Cut the yellow wire! (CAUTION - DO NOT READ IF EASILY BORED)

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TwoSheds
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Joined: Thu Jan 18, 2007 8:01 pm
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Full Name: Roger Watkinson

Cut the yellow wire! (CAUTION - DO NOT READ IF EASILY BORED)

Postby TwoSheds » Mon May 31, 2010 1:46 pm

There is something that I don't like about my 3.9 hotwire - the way that the ECU keeps the air bypass valve open when I lift off. This manifests it'self in two ways: 1) There is little or no engine braking. In fact sometimes it will actually pull against the brakes when stopping. And 2) If you dip the clutch and coast to a stop then - until you actually get below about 5mph, the revs stay high (1800rpm) and even when you stop they take several seconds to return to tickover.

Previously I have removed the wires from the speed transducer when driving off-road which prevents this annoying trait, however I can't do that when on the road since it also kills the speedometer.

Also previously I thought that the (electronic) speedo was driven by the ECU, but in this I was wrong... While trawling through the wiring diagram yesterday for something else, I noticed that the wire from the speed transducer does not just go to the ECU, it branches and the other branch goes to the speedo. Now since the transducer simply switches 12v on and off I reckoned that if I disconnect the wire from the ECU, the speedo would still work but the ECU would think I was stationary and give me proper engine braking again.

So - to cut a long story long - I cut the correct wire and it works. I now have almost normal engine braking and a working speedo. :)

Roger

p.s. If anyone is thinking of doing this, be aware that there are 3 yellow wires into the ECU so make sure you get the correct one!

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davew
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Full Name: Dave White

Postby davew » Mon May 31, 2010 2:45 pm

Can I be the first to point out that all you need to do is unplug the IAC stepper to get the same effect... you can then plug it back in if you need it.

TwoSheds
Posts: 1092
Joined: Thu Jan 18, 2007 8:01 pm
Location: M1/M62
Full Name: Roger Watkinson

Postby TwoSheds » Mon May 31, 2010 5:30 pm

I beg to differ Dave - you see with the yellow wire cut, I still get the idle control and maintenance, the correct setting while starting, and it opens up when running under throttle too. On the negative side it does take about four seconds for mine to drop from 1800 to idle when I lift off, but that helps prevent the lurch (transmission wind up) that you get when you lift off with the stepper disconnected.

I did try it with the stepper disconnected a few months ago I found it quite difficult to get the tickover correct when both cold and hot - to the extent that I even tried putting in a choke cable alongside the throttle (sort of a hand throttle) so that I could do it on the fly, but it was so crude in it's effect (try to gently increase the idle and it would rev the nuts off it) that I gave up and never fitted it to the new engine.

I just did a test run looking at drivability and performance and - within experimental error - the car feels exactly the same under throttle as it did before. Also my standard speed test (floor it at 30mph in third at one sign and see what speed it's doing at the 2nd) gave exactly the same as before. Rolling down hill to the roundabout near me and closing the throttle definitely produces significantly more engine braking immediately and all the way to a stop. Changing gear on the overrun is no more difficult either.

All in all I am pretty pleased with the result - it is so much better than standard that it makes me wonder why they built it like that in the first place!

Roger


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