View Full Version : How To:Add Resistors for Low Impedence/High Impedance Injectors *Approved*

02-05-2007, 05:58 PM
In this thread i will detail with photos what resistors to use for Nissan switch, how to install them into a project board, soldering and wiring.

So why would we add Resistors to our Fuel Injector Harness?

Well, alot of the larger Top Feed injectors are low impedance, and would other wise draw too many amps if directly connected to a 12v source. These injectors include but are not limited to the stock ca18det injectors, and the rb series 444cc's, and the sunny pulsar gti-r 444cc injectors.

A common problem when attempting any of the above swaps seems to be that pulling the drop resistor box with the engine in japan is often forgotten. Alot of people will get through a whole swap without realizing they are missing their drop resistor and find they have no injector pulse.

Commonly, the CA18DET Drop resistor will go for about $100 on ebay or elsewhere, and the DSM Drop resistor will go for about $70 new on some sites. Unfortunately these can be very difficult to find in a junkyard.

So, if you do not want to spend your time finding a dsm in the junkyard, or purchasing an expensive new one, this write up will help you wire up resistors to your efi harness.

This install can be done to almost anything including SR20DET and KA if you were converting to one of these injectors. To make this install work in this instance, trace your fuel injector live's back the harness to the section where the 4 wires junction to 1, and cut them apart. You would treat the 1 wire on the side of the junction as our yellow wire in our write up, and the other wires as the green wires.

Some quick math and theory.
High Impedance Injectors are wound more than Low Impedance injectors. The resistance of High Impedance injectors is much greater than low impedance, making the amp draw much less for high impedance, without using a resistor.

The down side to this, is this increases the Injector lag time of the solenoid, thus reducing minimum injector open time which effects idle in larger injectors, and effects the accuracy of these injectors at high RPM.

Low Impedance Injectors are deployed in this right. They are wound much less, yielding a lower resistance, but the solenoids are much faster, yielding a minimum injector open time MUCH lower than high impedance (.8ms vs 1.7ms typical). At idle with an 850cc, this can be 900 rpm or 1400 rpm.

Because Low Impedance injectors, have a lower resistance, they draw more amps. Like any electronic device, if a device draws too many amps, it will create an adverse amount of heat, and fail.

Typical High Impedance= 13.5-14ohm
Typical Low Impedance= 2-3ohm

High Impedance injector
14v/13.7ohm= 1.014amp
This is quite low.

Low Impedance injector, just "plugged into a high impedance harness"
This is far too high, and will cause the device to burn up quite quickly.

So, we can add more Resistance to our equation, to get the amps down nice and low. You should contact your Tuner, Retailer, or Manufacture, to figure out the right AMP value for your injector to operate at. You should also know the Resistance of the injector (though testing this is quite simple). My recommendation is 1.2-1.7amp, no more that 2amp MAXIMUM.

So if we wanted an AMP of 1.5
(Get the Notebook, pencil, and Calculator)

When plugged in 14v/9.4ohm=1.489amp

Viola! Our amp is now much lower, and injector/solenoids will not over heat.

This equation can be modified for any vehicle/injector, as long as you have correct data to fill into it, and know what you want your results to be. This will not work for Gain based injection. This is only for Saturated Injection.

Growing up, I always loved project boxes, so I jumped at the chance to use one again.
Lets build something!

Radio Shack
JK Electronics Garden Grove CA (http://www.JKElectronics.com)


*18 gauge wire (green)
*14 gauge wire (yellow)
*4 10watt 6.8 ohm resistors (2 packages)
*2 sizes of shrink wrap
*Zip Ties
*Double sided 3m tape
*Electrical Tape
*Proto Box and Board kit
*Male and Female Pin-able 6 pin connector.

*Flat head and Phillips Screw Driver
*Small Needle Nose Pliers
*Small Dikes
*Wire Strippers
*Soldering Iron
*Heat Gun

Optional Tools:
*12v Power Source
*Multi Meter

So we start by Clipping our Old Drop Resistor Box Connector on the CA18 Harness and pin and push our new Male 6 pin connector. There are 5 wires here. Doing this the same as our OEM Nissan connector, the 12v source is placed in the center Top, and the 4 injector wires are put on the outside flanks. Since all the signals will be merging on our board, remembering exactly which injector is which wire is relatively moot, just make sure you separate the 12v Source.

You can identify this by checking the pin-out on your ecu connector and color matching the color codes, or by running a test signal (12v) from your injector plug 12v source, then testing it on the drop resistor connector.

Ok, Open up the Resistors and project box. Carefully bend out the leads on each resistor and find a suitable pin to drop them through.

You should have all 4 resistors lined up like so. We will use 1 side of the board to wire up our injector leads, and use the other side to junction the signal back to our 12v wire.

This board is printed in pairs, so of each set of pairs, one hole will be occupied by a resistor lead, the other hole will be occupied by and injector signal wire. Insert the injector lead wire from the bottom up, 1 wire for each resistor in each resistors corresponding paired hole.

Apply a generous amount of FLUX. Flux is used in soldering to help the solder follow to where we'd like it to end up. So using your small flat head, take some Flux and apply it to where you would like your solder to end at. (IE: Resistor Lead, Injector wire, and printed board pair)

Using the soldering Iron, heat the lead of the resistor and stick the solder into the joint between the Lead and the board, do the same for the wire. Wiggle the wire and the resistor to see if they are soldered well, they should be made relatively immobile by a good joint.

Repeat this step 3 more times. Until you have injector lead wires paired to each resistor.

Do this only for 1 side of our resistors.

On the underside of the board, you will need to clip down the leads of the resistors to clear inside the box. Using these extra lengths, we can use them to make grouping bundles, to tie together the outputs of all 4 resistors to our 14 gauge wire(yellow).

This pretty much covers the completed circuit.

Now take the leads underneath and boot them with shrink wrap and shrink them down.

I used a small file to enlarge two holes in the board for a cable tie.

Apply some of our 3m double sided tape.

Now we will need an access hole in our project box.

Drill one out, make sure to use eye protection.

Place the board into the Project box, and run the leads through our access hole.

Use a small cable tie on the inside of the box, to keep the leads from being pulled out of the box too far. Cover the bundle of wires in shrink wrap.

A length about 12-14 inches should be more than enough.

Strip the ends of the wire.

Fasten the new pins, and push them into the connector in the opposite order that you did the male connector. If you are following me, injector inputs on the outside flank, 12v in the center top.

Now test the new circuit, using a multimeter, ground the multimeter to the 12v source, put the red lead into the 12v center junction wire, and use the hot 12v lead of the tester to test each pin. You should see whatever voltage is output by the tester (14 v in my case).

Now just plug in your box.

And find a good place to secure it and you are done.

Any questions just let me know.

[email protected]

02-12-2007, 11:06 PM
Ok, I finished it!

02-13-2007, 12:27 PM
Cool write up!

Here are a couple ideas for your article:

Might want to explain how you determined that 6.8 ohms was the correct resistance for the injectors you're gonna run... Thats easy to add an it will make your article really generic so anyone with any motor and any injectors should be able to figure it out with some easy math.

Also u might want to note the importance of ceramic 10w resistors instead of the little 1/4 watters.

02-14-2007, 09:25 AM
i agree with mello, other than that excelent write up man

02-14-2007, 05:51 PM
dam talk about a bitching ass job... Great write up! I know I will never attempt it after seeing how much it takes...plus I hate wires

03-26-2007, 05:48 AM

just proofread it really quick and add in that info mello88 mentioned.

03-26-2007, 02:36 PM
Ok, im going to re-lock it with the shocka!

I cleaned up the typo's, grammer, and formatting, plus added in the math for nerds like mello and I.