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Jaguar XJS v12 1988

Haydar Kirmani

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Hi everyone

I'm after recommendations for a suitable Link ECU for the following problem.

I have a jaguar xjs v12 1988. Last year I bought an EFi kit from Mobeck based in Sweden (www.mobek.com) to upgrade from my old factory EFi system. The kit included a Maxxecu Street with distributorless conversion. The kit included injectors, leads, coil pack, trigger wheel, idle control valve etc...

Overall the kit works fine except I'm not happy with how the idle works, as it seems a little off, never working exactly how I want it to, with fluctuations that don't reflect the settings. However the car drives 1000 times better than with the old EFi system, so I didn't make to much of this.

However, I live in a hot country (Pakistan) and so I decided to swap out the visco fan with a high powered Mercedes unit (600W). Originally we bypassed the PWM controller of the fan, so it would work using one of the custom outputs as a FAN controller, with a simple trigger at a particular temperature, in this case 86C. However the fan would then come on at full speed, with a noticeable load blip on the car. The fan would quickly cool the car down and turn completely off, however then the car would quickly heat up and again and then fan would constantly be flipping between on and off. Although this works fine, I'm a little worried that this constant high load on the car will stress the alternator and electronics. 

To this end I thought about using the fan's PWM controller so we could precisely control the fan speed. So there isn't the instant load hit on the car when the fan suddenly turns on at full speed and it just seems a more elegant solution. There is an option in the Maxxecu tuner to set the fan output to a PWM output, so thinking this would easily solve the problem we tried this. However in this mode the fan would work erratically, either full speed or rapidly cycling between on and off. We confirmed the frequency of the Mercedes fan, which is 10Hz, and also the duty cycle of operation, 10% off to 90% full speed. We even tested the fan in an actual Mercedes car and measured the frequency and duty cycle and they were spot on. When we measure the output from the Maxxecu, neither the Frequency nor the Duty Cycle is steady. In fact the duty cycle fluctuates widely. Interestingly, when we then checked the output for the idle control valve, that also wasn't steady! Maybe this explains the inconsistent idle! I contacted Maxxecu but their reply was that at a low frequency of 10Hz, there would be some fluctuations. I explained that when the duty cycle is set to 10%, it is fluctuating between 40% and 90%, which I regard as more than 'some' fluctuation. However just to check I set the frequency to 100Hz and still the duty cycle was all over the place. I think either the Maxxecu cannot handle PWM outputs or there is a fault in my particular ECU. Either-way, I'm not happy with their half hearted responses so have decided to find an alternative ECU.

So basically I need an ECU that can drive 12 cylinders (wasted spark acceptable) but also create steady PWM output. Sorry for the long message just wanted to explain my problem.

Thank you all.


Warm regards

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Storm, Xtreme, Fury can all do 12cyl wasted spark and either group or semi-sequential injection. For 12cyl with sequential injection it would have to be the G5 Voodoo Pro which will do 12cyl sequential injection and direct spark.

In terms of PWM fan control the standard fan control setup in Links is switched but you can use a GP PWM which would allow you to setup desired PWM duty cycle on a 3d table.

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How were you verifying the PWM freq/duty with the Maxx ecu?  You could be right that you have some issue with your particular ecu, but I would find it hard to believe that they could all be that bad in general, if it were they wouldnt even be able to drive a tacho, let alone boost control or NOS which must be pretty well tested functions with their drag racing customer base.  I've only ever used one Maxx ecu for a DSG car a few years ago and while I didnt find it to be exceptional at anything, it seemed to do most of the basics ok.  I didnt have any need to scope any PWM signals as everything seemed to work as expected.

I just dont wont you to go to the effort of swapping to some other ecu only to find the same issue apparent due to the way it is being measured or something. 

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Personally I don't want to go the effort and expense of swapping out the ECU but don't know what else to do. I have a multimeter with Hz and % option. When I use this multimeter to check the Frequency and Duty Cycle when the fan is connected to the Mercedes ECU, it is rock steady on the stated frequency of 10 Hz and as we input the duty cycle on the Mercedes diagnostic software, the multimeter matches the value exactly. Also, the fan behaves exactly as it is supposed to. It is off at 10%, then speeds up based upon increasing the duty cycle, right up to full power at 90%. When we use the exact same multimeter to check the Frequency and Duty Cycle on the Maxxecu, the results are as stated above, and the fan does not behave like it should!

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Im not really here to support a competitor product so I cant waste too much time diagnosing, but I cant help feeling the variability you are describing sounds more like a "floating input" issue or perhaps a highside/lowside drive incompatibility.  So I would dig a little deeper in this area before abandoning the ecu.  I believe the Maxx GP outputs are low side drive only and dont have internal pull-up resistors or flyback diodes, and the fan controller internal circuit is really anyone's guess.  Have you connected a pull-up resistor to the output?  If not, I would suggest trying a 1.5Kohm resistor connected between +12V and the output wire going to the fan controller.   A brief description of a floating input here:  https://www.mouser.com/blog/dont-leave-your-pins-floating

Be aware also, the commanded duty cycle in many OEM software and scantools etc is based on "percent high" (as are most multimeters), whereas many aftermarket ECU's with low side drive outputs, the commanded duty is "percentage low".  So you may find 90% in the OEM ecu is equivalent to 10% in the aftermarket ecu - assuming it has a pull-up etc.     

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