Jumping Speedo & ABS Light

[14.6@97MPH]

Next problem with my car ....

I've got a 98 LSC. After the car is warm (15-20 minutes) my speedo starts jumping around. If I'm going 60, it bounces down to maybe 40 and jumps back and forth between the speed I'm going and 15-20 mph slower. Eventually, after this happens a couple of times, my ABS light comes on. If I'm driving slow while this is happening and the ABS light isn't on yet, when I touch the brake the abs will come on (even though I'm not sliding). I can hear it and feel it in the brake peddle. The tach runs normal and the odometer puts miles on based on the speedo (not the speed I'm actually going).

I replaced my ABS module a couple of months ago with a junked one (new are not available), and that seemed to work fine for a couple of thousand miles. When that broke I did not have the jumping speedo.

I've done a lot of searching on this problem and I can't find a good answer. Where exactly does the speedo on a 98 LSC get it's reading from (I found a lot of posts saying 'i think', 'it could be', 'maybe', etc.???) I have not had the ABS codes pulled yet. If I do have them pulled will it give me the right answer?

Thanks,
Ken

 

[LaserSVT]

There are more posts stating that the 97-98 Marks do in fact get their signal from the ABS wheel speed sensors. Check both front sensors. See if the rings are dirty or if there is any dammage to the sensors or sensors plugs.

 

[DieselDan]

There are more posts stating that the 97-98 Marks do in fact get their signal from the ABS wheel speed sensors. Check both front sensors. See if the rings are dirty or if there is any damage to the sensors or sensors plugs.

Agreed.

The second gens do get their speedo signal from the front wheels, that is how when second gen owners do a gear swap, their speedo still reads correctly. My bet is a bad front sensor. Autozone or someone should be able to read the codes for you for free, my bet is on a front wheel speed sensor.

Good Luck!

 

[m_maker]

If I do have them pulled will it give me the right answer?

Thanks,
Ken

Yes, read, and more importantly monitor the sensors with proper scanner.

Check both front sensors. See if the rings are dirty or if there is any dammage to the sensors or sensors plugs.

While front sensors do send signal to the various systems, they have absolutely nothing to do with the speedometer on 2nd gen cars.

Agreed.

The second gens do get their speedo signal from the front wheels, that is how when second gen owners do a gear swap, their speedo still reads correctly. My bet is a bad front sensor. Autozone or someone should be able to read the codes for you for free, my bet is on a front wheel speed sensor.

Good Luck!

Again, the signal for the speedometer is being taken from the rear wheels on any 2nd gen car.

Moral of the story. Hookup a snapon or alike scanner, read the codes, and check the sensor readings from each wheel. You can check the rear abs rings and sensors for damage but a scanner will give you the answer right away.

 

[LaserSVT]

I dont know how true that is m maker.

First off notice I said it gets the signal from wheel sensors, not "front sensors". I said check the front sensors.

They are more prone to failure because of their dirty location and people can screw them up by breaking the lead when doing front end component work and stretching the cable.

I know they have something to do with readings for two reasons:
1. On my 97 my drivers front failed and at highway speeds the speedo would bounce about 5mph and occasionally I would get the ABS warning. Replacing that sensor corrected the speedo and ABS issue.
2. On my 98 I forgot to plug both sensors in and when Sapperfire and I went on the test drive the speedo worked when going 10mph but after about 30 MPH the speedo dropped to zero and I got the "check traction control" warning.

I believe they work in conjuncion. I think the speedo reading is based primarily on the rear sensors till 20 or 30 MPH then it reads all four. I can only base this on what I have seen.

 

[14.6@97MPH]

Yes, read, and more importantly monitor the sensors with proper scanner.



While front sensors do send signal to the various systems, they have absolutely nothing to do with the speedometer on 2nd gen cars.



Again, the signal for the speedometer is being taken from the rear wheels on any 2nd gen car.

So LaserSVT and DieselDan think it is the front sensors and M Maker thinks it is the rear sensor? This is exactly what I’ve been finding researching this………..there are many opinions as to where the speedo reads from in a 1998. And I know you guys know your Mark VIII stuff too!

If it is the front sensors, which one is it, the driver’s or passenger’s? It can't pull the speed from both, can it??

My Autozone doesn’t pull ABS codes, only CE codes My local garage can do it for some $$$. I just want to make sure I’ll get a good error code (not a bunch of them) if I pay him to pull the codes, or if there was an obvious fix to this based on the symptoms.

Ken

 

[Sapperfire]

uhhhh... the Vehicle Speed sensor is in the tail shaft of the trans

doing a gear swap shouldnt effect the speedo because -above-
gear swap would only effect the tac

 

[LaserSVT]

uhhhh... the Vehicle Speed sensor is in the tail shaft of the trans

doing a gear swap shouldnt effect the speedo because -above-
gear swap would only effect the tac

But the VSS has nothing to do with the speedo in second gens. That is for suspension and speed limiter.

 

[Sapperfire]

ok the one in the trans is used as the transmission output sensor, as like the 02-03 volvos, even thou its called a VSS, its primarily for the shift signal

ill have to pull up some schematics tomorrow to confirm all of this however

 

[mark82nv]

its like the cam position sensor it really has nothing to do with the cam either, why couldnt the comp take readings from all 4 and avg them? say after 20 mph or so ?

 

[m_maker]

I dont know how true that is m maker.

First off notice I said it gets the signal from wheel sensors, not "front sensors". I said check the front sensors.

They are more prone to failure because of their dirty location and people can screw them up by breaking the lead when doing front end component work and stretching the cable.

I know they have something to do with readings for two reasons:
1. On my 97 my drivers front failed and at highway speeds the speedo would bounce about 5mph and occasionally I would get the ABS warning. Replacing that sensor corrected the speedo and ABS issue.
2. On my 98 I forgot to plug both sensors in and when Sapperfire and I went on the test drive the speedo worked when going 10mph but after about 30 MPH the speedo dropped to zero and I got the "check traction control" warning.

I believe they work in conjuncion. I think the speedo reading is based primarily on the rear sensors till 20 or 30 MPH then it reads all four. I can only base this on what I have seen.

While the front sensors need to be clearly plugged in for the abs to pass the start up and self test (which happens within the speeds you indicated), they are not the ones responsible for the speed reading.
I am sure you had your mark viii on the dyno. Have you ever "driven" one on the dyno ? You can easily "zap" the speedo, while the front wheels are not even turning, let alone moving at the same speed.

there are many opinions as to where the speedo reads from in a 1998.

I didn't post an opinion, I posted a fact, but in the end it's your call

My Autozone doesn’t pull ABS codes, only CE codes My local garage can do it for some $$$. I just want to make sure I’ll get a good error code (not a bunch of them) if I pay him to pull the codes, or if there was an obvious fix to this based on the symptoms.

Ken

While reading codes is most likely going to pin point it, make sure they are actually going to take the car for a drive, while monitoring the sensors' outputs. This is why a proper abs scanner is required to easily diagnose abs/traction control issues in a 2nd gen car. You can guess a lot, but that's not the "economical" way of doing this, especially with the limited availability of 98 specific abs components.

But the VSS has nothing to do with the speedo in second gens. That is for suspension and speed limiter.

Yup, you got it.

 

[14.6@97MPH]

While the front sensors need to be clearly plugged in for the abs to pass the start up and self test (which happens within the speeds you indicated), they are not the ones responsible for the speed reading.
I am sure you had your mark viii on the dyno. Have you ever "driven" one on the dyno ? You can easily "zap" the speedo, while the front wheels are not even turning, let alone moving at the same speed.



I didn't post an opinion, I posted a fact, but in the end it's your call



While reading codes is most likely going to pin point it, make sure they are actually going to take the car for a drive, while monitoring the sensors' outputs. This is why a proper abs scanner is required to easily diagnose abs/traction control issues in a 2nd gen car. You can guess a lot, but that's not the "economical" way of doing this, especially with the limited availability of 98 specific abs components.



Yup, you got it.

M MAKER - Based on what you're saying the only problem this can be is a bad rear abs sensor?? It can't be the front sensors, and it can't be the VSS because those wouldn't make my speedo jumpy??

Either way, it seems at least everybody agrees, I'll have to get the ABS codes read at a garage.

ps - I didn't mean to use 'opinion' in a bad way, it's just there are different posts on this problem, on all the Mark VIII boards, for a Gen 2, with different causes as to why it happens.

Thanks,
Ken

 

[m_maker]

M MAKER - Based on what you're saying the only problem this can be is a bad rear abs sensor?? It can't be the front sensors, and it can't be the VSS because those wouldn't make my speedo jumpy??

Either way, it seems at least everybody agrees, I'll have to get the ABS codes read at a garage.

No, that's not what I said. Your problem is caused by a malfunction in the abs system (including speedo readings), no doubt. What is causing it that's another issue. You can inspect the abs rings first to rule out that possibility and it will cost you "zero dollars". Read the codes (there might be no codes stored though), and then data-log the sensors to verify they are all working properly. My bet is on one of the abs sensors, or possibly (very likely) the abs computer or it's wiring harness is malfunctioning.

 

[DLF]

Ken, If you want to bring it to me, I'm sure we can figure it out....

 

[14.6@97MPH]

Ken, If you want to bring it to me, I'm sure we can figure it out....

You have an ABS scan tool too!? Man, you have ALL the tools. I'll call or PM you next week and come down when you have time. Thanks!! - Ken

 

[DLF]

You have an ABS scan tool too!?

No, and I never said that I did. What I said was, that I think that we can figure it out.

 

[chicken]

Yes, read, and more importantly monitor the sensors with proper scanner.



While front sensors do send signal to the various systems, they have absolutely nothing to do with the speedometer on 2nd gen cars.



Again, the signal for the speedometer is being taken from the rear wheels on any 2nd gen car.

+2 if the speedo is jumpy its the rear sensors 99% of the time. mine used to do this when it got cold out. and every time i had a RR abs code, rings intact etc just a bad sensor in sub freezing temps. bad part is good luck finding one for a 98, its a one year only part and ford stopped making them a long time ago. my fix, park it in the winter

 

[14.6@97MPH]

+2 if the speedo is jumpy its the rear sensors 99% of the time. mine used to do this when it got cold out. and every time i had a RR abs code, rings intact etc just a bad sensor in sub freezing temps. bad part is good luck finding one for a 98, its a one year only part and ford stopped making them a long time ago. my fix, park it in the winter

You know how well those 99% fixes have been working on my car. lol