ABS Light on for 20 years, nobody knows what it is.

[ndcade0908]

So I recently bought a 1997 Mustang GT, auto, ABS equipped car. The previous owner told me the light has been on for 20 years, he's taken it to several dealers and other mechanics, and nobody knows whats wrong. Car drives fine, so I'm curious as to what this could be. I've done a bit of research and couldn't find any conclusive answers, so I'm making my own post to see what the Owners of these cars think could be the issue. Thanks for any advice or help.

 

[RAU03MACH]

i had one go hay wire on me once
a friend of mine that knows them well told me if no code shows up
and everything else checks out ok
the unit itself is the problem

 

[ttocs]

I thought that meant that the brake fluid was low? Or it could be a bad/disconnected sensor.

 

[RAU03MACH]

If all checks out what else would it be
I remember changing everything on one car
And the last piece was the unit itself
Got lucky at a junk yard
This stuff is obsolete

 

[cobrajeff96]

Have you tried simply inspecting the wheel speed sensors?

 

[ndcade0908]

i had one go hay wire on me once
a friend of mine that knows them well told me if no code shows up
and everything else checks out ok
the unit itself is the problem

Awesome! I'll keep that in mind when I grab my buddies scanner in a few days. Thanks for the response!!

 

[ndcade0908]

Have you tried simply inspecting the wheel speed sensors?

I have, nothing appeared wrong with them, but I spose I could be wrong. From what I've read though, if it were a wheel speed sensor the brake would be pulsing, which this car does not do.

 

[ttocs]

of course you should start simple and see if any codes come up. you don't need a scan tool for ABS codes (these notes taken from some other site at some point in the past).

Under the hood, on the drivers side, there is an ABS test port. It is mounted directly in front of the big black power distribution box. It is red and virtually impossible to not see. It says anti lock test right on it.

Push the little black tab down and remove the red cover thing.

You will see a black connector thing that looks like a pyramid with three wires. With the IGNITION OFF, run a jumper wire (short piece of copper wire is fine or a paper clip works well) from 57 (black ground wire) to 836 (orange/white diagnostic circuit wire).

Leaving the jumper in place, get into the car with a piece of paper and a pen handy. Turn the key ON BUT DO NOT START.

Your ABS light will now flash the codes for you. They are two digit codes, so it will flash a few times, pause then flash again. So, for example, six flashes, a pause, then one flash = code 61. If there is more than one code, it will pause again (long pause) and flash the next code.

 

[Mustang5L5]

From what I've read though, if it were a wheel speed sensor the brake would be pulsing, which this car does not do.

No, if it was a wheel speed sensor, the ABS system would record the error, trip the light, and turn the system off. And then you'd essentially be driving a non-ABS system. If you disconnect the battery to reset the system, a hard-fault with a wheel speed sensor will set the light immediately, but an issue with how the pulse curve looks may only trip the light when you start driving.

Short of reading the codes, the (second) best way to test wheel speed sensors is to unplug them, and test the sensor side with a multimeter and measure resistance. I forget what the value should be, but all 4 should be somewhat similar. An open wheel speed sensor would set the light immediately and would be easy to find with a multi-meter. The best way to test is an oscilloscope where you can hook up to the wiring and turn the wheel and see what the pulse looks like. However, not many folks have a Oscilloscope kicking around.


I believe 1998+ went over to the module based DTC setup, so the jumper method that was previously used may not work for you. You really need a good scan tool that can read module DTC's or use ForScan. A regular OBD2 reader won't read module codes.

Mustang 1998+ ABS Diag Info

The above link shows all the module DTC's that could potentially be stored. If oyu don't have any friends with a scan tool, you can download ForScan (www.forscan.org) and install the software on your computer. All you would need is some sort of PC to OBD2 interface. That will allow you to pull module DTC's.

Without pulling that code first, everything else is a guess. And as you can see by looking through those codes, a lot of them say to replace the ABS pump/Module.

 

[ttocs]

If you have a meter and know how to use it to test the sensors the rear sensors are fairly easy to reach under the rear seat. There should be two/two wire harnesses that go down under it. The front will require a little more work as you would have to either find the wires at the abs module in the console or remove the wheel and probably the fender liner to get access to the harness. Start simple

this might help a little

Mustang FAQ - Wiring, Engine, Air Bag, ABS Tech Home

Another thought, any chance the front wheels are maybe an inch smaller than the rears if they are aftermarket by chance? Some people like that look(I don't hate it myself most of the time) and it would confuse the abs module if this was the case as it thinks the rear wheels are turning at a different rate from the front. Not sure how big of a tire would be needed to throw it off but it is possible that a larger/smaller sidewall from front to rear could do it....

 

[ndcade0908]

No, if it was a wheel speed sensor, the ABS system would record the error, trip the light, and turn the system off. And then you'd essentially be driving a non-ABS system. If you disconnect the battery to reset the system, a hard-fault with a wheel speed sensor will set the light immediately, but an issue with how the pulse curve looks may only trip the light when you start driving.

Short of reading the codes, the (second) best way to test wheel speed sensors is to unplug them, and test the sensor side with a multimeter and measure resistance. I forget what the value should be, but all 4 should be somewhat similar. An open wheel speed sensor would set the light immediately and would be easy to find with a multi-meter. The best way to test is an oscilloscope where you can hook up to the wiring and turn the wheel and see what the pulse looks like. However, not many folks have a Oscilloscope kicking around.


I believe 1998+ went over to the module based DTC setup, so the jumper method that was previously used may not work for you. You really need a good scan tool that can read module DTC's or use ForScan. A regular OBD2 reader won't read module codes.

Mustang 1998+ ABS Diag Info

The above link shows all the module DTC's that could potentially be stored. If oyu don't have any friends with a scan tool, you can download ForScan (www.forscan.org) and install the software on your computer. All you would need is some sort of PC to OBD2 interface. That will allow you to pull module DTC's.

Without pulling that code first, everything else is a guess. And as you can see by looking through those codes, a lot of them say to replace the ABS pump/Module.

So out of curiousity, if I were to buy a 98 ABS module, would that not work in my 97? Or would I have to buy a 96 or 97 module? I'm just curious because I've become fairly confident that it's the module itself

 

[ndcade0908]

If you have a meter and know how to use it to test the sensors the rear sensors are fairly easy to reach under the rear seat. There should be two/two wire harnesses that go down under it. The front will require a little more work as you would have to either find the wires at the abs module in the console or remove the wheel and probably the fender liner to get access to the harness. Start simple

this might help a little

Mustang FAQ - Wiring, Engine, Air Bag, ABS Tech Home

Another thought, any chance the front wheels are maybe an inch smaller than the rears if they are aftermarket by chance? Some people like that look(I don't hate it myself most of the time) and it would confuse the abs module if this was the case as it thinks the rear wheels are turning at a different rate from the front. Not sure how big of a tire would be needed to throw it off but it is possible that a larger/smaller sidewall from front to rear could do it....

they are actually the stock 17 inch sport wheels.

 

[ttocs]

how do you know it is the module?

 

[ndcade0908]

how do you know it is the module?

all of the sensors themselves look fine compared to other ABS cars I've seen, and the wiring all looks fine. Like I said I'm 100% not sure, but I'm fairly confident just based on things I've read in this mustang forum and other mustang forums (all specific to 96-04 mustangs, talking about 96 or 97 cars). Based on my research and inspection of the car, there's no code from forscan, there's no error while driving, just the light, which leads me to believing that it's the module itself.

 

[Mustang5L5]

So out of curiousity, if I were to buy a 98 ABS module, would that not work in my 97? Or would I have to buy a 96 or 97 module? I'm just curious because I've become fairly confident that it's the module itself

I’d need to dig into it a bit, but I think 1998 was a transition year. 94-97 were 3-channel units and at some point in 1998 Ford went to a 4-channel system. 1999+ has traction control so not sure if those are compatible.

Can you see your ABS pump at the front pass corner? Do you count 4 outlet ports or only three??

 

[07GtS197]

Usually, when the abs units fail they throw a code. Something along the lines of communication failure. It is possible to fail without throwing a code I guess but I feel like if that the case another module would throw a code due to not being able to communicate with the abs module. I’m leaning away from the abs module but this is just me spitballing. Check for codes first.

 

[ndcade0908]

I’d need to dig into it a bit, but I think 1998 was a transition year. 94-97 were 4-channel units and at some point in 1998 Ford went to a 4-channel system. 1999+ has traction control so not sure if those are compatible.

Can you see your ABS pump at the front pass corner? Do you count 4 outlet ports or only three??

yep I see 3 ports, and the 98 pump has 4

 

[ndcade0908]

Usually, when the abs units fail they throw a code. Something along the lines of communication failure. It is possible to fail without throwing a code I guess but I feel like if that the case another module would throw a code due to not being able to communicate with the abs module. I’m leaning away from the abs module but this is just me spitballing. Check for codes first.

I will restate this but forscan showed me no fault codes, someone else in the thread said a no code ABS light is typically a bad ABS Module, I would guess because the ABS module has no way of communicating. As for a new module, I've done a bit of research and a new (used) module will have no issue communicating at all.

 

[07GtS197]

I will restate this but forscan showed me no fault codes, someone else in the thread said a no code ABS light is typically a bad ABS Module, I would guess because the ABS module has no way of communicating. As for a new module, I've done a bit of research and a new (used) module will have no issue communicating at all.

Perhaps, I’m basing my assumption on personal experience but I’m sure others are as well. I’m not saying anyone is wrong I’m just playing devils advocate before throwing parts at the issue. Hopefully a new abs module fixes the issue though.

 

[Mustang5L5]

yep I see 3 ports, and the 98 pump has 4

3 ports? One line out to each front brake and a single line to the rear? What's your build date on the door tag? Is this an early 1998?

The 3-channel ABS should have 5 brake lines connected to it. A 4-channel ABS will have 6.

Do you see an ABS test port under the hood like ttcos mentioned in post #8. Would have a red cover on it.




Essentially what I am getting at is that depending on which ABS system you have, there are different methods as to how to pull the codes. The older 3-channel style can use the paperclip or a specialized OBD2 code reader that can do Ford 90's ABS codes. The later 4-channel style was when ford started doing module based DTC's in which you can use forscan to read the module DTC's


It's touch to find info on the 1998 system and it appears to be a 1-year only system, with 94-97 3-channel units falling into one compatibility family, and the 99-04 units falling into another. QUite a few units on ebay

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