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Phil Stuart Tinkerer NSW Soarer 1x TT JZZ30, 1 x RAV4 and a pork chop
Posts: 61 Reg: 10-2006
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Friday, January 04, 2008 - 08:04 pm, by: Phil Stuart(Ericdog)
Hi Guys,
Having read much about the use of zener diodes as "overboost" fuel cut defenders (FCD's), I decided to do some testing. This applies to my 91TT, but should apply to most soarers near that era.
For those that don't know much about MAP sensors and FCD's, its probably better if you just do a search rather than me just repeating what has been said before.
For those that just want a quick run-down on the MAP sensor, the ECU (computer) in your car monitors the amount of boost pressure/vacuum using a device called a MAP sensor. Typically, this little sensor outputs a voltage between 0 and 5 volts depending on boost or vacuum in the inlet manifold. It will put out 2.5 volts when there is neither boost or vacuum, hence its output goes above 2.5v on boost and below 2.5 on vacuum.
If the ECU "sees" this voltage coming from the MAP sensor reaching about 4.82 volts, which I understand corresponds with about 14(ish) PSI boost, the ECU cuts the fuel to the engine, generally by shutting off the fuel injectors.
So, if you want to run boost around or above 14PSI, you will run into problems.
To stop this fuel cut from happening, many people suggest putting a 4.7 volt zener diode across the MAP sensor output, in the hope that the 4.7 volt zener will clamp the voltage at 4.7 volts.
This brings me to my tests:
What I have found is (from tests) that the output impedance (internal output resistance) of the MAP is about 2.2k ohms. This means that the MAP sensor does not have a lot of "power delivery" capability, and is easily "loaded down".
One test example was done with the MAP output at 2.5v (key in the on position, motor NOT running). By simply "loading" down the output of the MAP sensor with a 2.2k (2200) ohm resistor made the voltage drop to about 1.3v. The ECU does not load down the MAP voltage as it has a very high load resistance (>100000 ohms).
By applying pressure to the MAP sensor to bring the output up to 4.0 volts, putting a 4.7v zener across the output of the MAP sensor made it drop to 3.45 volts !!!
This drop in voltage is due to the zener diode having a small amount of leakage current.
Now, this will certainly make it work as a fuel cut defender, but the ECU will now be seeing a voltage that is not correctly proportional to the true pressure in the manifold, and could cause a change in performance.
So, in summary, although a zener diode will work as a fuel cut defender, I wonder what effect the incorrect MAP-to-pressure voltage being sent to the ECU will do.
There are many different types of MAP sensors, and some may have a lower internal resistance and may not suffer from this problem.
As another experiment, I figured I could make a "pseudo" zener diode, that has no (negligible in this application) leakage by connecting a number of standard 1N4001 diodes in series. I found that if you connected 8 or 9 (I found 9 the best) of these 1N4001's in series, it produced a very good 4.7(ish) volt zener diode and the best thing is that there was no leakage at all that loaded down the output of the MAP sensor, yet the diodes functioned fine to clamp the MAP output so that the fuel cut was disabled and I could run more boost successfully.
I know this sounds strange, as a 1N4001 has a 0.65v forward voltage, and that 8 of them would make 5.2volts, and it shouldn't work, but if you take into account the high internal output resistance of MAP sensors, it worked both on the bench in my workshop, and in my 91TT soarer.
If you want a reliable fuel cut defender, I would steer clear of zener diodes and go for the adjustable design such as the kit that jaycar sells, the circuit listed elsewhere here (do a search) or a commercial unit.
I hope this little piece of info, and testing has been informative.
