hi guys, im currently working on my 1990 sierra sapphire 4x4 cosworth, the abs light comes on with the ignition and stays on all the time, i have tested the following...

• all wheels sensors and pick up rings
• all the relays and fuses
• live and earth to the ecu
• all the hydraulic modlator solenoid valves, in cluding bridging the wiring to check the operation
• the pump motor
• the pressure switch
• the brake pedal position switch
• the brake fluid level warning lamp/switch

the only thing i havent done is connected a pressure guage to the accumulator to check the pressure is all correct but as the pump motor stopped pumping as the fluid got down to the max mark i assumed its all ok...!

im clean out of ideas guys, any help would be very greatful... cheers

 

when you say "I have tested the following......", how have you tested the fluid level/reservoir cap ? (the cap is a known cause of issues)

 

i basically made sure the reservoir was on the max mark and checked the light was off then removed the cap and checked the fluid warning light came on.. unless there is another way..??

 

no that's fair enough.

Next test :

Unplug the ABS ECU connector and remove the cover from the connector (undo screw and slide off)

With ABS system pressurized (ignition on until ABS pump stops running) check for continuity between wiring loom terminals 9 and 10 (ECU still unplugged).

 

ok with the ecu unplugged with the ignition on and abs system pressurized there is continuity between pins 9 and 10..! is this good or bad..?

 

It's as it's supposed to be.

Does the orange ABS light still come on if the ABS ECU is unplugged (switch ignition on after unplugging) ?

When you tested the sensors, did you check continuity, resistance or voltage ?

 

ok, with the abs ecu unplugged the light is still on.

i have test the resistance, voltage and continuity of all 4 sensors

 

unplug the green ABS relay behind the glove box (5 wires to it are 2xblue/green, 2xbrown and a red)

with the ABS ECU plugged in and ignition on, have you got 12V at both terminals 86 and 87 of the relay ?

 

ok at pin 87 i have got the battery voltage but at pin 86 i have only got 9.7v..!

 

that should be enough to energise the relay - try plugging the relay back in and turning the ignition on and off a few times whilst holding the relay - you should be able to feel it click, just to make sure the relay is working OK

next test - check for continuity between ABS ECU wiring loom terminal 18 (blue/white wire, ECU unplugged) and the main solenoid valve on the brake assembly under the bonnet (2 pin connector), and whilst you've got the connector off, check that the main solenoid earth does indeed have continuity to earth (brown wire)

 

yeah the relay is clicking everytime the ignition is turned on.

and there is continuity between ecu pin 18 and the main solenoid valve and also between the main solenoid valve connector and earth.

 

OK next, onto the wiring to the solenoid valve block on the passrnger side. Have you checked the continuity between the 7 way connector and the ABS ECU wiring loom connector terminals ?

Has the system ever worked correctly since you've had the car ?

What were the results of the tests of the 4 wheel sensors ?

 

ok i have continuity from the ecu to the solenoid valve block on all 7 pins.

the car is a friends of mine and yes the abs was working until light came on.

N/S/F - 1130 ohms - 0.149v (ac) (mate spinning the wheel)
O/S/F - 1108 ohms - 0.154v (ac) (mate spinning the wheel)
O/S/R - 1095 ohms - 0.155v (ac) (mate spinning the wheel)
N/S/R - 1095 ohms - 0.160v (ac) (mate spinning the wheel)

 

Starting to clutch at straws here now.

The test you did earlier was to test the brake fluid reservoir level according to the 3 pin connector.

I haven't taken a brake fluid reservoir cap apart to see exactly how many sets of switches there are inside, so let's see if the 2 pin connector works on different contacts within the cap.

Remove the 2 pin connector and join the wiring loom terminals together with a short piece of wire. Fingers crossed the light goes out.

ETA : Thinking about it, it should be OK as you've got continuity between ECU terminals 9 & 10

 

has any work been done on the car recently, either under the dash or bonnet ?

 

like you said as pins 9 and 10 have continuity, but i just bridged the 2 pin connector and the light is still on.

to my knowledge there has been no work done on this car since its service last year.

 

When the sensors were measured, were the measurements taken at the in-line connection or at the ECU connector ?

Testing main solenoid valve (wiring checks out OK, but you haven't tested the actual valve yet) : test resistance between ECU wiring loom terminals 11 & 18, with ECU unplugged. Should be 2 - 5 ohms.

 

i tested the sensors at the ecu and when i tested the main solenoid valve i tested the resistance across the valve and it was 3.8 ohms if i remember.

all the tests i have done so far are from my autodata cd.