What's This Relay?

Talk about the E-Type Series 3

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Nialls Shortbear
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#1 What's This Relay?

Post by Nialls Shortbear » Sun Dec 13, 2020 8:34 pm

Could anyone enlighten me on this relay please? Its on a US spec 72 OTS that had AC, located RHS of tunnel behind radio panel. I am quite sure it is factory wired. It is fed from the non fused side of F6 with a white wire to a line fuse, and onwards to the relay with a Light Green. Is it SBW related maybe??

This car has connections on the seat belt locks for the SBW, but if there was an SBW "unit" its either long gone or I havent found it yet and the wires are not the colour shown on the wiring diagram.

I am trying to iron out all the errors/missing info on the wiring diagram and create a correct one for the cars we have in colour with the long term goal being a massive laminated one on the shop wall which I once saw in a Delorean shop in Chicago and it struck me as a great idea. The diagram in the manual is an abomination, and while the one available here is much better, there is a lot missing.

Cheers.
NS
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vee12eman
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#2 Re: What's This Relay?

Post by vee12eman » Wed Dec 16, 2020 7:40 am

Hi Niall,

I have a series 3 FHC, RHD UK spec, no air con or SBW system and I re-wired it completely. The wiring diagram on here is the best I came across, and I bought both the Coventry Auto Components book (which I found incomplete, sadly) and a colour wiring diagram, I can’t remember exactly where from. The relay you describe does not seem to be in any of them. The wires on the non fused side of fuse 6 and 7 are sometimes connected to the opposite one to that shown in the diagram, but as they are linked anyway by a short white wire, this doesn’t matter electrically speaking.

The only white wire from 6/7 to a relay is to the air con relay connection W2, but no light green cables come from that relay. Another white cable leads to a line fuse behind the dash, this carries on with a white lead to the rear window heater/demister switch, on my car this is actually white/black, which is supposed to be the colour of the lead from the switch back to the screen. Possibly someone fitted a relay for the demister, which I would guess draws a reasonable current. Of course anyone could have modified the loom for many reasons.

A couple of other possibilities. The car may be wired with incorrect colours from new, it is a Jaguar and that would not surprise me. It could be to the air conditioning relay, but the system was later rewired with light green, alternatively the seat belt warning system may be wired with different colour to original but should take fused power from fuse 4 and 3. A close look at the wiring diagram shown on here reveals that three white leads supply power to the SBW system, one into the unit (at least that's how I interpret it) and one each to the SBW Gearbox Switch and the SBW relay, connection C3 (connections 1, 7A and 7C of the SBW system as shown on the diagram). I think these are pre fuse, but after ignition switch.

Hope that might help, here’s my colour diagram, printed on a plotter and somewhat difficult to roll open!

Image

I still prefer the black and white one on here, I find I very quickly remember the colours and codes and the drawing is simply easier to follow without the distraction of colour.

Regards,

Simon
Last edited by vee12eman on Thu Dec 17, 2020 4:21 am, edited 1 time in total.
Regards,

Simon
Series III FHC

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vee12eman
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#3 Re: What's This Relay?

Post by vee12eman » Wed Dec 16, 2020 7:52 am

Hi again,

I just checked and found that I actually have a slightly updated version of the .PDF wiring diagram on this site - I think it was me that sent it to David Jones originally. It came from the XKE data site and I praised it in an article in the club magazine. Shortly afterward, Paul Spurlock, who had drawn the original, contacted me through the club and sent the updated version which included the choke warning light and possibly a couple of other things. It’s on my PC in .TIF format so I’ll see if I can PDF it and send it in.

Regards,

Simon
Regards,

Simon
Series III FHC

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MarekH
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#4 Re: What's This Relay?

Post by MarekH » Wed Dec 16, 2020 9:49 am

The SBW box sits right in the middle of the dash between the two banks of fuses. It has a circular plug going into the box and a "Smiths" label visible on its front face. The relay you show is part of the SBW system and is shown on the s3 wiring diagram from xkedata.com /resources in the dashed box on the right hand side of the diagram. If you look at the wiring diagram, you'll see that it was designed to intercept the starter relay connection via the park/neutral switch relay winding grounding on automatic cars, so may be redundant or implemented via what the diagram describes as the gearbox switch if your car is manual transmission - see connection "7c" on the diagram.

In any case, you can bypass its effect by rearranging the four white/yellow wires under the dash and unplugging the round SBW plug if it is inhibiting your starter relay.

kind regards
Marek

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Nialls Shortbear
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#5 Re: What's This Relay?

Post by Nialls Shortbear » Thu Dec 17, 2020 11:00 pm

Thanks for these answers Gents.

I am somewhat mystified as to how a 3-pin relay works, does it energize the third pin to the power signal when on? I cannot find the pin out to that relay online.

Either way its solves my issue, as the SBW is long gone in previous owner wiring debauchery actions and I wanted to know if I needed to cater for it in my diagram.

Did the SBW have a "brain" in that box or is it a set of mini relays etc??

Cheers
NS

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MarekH
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#6 Re: What's This Relay?

Post by MarekH » Fri Dec 18, 2020 7:44 am

The three pin relays work by having c2 common with w2, so when w1 is given a polarity opposite to c2, c2 becomes connected to c1, otherwise c1 remains unconnected to c2. In the case of this SBW relay, it has a c3 contact instead of a c1, so presumably does the exact opposite to what a standard 3pin relay does. (You can always use a changeover relay in place of a 3pin relay, by joining c2 to one of the w terminals and leaving one of c1 or c3 always unused, so just think of them as cheap changeover relays which aren't as flexible as the proper item.)

The SBW did indeed have a brain. It had a very early (1970s) silicon chip which processed the seat belt inputs, the pressure pad under the driver's seat, the door switch and whether the ignition key was in the lock and output these to a buzzer, a light on the dash and a transistor which held the starter motor relay inhibited until the right conditions were met. I worked out the circuit diagram. It can be altered to just make the light light up the light if you really want to.

kind regards
Marek

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