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Discussion Starter #1
The title says it all. My ABS light is on. The pump is working and the pressure switch contacts are switching as they should. The fluid level switch is working properly. All the sensors are reading the correct ohms and I have also swapped them all out with good ones. All the sensor connectors read about 3.5 megohms back to the computer - so no broken circuits. I think that the problem must be either in the computer or in the wiring to the pressure and level switches. I have a spare computer that I think is good. So swapping that is the next step. Any other suggestions will be welcomed.
Thanks
 

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The ABS computer is located...

In the boot, mounted to a bracket, just forward of the electric antenna. You have to unfasten/pry clear the boot carpet to gain access.
 

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All the sensor connectors read about 3.5 megohms back to the computer - so no broken circuits.
Two things make no sense:
1. How can you have measured the impedance of the sensors at the computer if you don't know where it is?

2. If they all 4 read 3.5 MOhms, then they are not working. I think they (wheel speed sensors) should measure around 1000 Ohm. Yours measure 3500 times more...

Have you checked distance between sensors and emitter wheel? Maybe a bad/failing wheel bearing is at fault and cannot maintain consistent distance.

Have you swapped in a different pressure switch? I once had similar symptoms, and it was a bad pressure switch - basic functionality appeared ok, but some of the finer nuances of the switch were not working right.

Jes
 

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Discussion Starter #4 (Edited)
I disconnected the sensors at the connectors in the engine bay and under the back seat. I measured the resistance to the sensor plus lead at each connector and they were all about 1050 ohms. I then measured the resistance of the inputs to the computer + the leads and they were about 3.5 megohms. I did that because I suspected that I might have a damaged cable between one of the connectors and the computer.

I have the pin out details for the pressure switch. I confirmed that both switch contacts change state when the pressure goes from low to normal.

I have had sensor-emitter wheel spacing problems in the past but the symptoms were different. The ABS light would go out until the car moved, then it would go back on again. I have the manual for the ABS system. It says that on startup it checks for hydraulic pressure, level in the brake fluid reservoir and the sensor circuits. All of therse check out OK so why is my light on? Bad computer or maybe one of the sensor signals is not making it to the computer. Or maybe something else that is not in the manual or which I don't understand. I am an electrical engineer.
 

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At some point I was also measuring wheel speed sensor impedance all the way at the computer. I'm pretty sure mine measured 1000 Ohm there as well. So, if you are seeing 3.5 MOhm, something sounds fishy. My guess would be that you are measuring the wrong pins in that huge plug that goes into the ABS computer. I would still try change the pressure switch if you have a spare - those were the symptoms I had.

Jes
 

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Discussion Starter #6
Thanks Jes. I was not measuring impedances at the computer because I don't know where it is. I did all the measurements at the sensor connectors. I have a spare pressure switch but I have convinced myself that it is not the problem because I ohmed out the switch connections and I saw them change state between low pressure (pump running) and normal pressure (pump stopped).

Is the ABS computer underneath the radio or is that the cruise control computer?
 

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It is in the trunk nearby the power antenna. Pull the trunk liner from the left side in the trunk and you will see it mounted right by the power antenna. It has a plug of similar size as the plug for the L-Jet ECU.
Jes
 

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Discussion Starter #8
Thanks for the help.
 

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I like to see such a systematic approach to ABS troubleshooting instead of just converting to manual brakes as the default option.

We have several known-good ABS modules if it is determined to be faulty.
 

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Discussion Starter #10
Where are the fuses?

I think that my problem may be a blown fuse - one of the G126 "ABS system relays safety fuses" but I cannot find them. The manual shows them near the driver's left leg but I don't see them. Are they hidden behind a panel?

Thanks,
 

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Discussion Starter #11
I found it. That was the problem. Thanks for all your suggestions.
 

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Good to hear you got it sorted, and that it was not a major operation :) Imagine that someone of a less investigative mind could actually have taken the pass of converting to manual brakes because of a blown fuse...
Jes
 

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Ed,

How did you troubleshoot it down to a blown fuse? Process of elimination?

Great to hear you got it figured out though!!! Someday I will tackle my constantly on ABS light.... I need to take your systematic approach more often. I bought a new accumulator and a spare ABS pump to find out it was the darn strip fuse in the engine compartment that was giving me fits the last time I was messing with the Brake System. Brakes would go full hard...no boost.. under hard braking.. Turned out there was a hairline fracture in the strip fuse in the engine compartment. Under hard braking the fracture would open up but would be OK the rest of the time.
 

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Discussion Starter #14 (Edited)
I checked the resistance of the electrovalves at the master cylinder and they were ok. I then swapped computers - no change. I then ohmed out all the sensor and valve circuits at the computer connector and they were all good. I saw from the diagram that I should have 12 volts at pins 20 & 3 but there was nothing there. It could not be the big fuse because the pump works. Therefore it was either the G126 fuse or a relay. Fortunately I did not have to find the relay.

The irony is that I am selling both my Verdes just as I am gaining some expertise with the ABS.
 

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I think that my problem may be a blown fuse - one of the G126 "ABS system relays safety fuses" but I cannot find them. The manual shows them near the driver's left leg but I don't see them. Are they hidden behind a panel?
I found it. That was the problem. Thanks for all your suggestions.
HA! That's funny Ed. That is, funny after I tried to find them myself.
After much contortion, stretching, straining, and pushing stiff wires around, I think I found one of the two G126 relays/fuses and am able to touch it, but not unlock the connectors to allow the unit to be moved into an open area to allow removal of the fuse.

Other than just sucking it up and doing it, are there any hints? Did you remove the dash? Maybe remove the fuse door and drop the fuse box down might help?

The third 'free' relay (7.5A) appears easily behind the coin cubby but its removal does not appear to help with the other two fuses.

Yeah, it's been a long while for you, but with pain (ful access), there are usually strong memories.

Symptoms:
ABS light on suddenly and constantly. There is no power at the pump (unlike your case). The main (strip) fuse is good. The 'brake' light is on for the ARC though and I wonder if this indicated failure would inhibit pump power through the relay logic. Any thoughts here . . . anyone?
 

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Discussion Starter #16
I sold the Milano a few years ago and my memory is as unreliable as the car's electrical system. I think that the fuse was loose in the wiring under the left side of the dash.

Good luck with it.
 

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I don't know where the fuse you replaced is located. It's not in the fuse box from what you are saying. Is it in the jumble of wires behind the fuse box?
 

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I don't know where the fuse you replaced is located. It's not in the fuse box from what you are saying. Is it in the jumble of wires behind the fuse box?
On our 87 Platinum there are a few fuses that are loose under and behind the fusebox. And one plastic relay with a fuse on top.
 
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