ABS upgrades on 1987 XJ40?

So I’ve bought a Jaguar in which it looks as though a PO has upgraded the brake booster to a standard vacuum unit and then proceeded to remove the SLS components from the engine bay…pump and all. My understanding though is that the vehicle still needed the SLS pump to run the ABS correctly? The vehicle does have an ABS fault (among the many faults currently on display :neutral_face: but I’ve not looked into why as yet).

If this is the case, what options do I have for getting the ABS to function as it should? Can I install the later Teeves ABS system or another system from an even later model (say X300) and if so, what components would I actually need?

All my other cars (the ones I tinker with anyway) are pre-ABS and so I’m not familiar with how it works at all (let alone when part of a system like on the XJ40) but always happy to learn.

TIA for any suggestions/help you can provide.

Matt …

I replaced the hydraulic brake system on my 1989 XJ40 with a vacuum brake booster years ago. If I understand correctly how everything operates the mechanical hydraulic pump on the front of the engine only operated the hydraulic brake boost unit and the self leveling rear shocks. If the conversion is done properly that pump can be removed and a triangular blanking plate installed.

The ABS system consists of the sensors at each wheel, the ABS control module in the trunk, and the ABS modulator in the engine bay. The ABS modulator has it’s own internal electric pump. After the vacuum brake booster conversion (and being able to eliminating all of the now unnecessary plumbing from the old hydraulic assisted brake system) the ABS should still operate normally.

Exactly what warnings are you getting on the VCM.

1 Like

Hi Grooveman

I’ll have to check the actual message on the VCM next time I’m in the car but I’m sure it just says “ABS fault”. I have also noticed though that the sensor wires running to each wheel are in various states of ruin. I think these are to do with the brake pad wear sensors (also a VCM error at the moment) but not sure if they support the ABS as well?

Is there a way to get more accurate coding form error message somehow through the VCM? I’ll have to look into what diagnostics I can actually get from the XJ40 such as OBD etc.

Overall the brakes on the car and spongy and feel horrible but they do pull the car up. It tends to pull slightly right on hard braking as well. I have been chasing/confirming part numbers so I can just start with fresh rotors and pads, rebuild the calipers, bleed out the air with fresh fluid and see where it gets me.

There is also a brake line that has been cut and crimped that looks like it used to come from the front of the car (I’m assuming part of the SLS system) and disappears under the drivers side front mudguard. I’ll have to get the car up on stands and see where this line runs to. Maybe air is getting into the system through this crimped line and hence spongy brakes?

I just need to find a day where I can get under the car and have a look.

Hi Matt …

The brake pad warning is completely independent of the ABS system. It’s simply one continuous circuit that runs through all of the brake pads via the sensor wires.

There is no OBD type system on your car that you can access. I believe that only the dealers had the required specialized equipment to check certain systems and they have all gotten rid of it years (decades) ago.

What you see on the VCM is what you get no further information available. The only exception is the check engine warning, which has 24 faults (Haynes manual chapter 6 page 3)

If your brake pedal feel soft and slowly sinks it’s either air in the brake lines or an internal leak inside the master cylinder. If you’re not losing brake fluid I’d start by bleeding all 4 calipers starting with the furtherest away from the master cylinder.

I think your guess on the crimped line is correct. Just some left over plumbing from the hydraulic brake booster system that was removed.

Thanks for your replies Grooveman. I’m hoping to get some time this weekend to spend having a good look at the brakes. I’ll check the level and state of the fluid but will leave the bleeding until I have replaced rotors and pads and serviced calipers etc. So the ABS fault will mot likely just mean I have no ABS but this wouldn’t otherwise affect braking performance?

Thanks for attaching the service manual… some good bedtime reading no doubt.


Matt …

The “Anti Lock Failure” warning on the VCM can be caused by a number of things.

  • Problems with the ABS over voltage relay is the most common
  • ABS controller - not that common but that happened to me
  • Dirty ABS wheel sensors

Go to the spyglass (on the top right next to your icon) and type in “ABS light” plenty of info.

Yes your correct, the loss of the ABS system has no affect on normal braking.