How to make a resistor box for low impedance (ohm) injectors.
I got most of this info from Google searches and trial and error.
If you blow up your shit attempting this its not my problem.
If you can’t figure it out its not my problem.
If you need help filling in the blanks you should not be attempting this.
Basic/Much Over simplified reason you need resistors (or a driver box which I am not talking about here): Stock injectors and many injectors (about 950cc and less) are high ohm (high impedance), higher resistance require less current to operate and so the amp draw is much less (less heat), with no need for a resistor/s on the stock system. However, because Low Impedance injectors, have a lower resistance, they draw more amps. More amps equals more heat which our PCM’s drivers are not designed for. I recommend doing some research on the differences between Saturated and Peak and Hold Injectors and the differences / advantages / disadvantages of a resistor box versus a injector driver box.
Typical High Impedance= 13.5-14ohm
Typical Low Impedance= 2-3ohm
We tested the stock battery voltage on Kyle’s car and came up with 13.2 volts (so our system will put out a constant 13.2 volts while the car is running (+or- 0.8 volts or so).
Stock high impedance (ohm) injector example:
13.2v power / 13.3 ohm injector = 0.99 Amps draw (stock current).
Low Impedance (ohm) injector (this is from the test of the 1550 cc ones I have), if plugged into a high impedance harness without a resistor box:
13.2v/ 2.7 ohm injector = 4.89 Amps draw
This extra current (amps) creates HEAT and this is why if you hook up low impedance (Ohm) injectors into a stock harness without a resistor pack it will melt the PCM’s low side (ground controlled) drivers pretty damn fast (only fix once melted / blown is a new PCM).
Note: You should contact your Retailer, or Manufacture of your injectors to figure out the right AMP value for your injector to operate at. You should also know the Resistance of the injector (ask or test with a multi meter, testing is better as the manufactures specs is not the exact spec of your injectors because of tolerances within the manufacturing process).
Note: You probably don’t want to put more then a about a 1.2-1.6amp draw on the stock PCM (just guessing here) because the heat from extra current (stock is 1 amp) will affect the longevity of the PCM drivers,. Try to go with the lowest amp setup you can with the injectors working correctly.
Example I found my 2.7 ohm injectors (tested the ohm with a multi meter) needed a minimum of 1.19 amps to open/fire correctly. Generally the smaller low ohm injectors 1200 cc for example (mine are 1550 cc) will work with less current / amps (so something like a 10ohm resistor).
13.2v/(X+2.7ohm)=1.19AMP
13.2v/(X+2.7ohm)-1.19AMP = 0
X=8.4
13.4v/11.1ohm=1.19 Amp
Notice that in my first example 2.7 ohm injectors on a stock harness would have equaled 4.89 Amps and a fried/melted my stock PCM injector drivers. Once I added the resistors inline I get nice safe 1.19 Amps.
Supplies I used:
18 gauge wire
Radio Shack: Four 20watt (they can safely dissipate up to 20 watts) 8 ohm ceramic resistors (use the above calc to figure out what ohm resistor you should go with!) with a tolerance of 5%, when measured with a multi meter they were all 8.4 - 8.6 ohms. If you can find a lower tolerance (1%) that would be better as they will all be dead on the rated ohm’s. DON’T use a high tolerance then 5% though.
Water proof shrink wrap tubing
Zip Ties
Double sided 3m tape
Solder (get the nice thin, high lead 60/40% stuff)
Flux
Quality 3m Electrical Tape
Radio Shack Project Box
Wire loom (various sizes)
I also used a Mopar Harness repair kit but its not needed
Tools (I used more but this is all I can remember):
Screw Driver/s
Dikes
Wire Strippers
Soldering Iron
Heat Gun
Multi Meter
Drill
Drill Bits (I used a big one LOL)
FIRST: Test the Ohm’s of your resistors with your multi meter, I bought 5 and picked the 4 with the closest tolerances.
SECOND: SHUT OFF THE BATTARY (not needed but a good idea)
Picture of everything laid out:
Picture of project box wrapper:
Resistors I used (use the calculations to figure out what you will need):
Picture of were to set the multi meter to test the resistors before the wiring and after:
Project How To:
1. Measure can cut about 8 lengths of the 18 gage wire, I used four lengths x 2ft each.
2. Drill a hole big enough hole for 4 wires plus some extra room for some shrink wrap.
3. I lined up all 4 resistors and wires then trimmed some of the resistor wires, stripped the wires, added the flux (use lots of flux it helps the solder wick), crimped, soldered and then shrink wrapped first:
Crimped with flux and ready for solder:
Water proof shrink wrap:
4. Repeat step 3 for the other side of the resistors.
5. Install into box. Be careful with this step NOT to twist the metal from the resistors back and forth as it will weaken and SNAP and you will have to get a new resistor. I drilled the holes in the end of the box big enough to fit a little of the wider shrink wrap in there.
6. Test each resistor for good conductivity and the correct ohm (do this before and after they are soldered in):
7. Put the lid on the box (mine did not quite fit tight but it worked good anyway):
8. Open up the harness and find the four injector wires they are a switched 12v wire (green with orange strip). Join the side coming from up the harness (battery/fuse box) all together and flux/solder/add shrink tubing to the four wires going to the resistor box (also twisted together).
Note info about the green wire with orange strip: It does not matter what wire goes to what injectors because the all get constant power. The Injectors are switched on/off (controlled) by the PCM low side (GROUND) driver. Furthermore this wire if traced back of though the harness is actually a single wire that splits into 6 wires that power the coil, noise suppresser and all four injectors. You want to splice into it where it is four wires that are going directly to the injectors.
Here is were I spliced into the green/orange strip injector power wires:
How-To: Make/Install Resistor Box for Low Impedance (ohm) Injectors
Tuned by NemoFoss
Linear Mode
