Got two Zener diodes, which should I use on my 89 mini?

[Revdoc]

Ordered a pair of 4.7v 1w Zener diodes. Checked them using a computer power supply that put out 5.12v, one of them drops that down to 4.69v, the other only drops it to 4.96v or so.

So the question is, which do I use on the mini? I've heard conflicting reports over the 4.3v vs. 4.7v Zener's, which should be used on an 89 mini? Also, I know the MAP sensor voltage isn't 5.12v, so that's throwing me off as well. Do the Zener's scale voltage, or just cut it off?

Thanks,
Josh

[Aries_Turbo]

the zener just cuts off the voltage... however on a non T2 ecm i think you have to use the 4.3 v as the cutoff on a T1 ecu is lower than its T2 counterpart. or you could make spoolboys adjustable one... i heard it works real nice and you can dial it in super close to the cutoff so that the ecm will control fuel as much as possible.

Brian

[AZTurboMini]

I tried a 4.7 in my mini with no success. I put a 4.3 in and it works (most of the time).

[Revdoc]

Well, I guess I'll try the lower one, and see if I still hit cutout. If so, maybe the local RatShack will have a 4.3, I know they've got 4.1 but that may be too low.

[My96z]

Try the 4.7V one first, it it doesnt work go down to the 4.3V one. My 89 mini was happy with the 4.7V. AZ's van didnt like it. Seems each one is different...



Jeff

[Revdoc]

I installed the diode last night, seems to still run okay. I'm going to wire up the extra injector before I open up the grainger and test the boost cutout, just to be safe.

[AZTurboMini]

Quote:

Originally Posted by Revdoc I installed the diode last night, seems to still run okay. I'm going to wire up the extra injector before I open up the grainger and test the boost cutout, just to be safe.

Cool...let us know what happens.

[3.0L_V6_Duster]

so the diode increases the psi that the overboost kicks in at, which is 14.7 stock right??

then u use the grainger to control the boost to what ever you like?

or does the zener itself increase the boost??? and overboost stays the same?

[Revdoc]

The way it works is this. The MAP sensor reads boost or vacuum and outputs a voltage. ~0v is full vacuum, ~4.7v is full boost, or 14.7 lbs. The zener diode goes between the MAP's output and a ground. When the voltage from the MAP goes over 4.7v, the zener diode dumps the extra voltage to the ground. So the computer never sees more than 4.7v, and therefore never cuts out. The only problem is zener diodes, like all electronics are manufactured with tolerances, so they're not precisely set at 4.7 volts. And, it seems the cutout voltage is different from car to car. Sometimes it's 4.3v, sometimes it's 4.7v. So you don't really know until you install the diode and test it whether or not it's going to work.

Oh, and you don't want to install a diode that's lower than your cutout, or you're cheating yourself out of extra fuel. The computer adds more fuel for each increase in voltage from the MAP sensor, until it hits cut out. So you don't want to install a 4.1v diode when you could get by with a 4.3v or 4.7v, or else the computer will be sending only 4.1v worth of fuel.