1995 XJS 4.0 Coupe

I'm having a problem where my ABS Pump Fuse (30A) blows when the key is turned to the ON position.
Brakes are very hard but stops normally (though with a lead foot push on the brake)

Fuse does not blow when Pressure switch wire is disconnected.
I replaced the switch with a used one... Same problem.

I have not replaced the accumulator ball but I have a spare.

Has anyone experienced this symptom before?

 

It sounds like you may have a 'Short' in one of the Wires to the pressure Switch, so well worth checking it out.

 

That's what I thought But where do I start. I don't have the ABS wiring schematic.
I have the 1995 Revisions Update Manual but there's no section for ABS. All it says about that fuse location is "30 A fuse - WIRE~ Lt. green

 

Hi Jimmy

It's much more likely that the switch could be faulty, have you tested the Switch with a meter, when it is out of the Car.

 

a 30A fuse blow is highly suggestive of a pump motor failure. There's not a lot more in the circuit that can sustain this current load. The two primary possibilities would seem to be solenoid failure (shorting of the coils) in the valve body ($$$) or pump motor failure.


When I had that fuse blow, the motor had failed.


-M

 

i have the same problem as you do. I just posted the same question before seeing this. I will delete mine. I replaced my pressure switch and it didn't make any difference, blows the fuse immediately when the switch is turned to on. Mine started every once in a while the fuse would blow, then progressed to blowing immediately every time. I hope we can find a easy solution.

 

The usual things that fail in the ABS (judging by the posts in two Jag forums, anyway, combined with my own experience) are these:


Accumulator: needs replacing if car is >10 years old regardless of mileage.


Switch: if the accumulator goes bad and the car is driven like that for a long time, the pressure switch often fails next. This is usually due to an interior rupture, possibly rupturing the plastic housing as well, and leaking fluid. Aks me how I know. This is usually presaged by many intermittent ABS lights on the dash. In my case, I never blew a fuse.


Motor: Here, I blew a fuse as the first indication that the motor had failed. Replaced the fuse, and promptly blew it again. Replaced the motor, and viola, ABS back in operation.


There are other issues owners have had, too: leaky seals in the master cylinder; stuck solenoids in the master cylinder. Failure of the pressure valve that lies between the master cylinder and the rear caliper circuit. Valve is in the engine compartment.


Many problems with this system can be cured by changing the accumulator, and flushing the system COMPLETELY and putting in fresh fluid. THEN, follow the manual, and RELIGIOUSLY flush that system regularly. ABS systems do not do well with old, dark fluid. Causes all kinds of issues, most of them really bad.


-M

 

I just posted to jblevins2 about a eBay source for the ABS accumulator ball.
Just search for Delco ABS accumulator 25528382 on eBay.

These are newly manufactured units. Not ones made 10 - 15 years ago and
Sitting in a box on some warehouse shelf.
Still somewhat pricey at $199.00 + shipping from California. But, better than the
$300 - 500 prices for "new" units made several years ago.

Regards and POR - Press On Regardless.

 

Yes. Both switches are fine.

 

Thanks but I already switched out the ball. Problem persists.

 

Can't bleed the brakes unless the ABS pressure switch is working. It stays closed and doesn't allow bleeding unless powered up. So it's a catch 22. Can't bleed brakes to test switch, can't test switch without bleeding the brakes.

 

Let's work on this together. So far I've been told it's most likely a short coming out of the ABS relay that runs to the switch. Black is ground on the ABS pressure switch. There's also a white wire, purple wire, black w/ green stripe and green wire on mine. I'm not exceptionally good with electrical but if I test for continuity from the black (ground) to each coloured wire I should be able to find the short.

 

This is what I have so far. The wires to the ABS pressure switch are as follows. (Ignore previous wire colours)
Black- Ground
Black/Green Stripe -unknown
Purple Green Stripe -unknown
White/Purple Stripe -unknown
White/Blue Stripe -unknown

I connected my Continuity Meter to the ground post on the firewall above the ABS booster pump and the red line from the meter to each of the colour coded wires. The results are.
Black/Ground- BEEP (continuity)
Black/Green Stripe - first random numbers then it settles down to 1.
Purple Green Stripe -1291
White/Purple Stripe -1
White/Blue Stripe -1297

I'm assume that the 1291 numbers are volts ie. 12.91 volts.
I don't know what the 1 is.

From the Yellow ABS relay socket under the Passenger knee bolster there are 4 wires. Fat orange, fat brown, white with orange stripe and white with purple stripe (I'm assuming that this is the same White/Purple stripe on the ABS pressure switch)
I'm believe the fat wires are power and ground. With the meter connected to the same ground post I tested the two coloured wires. The results are.
White/Orange Stripe- Ground beep (which doesn't seem right)
White/purple stripe- 1

When I connect white purple stripe relay wire to white purple stripe at ABS pressure switch I get a reading of 716 (7.16v). shouldn't I get a continuity beep when connecting these two wires?
Can anyone explain what any of this means?

 

Disconnecting this wire will deactivate the pump relay.

Your problem is in the Pump area.

Trace the wires from the Pump, they run under the pump mounting bracket. Check the wiring harness is not shorting on the bracket, there is a cover on the bracket to prevent the wiring being chafed and rubbing through the insulation.

If the wiring is good my next suspect would be the capacitor across the pump, I have no idea where this is, I have looked around the pump and could not see it, It could be near the pump relay and fuses. I will have a look later, my car is high in the air ATM with my Trans swap.

Also check diode 2. If your DVM has a diode test function, it will measure 1.6-3volts one way and 0 volts the other way.

The numbers you got in continuity are Ohms 1291ohms ........ FORGET THIS ATM and do the above.

 

Thanks so much. I bench tested the pump. It's good. No chafing and runs on the car with power bypassed. The problem is definitely the ABS pressure switch wiring. I located the diodes. There's two in line off the yellow ABS relay. I'll test them on the weekend. Getting a ground out from the white pink wire off the relay is concerning but I'll test the diodes first. If one is shot it will allow voltage to leak backwards in the circuit causing the fuse to trip. I'm hoping it's not a pinched wire as the ABS wiring runs all over the place.

 

Deleted so as not to confuse.

Read next post

 

Thought I’d better explain why I said not to measure continuity. You are measuring continuity (resistance R) at WK pin 86 on the pump relay, this is the ignition feed so has 12v when the ignition is on.

What can confuse people is when the power is off and you measure R at pin 86 you are measuring the R to ground on the green path to ground via the pressure switch. What you are measuring is the DC resistance of the relay coil only a couple of ohms and a continuity tester will tell you it is a short circuit, this is OK and perfectly normal.

The pressure switch is NC (normally closed) on this line so when the ignition is turned on the pump will run until the pressure is built and this switch opens turning the pump off.

The 30A fuse is on the blue line. The ONLY things that can blow this fuse are:

- Bad pump motor – you have tested this and its ok.
- Diode 2 shorted
- Capacitor shorted (circled in red). The capacitor is most likely part of the pump so it may not be this if the pump is working.
- Shorted wiring between the relay and pump.

There is nothing else in the circuit that can cause this fuse to blow.

Hope this helps and clears up why not to test continuity unless you know what you are measuring.

Disconnect the pump at the connector on the bracket, does the pump fuse still blow?

If yes then its shorted wiring or diode.

 

That was extremely helpful. Now I know that the WK wire is normal. I was worried it was shorted. Plus your schematics are much better than mine. I have the Jag 95 XJS electrical guide. It shows the entire system on one sheet. Very confusing. I'll test the diodes. Any idea what spec the diodes are?

 

Diodes. Depending on your DVM

If your meter has a diode test function (it will have a diode symbol) the diode should read 0.5-1 volts in one direction and infinite the other.

If your meter does not have a diode test function use ohms and it give a reading (this will depend on the diode type) one way and be infinite ohms the other.

The first 2 pics are my meter in the diode test function.

The 2nd 2 pics are my meter on the ohms range.

You should see reading something like these, they may not be exactly the same as each diode will read slightly different.

 

Teves mk II abs manual may be helpful