Technical Information Low and High Impedance Injectors

There are two basic types of injectors. Low impedance injectors with a resistance of around 2.5-3 ohms, and high impedance injectors of around 12 ohms. This can easily be measured by a multimeter across the two electrical terminals.

Early Hondas used low impedance injectors with a resistor pack. From about '92 to '96 Honda transitioned from low impedance to high impedance without a resistor pack.

Illustration - 92 Accord resistor pack (on the left)

Over the years we have seen many low impedance injectors installed without a resistor pack, and many burned out OBD 1 ECUs (and several RSX-S ECUs) as a result. Why? If you look at the specifications of the injector driver package, it indicates it can flow the amount of current required for low impedance injectors. (Top specs highlighted in red.)

The clue is in the bottom specification Tj of 150 degrees C. This relates to the absolute maximum junction temperature of the transistor pack. The junction temperature is the temperature of the transistor itself embedded in the package. There are 4 of them in this particular driver package (for the RSX-S ECU). Click picture to enlarge.

What affects reliability is heat, which is a by product of the current flow. The greater the current, the greater the temperature. Reliability takes a long time to measure. Temperature is straightforward.

Test Equipment and Procedure

  • A car battery at and charger @ 13.1v was used to directly power a RSX-S ECU.
  • A logic probe was used to generate a 50% duty cycle pulse at 50hz, equivalent to 6000 rpm engine operation.
  • An Omega 817A digital thermometer was connected to the STA646C
  • The injectors were cooled by immersing them in water.
  • Stock 12 ohm injectors were connected and the temperature rise graphed until the temperature stabilized.
  • The transistor pack was allowed to cool.
  • Aftermarket 2.5 ohm injectors were connected and the test rerun.


Time (minutes) 12 ohm injectors (temp in Celcius) 2.5 ohm (temp in Celsius)
0 31 36
1 52 74
2 55 154 (Exceeds specification)
3 57 180
4 59 179
5 61 174
6 62 200
7 63 ECU blown

The transistor pack is soldered to the circuit board, so heat is being removed by conduction and convection. The temperature probe was connected to the composite part of the case, which does not conduct heat as fast as metal or semiconductor. So, internal junction temperature will be higher than the externally measured temperature.

Watch the final minute of this ECU's life: Click here (5 mb Quicktime movie)

The temperature was so hot, it melted the solder joints of the transistor pack. It was removed using only pliers and no soldering iron. Our soldering iron was set to 350 degrees C to install this transistor driver package.

This test was repeated three times with similar results. At about the two minute mark the external temperature of the transistor driver exceeded the manufacturers ratings. Between 7 and 10 minutes the driver package blew.

Factors that would increase the temperature from this test when run in a car:

  • There was no fuel pressure behind the injectors, so they were very lightly loaded.
  • A climb up a hill. (Increased RPMs and duty cycle)
  • A top speed run (Increased RPMs and duty cycle)
  • Car battery voltage would be higher than this test (13.8v vs 13v)

Factors that would lower the temperature:

  • Partial throttle - low duty cycle (10-40%)
  • Deceleration Zero duty cycle

Injector temperature

Not only do the injector drivers get hot, but the injectors themselves heat up from the current passing through them. We ran one three minute test with the injectors in open air and not being cooled by water.

  • After 3 minutes the stock injectors were 40 degrees C.
  • After 3 minutes the 2.5 ohm injectors were 100 degrees C.

Injectors are normally cooled by fuel. What these figures show is that the 2.5 ohm injectors without a resistor pack run hotter heat the and fuel more than stock injectors.


  • Running low impedance injectors without a resistor box will substantially increase the temperature of the transistor driver IC due to the increase in current flow.
  • The temperature increase will depend on your driving.
  • If that temperature remains high enough for long enough the driver transistor will be damaged.


Do as Honda does. Run high impedance injectors, or if you must run low impedance injectors, use a resistor pack.