Solid State Relays (SSR)

Solid State Relay and diagram

Can the uActivate® be used to test solid-state relay circuits? The answer is yes, but there are important factors that need to be considered for successful connection and testing.


Solid-state relays (SSR) are activated and switched electronically, with no coils or mechanical components. This offers many advantages over mechanical relays. For example:

Safety: No arcing across mechanical contacts - explosion-proof.

Durability: With no moving parts to fail ormake noise, they have a long service life. SSRs can be used to control lights, windows, and seats.

Advanced Control: Along with operating as an on/off relay, SSRs can also be pulsed to control current - controlling inrush current as well as dimming lights or adjusting motor speed.


The first step is to ensure you are dealing with an SSR. This can be determined by:

    [1] Reading description of circuit operation found in your diagnostic information. Does it mention an SSR or special operational features of an SSR, such as in-rush current control?
      [2] Examine any markings on the actual relay. Most relays have an internal schematic printed on the side An SSR, will use a solid state electrical component symbol such as a MOSFET, instead of showing mechanical contacts.
      [3] Check the circuit wiring diagram. If the circuit diagram shows a coil and mechanical contacts then perhaps it is meant to have a standard relay.

      These three steps are important for SSR identification. They will also confirm whether the right relay is in the circuit.


      Amperage - Is the maximum circuit amperage within the uActivate® 25 amp rating?

      Advanced circuit control
      - Does the SSR provide advanced circuit control such as controlling in-rush current or speed control? If so, keep in mind that the uActivate® is either 100% on or 100% off.


      In many cases, SSRs are the same size and form factor (body style) as standard relays. But be aware that the terminal positions and assignments/designations may be different! Using an incorrect cable can result in damage to the circuit controller. Therefore, it is important to investigate and qualify the terminal arrangement. 

      Another major concern are the numbers used for terminal assignments. In some cases, the terminal numbers don't conform to any of the traditional automotive relay terminal designation standards such as ISO 7588, DIN 72651, DIN 72552 or the British standards.

      Consider this '03 GMC Envy headlight circuit diagram, which includes an SSR. Notice that switched power for the SSR is designated as pin 2 and the normally open switched contact is pin 4. Based on the DIN 72651 standards for a traditional relay, pin 2 is coil control (85) and pin 4 is the normally closed contact (87a).

      Chart of automotive relay terminal number assignment and designation

      This example illustrates the importance of using the circuit diagram as the primary guide when selecting and connecting cables. In this case, the best cable is the uActivate® Universal Cable!

      There are also aftermarket SSRs that are designed to be a direct replacement for standard coil-based relays. As direct replacements, they will have the same terminal assignment/designations and therefore can use the standard uActivate® relay cables.

      Expect the unexpected. Do your due diligence.
      Always compare your connections to the circuit and relay schematics.


      Refer to your User Guide for more information.