]

Haha, the problem has been found and fixed, but it's one of them problems that was staring at you in the face that any one could of made

[Alun450]

Mine seems to suffer the same issue, so what's the obvious solution then Alex?

[Andav469]

The problem was the fuel temp and coolant temp senders were crossed.

The big mystery is, the same high idle issue was there before I changed my engine and first happened in November after the car had been laid up for four weeks and the wiring had not been touched at that point!

[Alun450]

Tvr enigma ? How's the new engine.

[blitzracing]

The more I found out about the ECU, the more that it seems can go wrong with the idle control- Im not surprised that the idle control software has been tweaked many times. You have lots of inputs that lift the idle.

ECU inputs:

Throttle pot.
Speed pulse.
Air Con.
Heated screen (Range Rover).
Engine temp (cold reading).

Mechanically:

Stick stepper motor operation
Mixture control- ie air leaks or wrong initial mixture due to a wrong sensor input like AFM.

Ignition timing and Air bypass valve setting- the timing has a huge affect on the RPM at idle, and consequentially the amount of air needed to keep the idle stable.

Potential misfires or general poor running.

At least with diagnostics we can now get if the ECU is in idle mode and more importantly what the target idle RPM is- as this changes with the ECU sensor inputs, so at least you can tell if its the ECU holding the idle up, or something mechanical.

[EGB]

blitzracing said:

The more I found out about the ECU, the more that it seems can go wrong with the idle control- Im not surprised that the idle control software has been tweaked many times. You have lots of inputs that lift the idle.

ECU inputs:

Throttle pot.
Speed pulse.
Air Con.
Heated screen (Range Rover).
Engine temp (cold reading).

Mechanically:

Stick stepper motor operation
Mixture control- ie air leaks or wrong initial mixture due to a wrong sensor input like AFM.

Ignition timing and Air bypass valve setting- the timing has a huge affect on the RPM at idle, and consequentially the amount of air needed to keep the idle stable.

Potential misfires or general poor running.

At least with diagnostics we can now get if the ECU is in idle mode and more importantly what the target idle RPM is- as this changes with the ECU sensor inputs, so at least you can tell if its the ECU holding the idle up, or something mechanical.

Good summary.
Methinks, By stick stepper means a lazy stepper, communicating lazily with lazy ECU. Mine is lazy on idle and slowing down when engine is not fully warmed up. When hot 70-85 C it's ok.