Lextreme II
Active Member
An Article by:
Phil Bradshaw
[email protected]
1990-1992 Lexus LS400 Inability to Rev on Factory ECU Conversions
Over the years I have wired a whole bunch of Toyota ECUs in various conversions, with the last few years focusing largely on the 1UZFE, with over 25 of them now completed. This hobby is getting out of hand!
One problem I faced was a propensity for the 1990-1992 vintage LS400 ECUs (with 4 plugs, pictured) to not run properly. Somewhat frustratingly the engine would start and idle like a dream, but just not rev out, with the engine effectively hitting a wall at around 2000 rpm.
To make it even more challenging, the problem did not occur with all the ECUs of this vintage (albeit about 70% of them exhibited this characteristic). Even more annoying was that this problem did not occur with the later model ‘bolt type’ ECUs or any other ECUs from the same vintage such as 1UZFE Crown.
My initial suspicions were the issue was related to the lack of an auto trans in conversion, but this soon proved to not be the case. Likewise so much for my theorising that the lack of an ABS or Traction Control System was at fault.
My solution had been to rewire the engine to take a different ECU (such as a Crown).
Long story short, my understanding now is that the inability of the engine to rev out is actually a factory installed feature…
Until this week I was under the impression that fuel injection cut only occurred when either the vehicle reached the pre-programmed speed limiting (around 180km/h or 110 mph for JDM vehicles) or if you hit the rev limiter (around 6750 RPM as best I can tell). It would appear that this vintage LS400 also has fuel cut such that you can’t rev the engine without the vehicle being in motion. Toyota probably decided it wasn’t in keeping with the image of the vehicle!
The fix appears to be fairly simple – the ECU just needs a speed sensor signal (SPD). Don’t ask me why the other ECUs don’t seem to be as fussy. That said, I have also recently heard that the intermittent stalling problem some 1UZs experience when returning to idle is also solved by the restoration of the speed sensor signal. My 1UZFE Supra (with a Crown ECU) does this, and I will look into hooking this signal up in the future to see if it makes any difference.
That said, it will also most likely result in the return of 180 km/h speed limiting, which isn’t really an issue for normal road use. It would also be easy to have a switch to stop the signal for race use.
Ordinarily the speed sensor signal comes from the speedo head, which has a switch on the back of the unit that utilises a rotating magnet (running at speedo cable RPM) to close a reed switch to earth twice per rev of the cable. The reed switch effectively intermittently grounds ECU terminal ‘SPD’. The engine will only rev out when this intermittent signal is being generated; i.e. the vehicle is moving.
So, the simple fix is to utilise the standard speedo head in the recipient vehicle (most if not all Toyota non electronic JDM speedo heads have this switch), otherwise find a suitable aftermarket signal generator. Just remember that the signal is a switch to ground, not a +12 volt pulse.
Note that thus far I have only tested this on a very much under construction air bagged Hilux Mini Truck project, with a temporary, very dodgily wired engine harness. I hooked up a suitable speed sensor which I spun with a variable speed battery drill. With the sensor not rotating the engine would miss and not rev as previously experienced; with the sensor rotating (even at a relatively low speed) the engine ran fine all the way up to normal rev limiting.
As the truck will be some time before it is able to be driven to fully road test this solution, I figured it best to get the information out now. I am pretty confident this is the solution, and would appreciate any feedback to the above email address.
The good news is, if you are running the engine with a full auto trans installation, you can use this same signal to get the trans to shift.
Happy revving!
Phil Bradshaw
[email protected]
1990-1992 Lexus LS400 Inability to Rev on Factory ECU Conversions
Over the years I have wired a whole bunch of Toyota ECUs in various conversions, with the last few years focusing largely on the 1UZFE, with over 25 of them now completed. This hobby is getting out of hand!
One problem I faced was a propensity for the 1990-1992 vintage LS400 ECUs (with 4 plugs, pictured) to not run properly. Somewhat frustratingly the engine would start and idle like a dream, but just not rev out, with the engine effectively hitting a wall at around 2000 rpm.
To make it even more challenging, the problem did not occur with all the ECUs of this vintage (albeit about 70% of them exhibited this characteristic). Even more annoying was that this problem did not occur with the later model ‘bolt type’ ECUs or any other ECUs from the same vintage such as 1UZFE Crown.
My initial suspicions were the issue was related to the lack of an auto trans in conversion, but this soon proved to not be the case. Likewise so much for my theorising that the lack of an ABS or Traction Control System was at fault.
My solution had been to rewire the engine to take a different ECU (such as a Crown).
Long story short, my understanding now is that the inability of the engine to rev out is actually a factory installed feature…
Until this week I was under the impression that fuel injection cut only occurred when either the vehicle reached the pre-programmed speed limiting (around 180km/h or 110 mph for JDM vehicles) or if you hit the rev limiter (around 6750 RPM as best I can tell). It would appear that this vintage LS400 also has fuel cut such that you can’t rev the engine without the vehicle being in motion. Toyota probably decided it wasn’t in keeping with the image of the vehicle!
The fix appears to be fairly simple – the ECU just needs a speed sensor signal (SPD). Don’t ask me why the other ECUs don’t seem to be as fussy. That said, I have also recently heard that the intermittent stalling problem some 1UZs experience when returning to idle is also solved by the restoration of the speed sensor signal. My 1UZFE Supra (with a Crown ECU) does this, and I will look into hooking this signal up in the future to see if it makes any difference.
That said, it will also most likely result in the return of 180 km/h speed limiting, which isn’t really an issue for normal road use. It would also be easy to have a switch to stop the signal for race use.
Ordinarily the speed sensor signal comes from the speedo head, which has a switch on the back of the unit that utilises a rotating magnet (running at speedo cable RPM) to close a reed switch to earth twice per rev of the cable. The reed switch effectively intermittently grounds ECU terminal ‘SPD’. The engine will only rev out when this intermittent signal is being generated; i.e. the vehicle is moving.
So, the simple fix is to utilise the standard speedo head in the recipient vehicle (most if not all Toyota non electronic JDM speedo heads have this switch), otherwise find a suitable aftermarket signal generator. Just remember that the signal is a switch to ground, not a +12 volt pulse.
Note that thus far I have only tested this on a very much under construction air bagged Hilux Mini Truck project, with a temporary, very dodgily wired engine harness. I hooked up a suitable speed sensor which I spun with a variable speed battery drill. With the sensor not rotating the engine would miss and not rev as previously experienced; with the sensor rotating (even at a relatively low speed) the engine ran fine all the way up to normal rev limiting.
As the truck will be some time before it is able to be driven to fully road test this solution, I figured it best to get the information out now. I am pretty confident this is the solution, and would appreciate any feedback to the above email address.
The good news is, if you are running the engine with a full auto trans installation, you can use this same signal to get the trans to shift.
Happy revving!
