Fleet Maintenance - 44

TRAINING

What is a
bistable relay?
Even if you've never heard of
bistable relays, you have likely
diagnosed circuits affected by them.

»»Fig. 1: Suppressed relays have a diode
designed to suppress voltage spikes.
These types of relays are either diodesuppressed or resistor-suppressed.
Photo courtesy of K&D Technical Innovations

Have there been instances where you or a
technician may have removed or replaced a
relay, only to find the relay's sides were bulged
out and the plastic was melted?
This bulging can happen when the coil has to
be energized continuously. This process allows
the temperature to rise in the relay and in the
computer controlling the relay current - either
the engine control unit (ECU) or other control
units. For example, the computer controls
multiple conventional relays that require
continuous current to the coil in order for
the relay to stay active. With a bistable relay,
however, once the relay is set (contacts closed)
the current is removed from the coil and the
relay stays in the set position.
Most technicians have a drawer filled with
all kinds and types of relays used for testing.

By Keith Littleton,

OWNER, K&D TECHNICAL INNOVATIONS
K&D Technical Innovations (kdtechnicalinnovations.com) is
a service provider offering training solutions for industry
and education. Littleton specializes in CAN communication issues and lab scope diagnostics, and is the current
station chair for TMC SuperTech's electrical test station.
Littleton holds numerous ASE certifications, as well as
nine Toyota certifications and 11 GM certifications.

By George Arrants

TRAINING CONSULTANT, K&D TECHNICAL
INNOVATIONS
George Arrants is the training consultant for K&D
Technical Innovations. As an automotive education consultant specializing in ASE Program Accreditation, Arrants
works with instructors and administrators to develop
partnerships with local business and industry through
program advisory committees. He is past chair of the
Technology and Maintenance Council's TMCSuperTech
- the National Technician Skills Competition - and the
TMCFutureTech - the National Student Technician
Competition. His entire career has been in the automotive service and education industries. He works with the
ASE Education Foundation as the Medium/Heavy Truck
Alliance manager.

44 Fleet Maintenance | December 2019

When it comes to bistable relays, replacing a
bistable relay with a conventional relay, or vice
versa, could wreak havoc and cause expensive
damage to your vehicles and fleets.

Some background

It is first important to understand that there is
a difference between suppressed and non-suppressed relays. The most popular versions of
suppressed relays are either diode-suppressed
or resistor-suppressed - with either a diode or a
resistor, respectively, built in - to help suppress
voltage spikes (See Fig. 1).
(For more information, read the March 2018
Training column It's not okay to just swap relays
at VehicleServicePros.com/20992675)
Technicians should always bench test every
relay, including suppressed relays. Technicians
can test if a relay is suppressed by using a
digital multimeter (DMM) with the ability to
capture a fast-enough sample rate, in order to
capture the high voltage created when the coil
in a relay is turned off (See Fig. 2).
Some of the more expensive DMMs are not
fast enough to capture this. In order to complete
this test, you will also need a meter with a "Peak
Min/Max" function or a "REC" function.
Think about what this voltage could do to your
systems. Even using the correct suppressed relay

»»Fig. 2: In the training provided by
K&D Technical Innovations, there is a
test that technicians can do using a
multimeter with a fast-enough sample
rate to capture the high voltage created
when the coil in a relay is turned off.
Image courtesy of K&D Technical Innovations

does not provide guaranteed protection; the
circuit of the suppressed relay could open or,
even worse, short.
It is extremely important to test, replace, and/
or use the correct relay for the correct application.
Do not let anyone tell you that if the relay fits in
the same cavity, it will work - even when you
just need to test a known good relay to see if the
system works. The consequences could mean
having high voltage introduced into the circuit.
If you are replacing or testing a relay, be sure
to use the schematic on the outside of the relay
to confirm that you have the right relay going
into the correct cavity (See Fig. 3).

The differences

Bistable relays use much less current, or
amperage, which in turn create less heat and
allow the vehicle's computer to operate at a
cooler temperature.

http://www.kdtechnicalinnovations.com http://www.VehicleServicePros.com/20992675

Fleet Maintenance

Table of Contents for the Digital Edition of Fleet Maintenance