I had my 1994 XJS 2+2 to the mechanic today regarding issues with the rear quarter windows and the top not functioning. He told me that there is no power to the top switch and that he had no idea how many shop hours it would take to track down the short. I thought i'd bring the car home and see if perhaps I could save a bit of cash by trying to locate the problem myself. It seems from looking at the wiring diagram and reading the posts that the thermal breakers might be a possible solution. Can anyone post a pic of what these look like? Should these test zero resistance across them like a fuse if they are working properly?

Any other input on "typical problem areas" to look for a short between the power source and the switch

Thanks,

Lionel

 

I don't have photo, but they should be on the cable just in front or under the relay panel in the trunk with the wire colours Green Brown and Green Purple.


Hood Control Module and Relay-Convertible - Parts For XJS from (V)179737 to (V)226645 | Jaguar Heritage Parts UK

 

That diagram is very helpful. Thank you.

 

YycXJS

I'd be very surprised if it's a "short" to the switch.

The typical problems for the hood and rear windows not working are:

- gummed up switch
- One of the three inputs not sending a signal to the hood ecu to allow the switch to then operate. These are: Handbrake on, Gearlever in Neutral or Park, Ignition in Pos II. It's nearly ALWAYS the handbrake switch not connecting which causes the problem.

If your garage doesn't know how the hood works on an XJS, I'd be going somewhere else. The easiest way to check, assuming you've done the obvious checks, is to just use a multimeter on teh plug to the hood ecu. It's a 5-minute execise. Check the wiring diagram in your workshop manual for the pin-out. (And if you haven't got the manual, definitely go an buy one!)

Good luck

Paul

 

So I did a bit more checking tonight and so far this is what I have found:

1. The socket where the top switch plugs into has power on slate/white as well as on slate/red. Black is grounded. There is power to this socket regardless of the position of the hand brake.

2. The top switch closes the circuit between pin 4 and pin 1 when the switch is pushed to put the top up and between pin 4 and pin 2 when the switch is pushed to put the top down. This seems to make sense that it is working.

3. Both thermal breakers show zero resistance the same as an intact fuse would.


Paul, you make an interesting point about the neutral switch and the park brake switch. Lately I have noticed that the brake fail light comes on and stays on for a bit and then shuts off once the car has run for a bit. I have also noticed that the car has to run a bit before I can shift out of park (especially if it has sat for a few days). Would those also be symptoms of the ECU problem?

I have been working off of a wiring diagram that shows an ECM. Is this the same as the Hood ECU. It has pins numbered 2, 3, 6, 7, 8, 9, 10, 12, 13, 14, 15.

My Hood ECU looks similar to a sealed unit with a couple of screws on it, will it just open if I undo the screws? What readings am I looking for with the multimeter.

Thanks so much,

Lionel

 

Lionel,

The hood switch sends a signal to the hood ecu. You seem to have established that the switch is functioning ok. The hood ecu checks for input signals on the 3 points previously mentioned. If these conditions are met then it provides output voltage to hood circuit and rear windows.

You don't need to take the ecu apart. Just check (by probing the back of the plug) that the handbrake switch and transmission switch are providing the signal. Wiring diagram is in the manual.

To eliminate the usual problem cause, just disconnect the wires at the handbrake switch and bridge them.

Good luck

Paul

PS Problem moving from Park implies a problem footbrake switch or transmission rotary switch

 

Hi Paul, Just to double check that I am reading the diagram right, The parking switch connects via the Black/white wire. When the hand brake is engaged the meter should read 12v. The transmission switch connects via the black/green wire . When the transmission is in Drive or Reverse the multimeter should read 12v

 

Many thanks Paul!! Problem solved!! It was the hand brake micro switch.

 

Lionel,

Glad you got it sorted! Really pleased to have been of some small help.

Paul

 

i may need that diagnosis. what do you mean it was the handbrake switch? i have a 95 xjs convertible 2+2 that neither of my quarter windows will go down and neither will my hood. i can let the hood up by pressing the roof switch but it wont down except for manually. i assume my quarter windows would come up if they were down but since they are stuck up i cant test that theory.

 

If the pump runs one way, that sounds like the control unit for the top, not the micro switch on the park brake. If it were the micro switch, it wouldn't work at all.

 

what control unit are you referring to?

 

The DAC11333 hood control module. There is another thread discussing this, and it turned out his power top switch had a problem.