[Diy_efi] Triac versus Relay - Knowledge please!

[Daniel R. Nicoson]

My car is a 1994 Mustang GT with T4M0 EEC.  Street driven with full EGR &
Thermoactor operation.  I don't pretend to be able to properly design an
electronic circuit.  I understand On-Off, Good- Ground, Bad-Ground...

I have a permanently mounted DIY-WB that is powered up via a relay any time
the engine has "power-on" to the computer (EEC pin 57 hot, Vehicle Power -
VPWR).

This weekend I am adding a "Remote Data Log Relay" to reconfigure the EGR
sensor circuit for data logging at a flip of a switch.

Both relays are triggered by EEC pin 57 and I have several concerns:

1) Can these relay coils induce a voltage spike harmful to my EEC anytime
during their operation?   Starting up?  Turning off?

2) The relays are not shielded by metal boxes (both mounted in plastic
boxes), can EMI be a factor from CB radio's, ignition or anything else?

3) Would a triac be an appropriate replacement to the electromechanical
relay's?

4) Do I risk overloading this circuit that feeds the EEC (amp load or
voltage drawdown)?

The first relay that I have been using successfully for 6-9 months turns on
the DIY-WB system (I think 2 amp max load?) was chosen because it was on the
shelf at Radio Shack and seemed like it would handle the load:

RS# 275-226, SPST Contacts 30 Amp at 12 Volt DC, Coil Voltage 12volt,
Pull-In Voltage 6volt, Coil Resistance 66 Ohms, Nominal Current 160 mAmps,
Drop-Out Voltage 3.6Amps

Should this relay be replaced by a triac?  I have one in hand, RS# 276-1000,
PIV 400 Volt, Forward Current 6 Amps, Gate Current 25 mAmps, Gate Voltage 3
volt.

If I replace the relay with this triac do I need a large heat sink on the
triac?

The second relay when un-powered allows all three wires going the EGR
position sensor to stay in their stock connection.  When powered (activated
by a switch for data logging) the relay disconnects two of the wires
(circuit 359 & 351, EEC pins 46 & 26) and connects the DIY-WB to the EGR
valve position sensor infeed to the EEC (circuit 352, EEC pin 27).

This second relay is RS# 275-214A, 12VDC PLUG-IN RELAY, continuous coil
voltage 13.2 VDC, coil resistance 160 ohms, pull-in voltage 9 VDC, contact
rating 5Amps at 125 VDC, nominal current 75 mAmps.

I haven't looked for a solid state replacement for this relay.

Any thoughts, suggestions?  I don't want to fry my EEC because of a voltage
spike and the omission of some 5 cent piece that would have protected
everything.

Thanks,

Dan Nicoson


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi