The relays on my car have something like a diode across 2 of the contacts. What is the reason for this ?

 

The short answer is diodes allow current to flow in only one direction. Positive to negative or negative to positive.

 

They are needed in most modern applications where things are electrically controlled. Allowing AC current in the system creates noise, faulty signals to computers, etc.

 

Ok. thanks guys. But these seem to be across the + and - relay power contacts 85, 86. I'm adding a few relays to the car and wonder if l need to add a diode too. Its something l have never had to do in my other car.

 

A lot of your existing relays *won't* have the diode. In most cases where the diode is used I think the purpose is to block voltage spikes.

For most typical accessory additions I'd say a diode isn't needed.

Cheers
DD

 

Usually the relay terminal numbers 85 and 86 refer to the coil connections.
The diode across the coil is to prevent a reverse voltage spike from being applied to the wire connected to the +ve coil terminal when the 12V power source is removed from the relay coil. This voltage spike is caused by the collapse of the magnetic field created by the coil during it's powered up state. These spikes can harm sensitive electronic devices in the vehicle so I would look for the type of relay with an inbuilt diode to install for any future additions.
In the attached example, 85 would be coil -ve and 86 coil +ve for normal relay power up. In this state, the diode is not doing anything.
When the relay is powered down, the diode conducts to snub out any reverse voltage produced by the collapsing coil magnetic fields.

Clear as mud?

 

Just buy whatever replaces the relay. For example, one of the two rear trunk relays has a diode and the other doesn't. Just go with the part #

 

The 85 and 86 are the low power contacts from the switch. The current to the device being activated flows though the other two (87 30), as EXEF's diagram shows in post no. 6.

 

All of the relays I have looked at on my car also have diode across the relays. EXEF has nailed the reason. These are called "Flyback" or "Snubber" diodes and they create a current flow path through the coil and diode only when the coil is switched off.

So it is important that the + and - go to the correct terminals on the relay. If you get + and - incorrect the diode will conduct and at best prevent the relay from operating.

The coil in a relay acts the same as an ignition coil, so when the relay is switched off the coil "generates" high voltage and can cause an arc at the switch which significantly reduces the life of the switch. It can also effect other electronics such as ECU's.

 

Wow thanks guys, useful info. I am using the relays to operate the rear windows with a separate switch. As noted the original set do have the diode as they go to the roof ecu. so l guess l should do the same.
Anyone know what diode l need ?

Anyone breaking a convertible ??

 

A really common diode is a 1N4001, I use 1N4007's because I have heaps of them. The only difference is the voltage rating 1N4001 is 50volts and 4007 is 1000volts

How are you planning on wiring the relays?

 

I had the idea to use the roof switch to operate the rear windows. As the ecu controls the neg side of the roof/window relays. I have a push on contact switch next to the roof switch, this switch controls a relay in the trunk that changes the roof switch wires over from the roof control through 2 changeover relays to up down window control.
I have unplugged the control wires from the ecu plug and using new wire extended these to go through switching relays then back to the connector. These then feed the neg side of the window relays and up/down they go, as this is all Neg controlled, this also means that the relay will send neg. feed to override the shift lock and handbrake lock so I can use the windows whilst driving.
So while I push the switch, the roof switch will now control the windows, then when l let go it controls the roof.
If you can work out what l mean.....looks better in my head than on here. :-)

That's the theory anyway. .......And can all be reset back to original with little work.

Oh and l have the rear seat conversion so all the roof stuff is in the back. Hardest bit so far is routing the push switch wires to the boot/trunk. What a pain in the **** that's being. Unlike my Stag, the XJS isn't open over the wheel arch.

 

yeah...works as planed.
It really help that my local breaker had the pump wiring and relays laying in the trunk of a '94 breaker ($35.00) car so all done and with the original relays it even looks original.
Seems that later cars than mine don't have the diodes ....