Starter relay

Should the relay that diables the starter have a diode.  If so what is it's purpose?

It's always a great idea to add a diode across the coil of a relay if it does not have one built in. Its purpose is to block the back feed (happens when coil loses voltage) from the coil finding its way to sensitive electronics within the car. relays without diodes often can cause popping sounds in a mobile audio system and more.

The only thing I would like to add is the following. There are some basic things that any good installer / shade tree mech should take into consideration from the out-start, of any new project that will be interfacing with the vehicle's electrical system. With regards to the starter inhibit relay: A) Always select the proven & field tested relay brands made from Bosch, Potter Brumfield, etc. The MFR (mean failure rate) of these relays are very low and are easily found in all local markets for a reasonable price. B) When in doubt, purchase the highest amperage relay you can find for your application. Most users apply a standard 20/30 amp Bosch relay. I would offer to you that if you spend the extra $1.00 today for a 40 amp relay now, will result in 5 extra years gauranteed in service life for you, and the security system, when interfacing to a high amperage circuit, such as the ignition harness. Those who live in area's of extreme temperatures of hot and cold will reap the benefits more so. As when the temperature increases / drops the *Positive Temperature Coefficient* will increase or decrease the resistance in a conductor. ie. When the temperature gets very hot, normal conductors like copper increase in resistance, using up less current. In the same token, when the temperature drops, the resistance drops, and the current increases. Meaning that during the coldest days of the year, that little 20/30 amp relay is being asked to supply short and brief spikes of current of excess of 35-50 amps. Thus, causing premature wear, fusing, and in-opportune moments of distress on the side of the road, due to a stuck relay. :( C) Always use the available 5 wire pig tail connectors to mate to the relay, unless you know how to properly crimp, and use the proper guage of wire to the relay. Countless installs fail somewhere down the line due to poor crimping of terminals to the wire leads, or from systems melting down, do to improper wire diameter. D) Secure all relays to a firm and steady place in the vehicle. Vibration is one of the many things that affect relays, other than using the wrong amperage. E) Aviod placing the relay near any device that may emmit EMI / RFI. As the coil can be made to *sing* ie. Relays that click on their own F) The most obvious thing is to only use a sealed / external relay for the engine bay which has a a sealed end cap for that soul purpose to guard against moisture contamination, and internal relays for inside use. G) All relays should be tested prior to installation, regardless of the brand, to ensure QC of the relay to power the item in question. A little pre-testing and planing will save hours of stress and hair pulling. H) All relays have 3 important current ratings which you should be aware of, and take into consideration if the application is critical or where the load current is high. 1) In rush *make contact* capacity 2) Normal or continous carrying capacity. 3) The opening or break capacity. The *pick up voltage* or where the relay will energise resulting in a contact to switch is important in a starter inhibit. Why? If the battery is very low below the *Pick up voltage* the relay that is suppose to protect your vehicle will not engage to release / engage depending upon N.O. or N.C. configurations. Just food for thought for those who wanted to know more. Regards EVIL Teken...