There are two relays in the harness bundle between the separator panel and the rear window, on the LH side directly below the convertible top main hinge, above the gas tank cover.

The written info i have says all relays are either in relay groups in the trunk, or under the hood, or under the dash.

Nothing about rogue relays dangling from the wiring loom behind the drivers seat.

Are those relays from H&E, or was the previous owner doing more strange stuff to the wiring harness?

Trying to get the brake lights working, now. I am just starting on this episode, have to check the brake light switch, and find the factory brake light relay,and that light-burned-out box, which is probably in the console.

Doug

 

I kept most of the wiring harness from the H&E I disassembled a couple years ago. I'll try to dig it out and lay it in the driveway to see if I can identify what you are asking about.

 

H&E only modified the wiring harness in the center console to add two switches for the 1/4 windows and the up/down switch for the top . As far as I know, Nothing was changed under either the left or right side of the dash.
In the trunk area, that's another story. Since I have all of the electrical circuits working on my car (it was not easy), I will take some pics and label the connections for the various circuits.
The brake light circuit is typical to all 80's MY XJS. Once you eliminate the Brake pedal switch, the brake circuit continues in the right hand trunk area and the 3rd brake light circuit added on the XJS that did not come with the 3rd light feature.
On my car I have the speedometer circuit harness tucked up under the fender where the relay set for the convertible top is located. I haven't been there since I fixed all circuits 2 years ago, so it is hard to visualize what you are posting about.
What relays you have dangling behind the driver seat? A picture please:

 

I will see about getting pic of the harness mess below the top pivot tomorrow afternoon.

Onto the brake lights...

The brake lights work from the brake light switch back (long jumper wire i made) , the brake light switch probably works also, but there is no power to it with key in any position.

Now, onto fuse boxes, the two fuse rows on the LH side under the hood up near the front.

Only the rear most fuse on the inside box has power to it, key on or off, no power to any of the other fuses key on or off. I believe one of those fuses is brake light power. Unfortunately, the fuse box cover says to see the manual for fuse position,and the manual says to see under the fuse cover for fuse ID.

the 1989 manual ( Fig 9.1) says position #5 is brake lights, but there is no power to that fuse with key ON. That Fig 9.1 shows power flow Key on---> fuse---> brake light switch--> brake relay--> brake light failure unit--> brake lights.

If you know what the other position fuses are, that would be good info. There is also a relay mounted to the LH fender about 8" back from the fuse box, that i think may be the brake light relay, based on the green and green/purple wires going to it.

Another question, there is switch mounted to the LH side of the brake pedal pivot tower the master cylinder/booster connects to, there are couple yellow wires to that, any idea what that switch is?-

Thanks,
Doug

 

I also noticed a red capacitor connected (looks factory) across the brake switch terminals, not sure what function that serves.

Doug

 

Both my XJ6 and XJS have that capacitor. (Some say Diode) connected to the brake switch. Reason? To me unknown.
The XJS brake circuit starts at the brake switch > Brake relay located on the right side fender > Brake light failure unit under the center console then to the rear of the car.
As far as the underhood fuse box; there are two, Fuse box A (black) and Fuse Box B (White)
Fuse Box A
1. unused
2. 7.5A Right side Low beam
3. 10A Right side High beam
4. 7.5A Left side Low beam
5. 10A Left side High beam
6. 15A Horn
No power to fuses 2,4, until you turn the lights 'ON' power to Fuses 3,5 when high beam is activated.

Underhood fuse boxes
Fuse Box B (WHITE)
1.Washer Pump
2. Washer Jets (Heated)
3. Horn/Fan relay
4. Unused
5. Radiator Fan
6. Unused
Note : on my car I am only using #1 for the Washer pump
In the trunk area I do not have or see any relays behind the trunk hinge.
If you need any pics of the wiring in the trunk, just ask.

 

Thats exactly what mine look like. Thanks for telling me what those fuses are.

I don't see how the white fuse box positions 2-6 could be used as there are no electrical sockets to receive the fuses, those are just empty holes.

Do you have a blue, square "relay" aft of the fuse box? I am starting to think that may be the hazard flasher.

Have to look for the power interruption elsewhere, i guess. Maybe I will change the main power relay thats over the ECU in the trunk, and start there, as its a double pole relay, one leg may work, one leg may not. Then check the fuse panel under the steering wheel, but none of those fuses have "Brake lights" listed as being in that panel. Maybe they call it something else

I will take pics of my trunk relay set on the LH side, four relays, the H&E docs show those as being top up, top down, low fuel warning , and power. The two relays under the RH hinge are listed as fuel pump(s) and power

Doug

 

Circuitry on the white fuse box is market dependent. (Europe) Most US cars do not have the circuitry to the white fuse box. Mine only has one circuit to it and it is for the washer pump.
Most posters say to ignore the white fuse box.
You are correct. There are 2 relays on the right rear trunk area. One is the MAIN relay which is connected to the fuel pump relay. If the Main relay is faulty the fuel pump relay will have no power so the fuel pump will not work and of course if the main relay is good but the pump relay is faulty you will have the same result. That is the standard setup for the XJS.

right side trunk area with main and fuel pump relays. Main and pump relay cannot be interchanged. Main relay has a diode in it,
The main relay is marked with a diagonal stripe on it( look closely at the silver relay to the right with the red connector, you will see the stripe). unless it was replaced with an aftermarket one.
On the main relay you will see an Orange wire which goes to the ECU. This is the ECU ground trigger for the fuel pump.

 

Here are some pics.
First two are self explanatory. Third is the blue flasher, i am positive it is the hazard flasher, but its under the hood, not attached to the main fuse panel under the dash like the handbook says it is. There is another silver round flasher on the main fuse panel that is probably for turn signals only.
Last pic is the mystery switch on the brake pedal tower.
Car was built 11/87 and its wiring seems to match the MY 1988 1/2 schematic in the service manual.

I found the brake light fuse, having better light to read the panel cover the info, helps.

Doug

 

I found part of the problem.

Fuse was blown. But why was the fuse blown?

New fuse, turn signals worked fine, step on brake, fuse blows again.

Now to find that problem.

I will jumper the brake lights from the battery, again, but this time with fuse in the jumper connection and see if the problem is the brake light switch, or something in the wiring to the brakes.

Edit: I wonder if that capacitor or whatever it is, is bad.

Doug

 

May be the brake relay, which seems to be one of those with a diode, as it has the diagonal yellow stripe.

There is another relay next to it, so I am not sure if I have the correct relay.

Is your brake relay a diode relay?

Ohming the relay shows 90 ohms one way across the coil, ohming it the other way shows 15 ohms across the coil.

Ohming a new five pin relay (which won't work, posts are in the wrong position), I got 90 ohms each way across the coil.

Doug

 

What i have found out...

That relay for the brake lights is no longer made by anybody. It not only has a diode, it also has a resistor in that circuit. For some reason.

I talked to friend of mine who is EEE, who built stuff for the govt he can't talk about... He said you don't really need the resistor, just diode only relay will work.

The purpose of the diode, is to prevent a voltage surge back into the harness when the relay coil field collapses. i don't fully understand how that works when the diode is in parallel with the coil, but whatever, he is EEE, I am Mech/ChE, I don't do electrons very well

Not all relays on this car have diodes.

the power to the brake light relay also has power line direct to the ECU, so maybe thats why they don't want voltage spike, so the ECU doesn't spook Or maybe its an unneeded Lucas-ism.

I will try to find a diode only 4 pin relay, I could also get a std relay and put diode in parallel with the coil, if I chose to but....


The relay system seems to have been designed to handle current for not only the two rear brake lights, but also the four high mount brake lights my convertible no longer has.

I also put in LED brake bulbs, so I am thinking I don't need a relay at all, I can just make a spade jumper and goes directly from relay coil feed in, to brake lights out, as there is no current draw with LED lights, esp with only two LED bulbs instead of six incandescent bulbs.

Another friend of mine suggested I just buy a Painless wiring harness for muscle car streetrod and replace the Lucas harness. The funny thing is, that crossed my mind before he suggested it.

Doug

 

I have that Blue relay disconnected and the flashers still work.
At the brake pedal you have two switches. Brake Switch(bottom) and the CC switch (top)
Put back the original bulbs on the brake lights and see if the fuse still blows when you depress the brake pedal. Focus on one side at a time. See which side of the brake circuit is shorting to ground when you apply the brake. The left/right split is in the trunk.
I will remove the trunk trim tomorrow to expose the wiring and take some pics to see if that will help.
BTW: Did you get your fuel pump/sender unit situation resolved?

 

What is blowing the fuse is ONLY between the brake pedal (out) switch,and the relay, or possibly the cruise control box.

The brake lights were totally out of the circuit when i jumpered +ve on the battery to the "out" side of the lower brake switch (green/purple wire) and the fuse in my jumper wire, blew. The short isn't in the lines to the brake lights, its either the relay, the line to the relay, or or the line to the CC unit (or maybe the CC unit, I haven't located yet) My wiring diagram shows the same green/purple wire that goes the the relay also goes to the CC box, there is no separate switch for it, that I can find.unless its that switch in my pic #4, but it has yellow wires. Had the brake pedal been depressed, power would have gone the other way to the brake lights also, but that wasn't the case.

The LED bulbs aren't the problem, fuse blew with regular bulbs

I suppose that capacitor across the brake light switch could be shorted, allowing current back through the brake pedal switch to feed the high current side of the relay, also, but if the cap is OK, then that just leaves the control circuit.

So, diagnostic method would seem to be.

1) Ohm the capacitor.

2) With relay removed, jumper wire, with fuse in the jumper wire, from +ve to the "out" side of the relay socket to double verify the brake wiring is OK. If fuse doesn't blow, that wiring is all OK.

3) Stick a normal relay in the socket to see if the #5 fuse blows, if not, its the old relay thats bad.

4) If fuse blows with new relay, find the CC box and disconnect it and try again. If fuse blows with new relay and CC unit disconnected, its the wire from the brake switch to the relay socket.

Both fuel pumps work, both tank senders work, fuel gauge now shows some amount of gas in the tanks. Thats all buttoned up, new tank connector hose, and new fuel transfer hose from lower pump to upper tank..

Doug

 

Alas, this is going to require much deeper digging in the dash.

With every single relay removed from under the hood, except the rectangular silver box on the rear corner of the RH fender (do you know what that is? Headlamp relay, maybe?), I sacrificed a couple fuses and the brake lights lit briefly before blowing the fuse.

I have a real strong suspicion, the previous owner rewired this somehow to bypass the under hood brake relay. The other relay next to what I believe should be the brake relay seems to be the A/C compressor clutch based on it energizing when jumpered.

It makes things a lot harder when you have a wiring schematic, with components placed randomly on the schematic, vs a wiring diagram that has the components placed in pretty good rendition of their actual location.

Doug

 

STARTER RELAY - Rear RH fender
HEADLAMP RELAY - LH fender
ECU plus other wiring harness runs through on right hand side.
Left hand side has a lot less wiring.
If you remove the kick panels on the left and right side you will see the harnesses. Some of the harness is a pass thru, others are connected to a black wiring block in the cavity you see when you remove the kick panels. The wiring blocks are held together with screw/s. Remove the screw/s and you can separate the harness The connector pins can only fit one way so you can't mess it up. They do corrode. The main harness exits through a hole in the RH fender. It is the biggest rubber grommet you will see there.
There is also another harness that runs from the back along the transmission tunnel and can be seen under the center console.This harness takes wiring from the RH and LH side of the trunk area. This harness then splits in two at the center console area. one to the right hand side underdash and the other to the left hand side underdash

 

Thanks.

I checked the brake light relay connections on the RH fender, only the ground wire is connected to ground, the other three wires read infinite ohms and or zero volts, so they have been cut and rerouted someplace under the dash. I think the PO couldn't find that replacement relay (none are available, exactly like it), and did some other fix.

oh, joy. Thus my question about harness locations to track this down.

I guess I will lift up the console plate and check there and remove that brake failure light module at the same time.

Doug