Main relay reliability test ( lasting 14 years max, California region)

Overview: In this experiment a bad main relay is used to recreate an early failure to prove that desoldering is required. The bad relay is used to help speed up the experiment.  Normal relays won't behave this way. 

In this experiment we will run the vehicle until the car stalls or won't start under an extreme condition of heat and abuse. As you can see several months later the relay will fail as this graph records the event.

chart mainrelay

This graph shows a length of a few seconds and how the current behaves before the vehicle stalls in traffic. Usually the car won't start unless the vehicle cools down or a key pressed against the solder joint or joints.


The chart below show how different methods and solder materials are used in this experiment. Notice that none of the mainrelay use plain solder. They are all rosin core solder. A few websites are misleading when all you need to do is to reheat the solder joints without flux and new solder. 

Test based on solder application.

Type of solder Desolder Solder
Num of days 
before failure
Stalled in 
Solder Iron/ Watts Joint areas redone
40/60 Normal Rosin core No Little about 90 Yes Control temp/600C  Small terminals only*
High silver Rosin core No large about 120 Yes Control temp/600C Small terminals only
High silver Rosin core Yes correct about 1.5 years No, but failed to start in one occasion for 2 seconds Control temp/600C Small terminals only
40/60 Normal Rosin core Yes correct 2 year and counting No Control temp/600C Small terminals only

* Small terminals only = this is part of the circuit board where the small terminals characterized by the small amount of solder used. This is not the section where globs of solders are used located on the larger terminals on the edges of the circuit boards. These terminals don't usually fail.

*Solder Amount =  how the solder is applied.  Little means the same amount similar to the factory amount. Large meaning globs. Correct meaning 40-70 degrees from horizontal. 

How the experiment is conducted. A logic electrical configuration is set up to automatically kick in the backup relay in an event the test subject main relay fails. A warning is indicated for the driver. Then the relay is check to see which joint had failed simply by pressing the tip of the car key on the joint and attempt to start the car. If the car starts then that joint is logged. This has to be done quickly otherwise the joints will cool off. The failed joints are usually in or near the center of the circuit board.


  • Remember that other factors which affect the reliablity isn't shown here. Such as how the relay is not suspended or coushioned during soldering. This prevents the terminals from shifting during expansion contraction.


    Add your comments, suggest corrections or clarifications. Thanks.