An ISO 9001:2000 Registered Company

Technical Service Bulletin
TSB # T2008-02 09/04/08
Product: All products manufactured by InterMotive Vehicle Controls.
Vehicle: All vehicles.
Issue: The use of a relay without diode clamped Transient Voltage Suppression (TVS) can
cause very large voltage spikes when the relay coil is deactivated. These voltage spikes may
damage or cause intermittent behavior of on-board vehicle electronics. It is recommended that
only relays with diode clamped coils be used. Resistors and other methods of clamping are not
as effective at eliminating voltage spikes and are not recommended.
Action: Install only relays with diode clamped Transient Voltage Suppression on vehicles to
improve the vehicle’s electronic system reliability. Care must be taken to observe proper
polarity when connecting relay coils which are diode clamped. The diode “arrow” or “cathode
line” should be towards the connected positive voltage.
Additional information: The coil of a relay is an inductor which can create large voltage
spikes when it’s current is shut off (think of an spark plug ignition coil). The collapsing
magnetic field will send a large voltage spike back into all connected circuits which can
propagate throughout the vehicle’s electronics, causing damage and/or intermittent behavior.
Relays are readily available with a clamping diode, which will prevent high transient voltages
from damaging solid state semiconductor devices which are increasingly used in modern
vehicles. Relays with resistor Transient Voltage Suppression are not as effective in clamping
high voltages, and are therefore not recommended. Instead, use diode clamped relays. Most
relays have a printed diagram on the case, which indicates if the relay has diode Transient
Voltage Suppression.

85

A relay with diode Transient
Voltage Suppression (shown in
red)

30

Diode

86

986 South Canyon Way
Colfax, CA 95713

www.intermotive.net

87

Phone: (530) 346-1801
Fax: (530) 346-1812
9/5/2008

SAE recommendation:
www.sae-a.com.au/fsae/downloads/newsletters/fsae_09_2005.pdf
“If you are using relays in the circuit to switch fans etc by grounding the relay trigger circuit
through the ECU, make sure you use a protected relay (Inbuilt resistor, diode or capacitor as
appropriate) in order to protect the switching circuit in the ECU.” (Emphasis added)

Ford recommendation:
https://www.fleet.ford.com/TRUCKBBAS/non-html/Q130.pdf
“Purpose: This bulletin is an attempt to provide a source of general best practices for
modifications and/or interfacing with any Ford Truck electrical system….
All electrical devices that have inductive loads (i.e. electrical motors, relay coils, PTO clutch
pump clutches, etc.) must be provided with isolation discharge paths for the reverse voltage
generated when power is disconnected from the device. The disconnection could be an
intentional disconnection (i.e. a switch, relay, timer, etc.) or an unintentional disconnection (i.e. a
blown fuse, damage to a device in series, disconnection of a wiring harness connector, etc.).
Either case will cause a large reverse voltage pulse of very short duration to be sent through any
connected electrical conductor. The short duration precludes the use of fuses as protective
devices. Additionally, any connection between a device with an inductive load and the OEM
electrical system should be isolated with a relay. Contact the manufacturer of the electrical
device for proper instructions to add a discharge path to their device.” (Emphasis added)
Relay manufacturer recommendation:
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf
“The Need for Transient Suppression
When an electromechanical relay is de-energized rapidly by a mechanical switch or
semiconductor, the collapsing magnetic field produces a substantial voltage transient in its effort
to disperse the stored energy and oppose the sudden change of current flow. A 12VDC relay, for
example, may generate a voltage of 1,000 to 1,500 volts during turn-off. With the advent of
modern electronic systems, this relatively large voltage transient has created EMI, semiconductor
breakdown, and switch wear problems for the design engineer. It has thus become common
practice to suppress relay coils with other components which limit the peak voltage to a much
smaller level.” (Emphasis added)
Additional Reading:
http://www.intermotive.net/Tech%20Tip/Tech%20Tip%20-%20April%2008.pdf

Technical inquiries – InterMotive Technical Support (530) 346-1801
986 South Canyon Way
Colfax, CA 95713

www.intermotive.net

Phone: (530) 346-1801
Fax: (530) 346-1812
9/5/2008