Hi everyone, I have a 1988 xj6. I have a abs brake failure light on. I have checked the accumulator, abs relay, and the speed sensors all tests out good. I dont know how to test the brake pressure switch or the abs module.. can anyone help me diagnose these? please and thank you

 

What about the 'overvolt relay' in the boot????

bob gauff

 

thanks for the thought bob, the overvolt relay is a protection for the abs, it only lets the abs work when sufficient voltage is present 13-15 volts. i solved my own problem on this by simply cleaning the speed sensors. the rear sensors are left wide open and were covered in grime. I cleaned the two rears with a pick and brake cleaner and didnt even have to remove them. the front two were easily removed and cleaned. just a side note on that abs overvoltage relay. your abs failure light will come on when you first start your car and remain on until your alternator starts its charge. thanks again and sorry i solved it on my own

 

Thanks for posting the problem and the resolution. Every little bit of knowledge helps!

 

The overvolt relay is often at fault because the circuit board solder joints crack and the ABS light comes on.

Usually you can just resolder the board and reinstall the relay.

The relay protects the ABS module by means of a 'CROWBAR' circuit. It will 'blow' the low amp fuse if the Zener diode detects an OVERVOLT situation.

I have several old relays in a drawer in my toolbox that have cracked solder joints.

Every so often I pull one out to repair an old 88/89 Xj40.

bob gauff

 

Good to hear such an important failure was solved and thanks for your updated message .I remember my '88 used to trigger ABS light too . Actually what was it not throwing

 

hi, everyone I also have a problem witf my 1986 XJ40 the display tells me that i have a antilock failure,
I already cleaned the wheel sensors and checked the overvolt relay, that is ok.
Is there anything else i can check to solve this problem.
Who can give me a good advise

new memeber Gerrit from the Netherlands

 

Hi don't mean to hijack the thread but I was searching anything on over volt relay and came across it mentioned here.

I have noticed that the one in my boot/trunk is missing.
When I replaced it I found my instrument panel stayed lit up even when the ignition was off and keys removed...

Also found that once the car was started it would not turn off even with the ignition turned off and key removed.

I still have no idea what the over volt relay is for or whether it is necessary.
But if it is causing those side effects.... I assume it was removed for that reason...

I can not swear by it, but I believe the delay on the interior light also stopped working around that time. As it worked fine prior to then, whether the relay has anything to do with the light delay is beyond me other than maybe overloading the circuit.

Can anyone shed any light on this for me please.

 

Red
I had an ABS fault and Removed the ABS Relay. A tall white relay, rather expensive
to buy a new one.
It cannot be replaced with an ordinary relay. The diode in mine was bad/melted ( PN#MR2150). While I was in there I soldered all the joints. It did make my ABS fault go away.

 

Is there a reason why a normal relay can't be used to build this type?

 

KC7
I guess you mean the Black ,Green, Blue or Yellow Relays?
I have put a IN4007 (hysteresis clamp) diode in the Black ones.. To allow substitution for the Blue one in the Blower. This could be done because the diode is the only difference between those two.
the O-voltage Relay is a whole new animal , in function and form.
Google MR2150 ..this is one component in the relay that the spec sheets, along with viewing the schematic for the ABS modulator.will help you understand what is going on in the realy.
To the best of knowledge you cannot use another relay in place of this one.
Will send a pic of Overvoltage relay all opened up to my album if wanted .

 

please do.. I will look up that part number to see what is going on in the White relay.

Being the DIY builder and electronics nut I am.

 

Looking at my relay, the fault is that. The inside has water damage and very corroded.

Now to find a circuit diagram of this white box.. There is no fancy parts in here. I would use the board on mine.. but all the traces are corroded off...

Will post a work around of this when I finish building it.

the other thing is. when I google this part I do not find any responses for hella 4ra 004905-01 (at least that is what was in my car)

 

KC7
I incorrectly gave you the wrong part mumber for the diode...MR2520.. the datasheet from Motorola is extremely Informative. It will even explain how it is used in Automotive systems The pn# for the relay is ..DBC4282 ..."Overvoltage Transient Suppressor "and the cheapest price right now is Brokenkitty.com 40.00
I posted a quick drawing of it from 13 years ago. Will get a pic as soon as possible posted to album.

MR2520L datasheet pdf datenblatt - Motorola, Inc - OVERCOLTAGE TRANSIENT SUPPRESSORS 2.5K - 10K WATTS ::: ALLDATASHEET :::

 

I might be able to pull them from the scrap micors and MSR radios.

 

just viewed the relay online and they are no less than $100....
expensive little relay.
I think I will leave it out for a while to come yet.
Unless I come across one at a wreckers or something

 

This is why I am looking into a revised relay (or circuit) to replace this part.. 100 bucks for what was 5 10 dollars in parts is crazy...

I do understand the Motorola part has a lot to do with this I suppose.

 

KC7
I sent you a 40.00 quote from ...... Broken kitty.com
That one diode cost me $11
Also if you look at the pictures I posted in album , you will see where I built up the traces with solder on the bottom of the board.
13 years ago and still no problems.

 

Cool... I have to rebuild it from zero. I have no ideal, how it got so bad? It looked like it had sat on the bottom of the sea for 20 years...

Makes me start to worry about other issues that are going to creep up.

cool.. found the drawings.. I can build this from parts in the junk boxes.. I have a few of the diode from Motorola power supply's. Seems they used them in the older radio systems from the 70's..