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Discussion Starter #1
So, I recently purchased my '08 Exige to replace my unreliable track day car with the idea that it would be significantly less maintenance/have fewer issues and have been loving it so far. However, tonight on a spirited drive, I decided to see just how fast I could bring it to a stop from 80 mph by depressing the brake pedal to the floor (i.e. panic stop). The abs came on (as expected) and it seemed to stop very well. Then, a few miles later, I braked somewhat lightly turning onto my road and the abs seemed to come on (made a clunk in the front accompanied by a shutter in the pedal). Thinking i must have gone over a bump or some loose sand/gravel causing the ABS to actuate prematurely, i tried stopping again with light pedal pressure going straight and sure enough, same thing. When i got home, i bled the brakes and checked all the suspension components in the front end for damage/proper torque. everything appeared to be fine. i took it out again (after getting one or two bubbles out of the front calipers) and sure enough, same thing. It also seems to come on very unevenly as the car will pull slightly to either side (depending on how the abs decides to engage). Could it be the ABS pump? Maybe an ABS sensor is getting gummed up with brake dust? how do i go about diagnosing the issue? there is no ABS light on the dash. the car has less than 5000 miles on it and is bone stock. Thanks in advance for any help!!
 

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Discussion Starter #5
So, I unplugged the ABS wheel speed sensor on the front passenger side wheel, and the problem seemed to go away. In fact, the car seemed to stop much better as I could easily threshold brake (as opposed to with the "assistance" of the ABS). I would just as soon keep the ABS unplugged at all times because of this (I much prefer driving without it as i think it can be dangerous in some situations) however, it seemed to affect the functionality of my traction control. Whenever i would make a slow left hand turn in 1st or second gear, the traction control would kick in, even if i was moving 6 mph and applying less than 10% throttle. It did this even with the traction control turned off. Did i unplug the wrong wheel speed sensor, or is this to be expected?
 

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The traction control issue you are experiencing should be expected with a front wheel speed sensor removed. I assume you get an amber abs fault light with the sensor removed? You get the traction hit turning left because with the right front sensor removed the front wheel reference speed cannot account for the turn and the traction system correctly believes the rear wheels are slipping. As far as your original issue were the events at low speed? You mentioned one event while turning, is it consistent with turn direction? Do you have any play in a bearing? It looks like the wheel speed sensor is integrated into the bearing so unless there is a sensor harness issue this may be due to bearing play. Are you running stock size tires? ABS and traction control isn't smart enough to tell when tire rolling circumference changes.

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Btw, I would not choose to drive on the street without ABS, it is far better at controlling wheel lock up than you and will absolutely save your a$$ in an emergency situation. And before you give me all the reasons why i am wrong, I have been doing brake system development for OEM car manufacturers for over 15 years.

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Discussion Starter #8
Btw, I would not choose to drive on the street without ABS, it is far better at controlling wheel lock up than you and will absolutely save your a$$ in an emergency situation. And before you give me all the reasons why i am wrong, I have been doing brake system development for OEM car manufacturers for over 15 years.

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I can't disagree with you at all, especially when it comes to adverse weather conditions and panic braking. However, I feel strongly that I can stop/control the car a lot faster/better without ABS when the ABS is faulty (i.e. this situation). If it were fully functional, I would absolutely leave it alone. Now to answer yor questions:

I am running stock tires on monolites. The rims are slightly wider than stock/have different offsets, but given that it is the stock tire, the diameter/circumference is unchanged. It seemed to do it at low speeds, while turning and going straight. Also, there doesn't appear to be any play in the bearing. Could tire pressure be a major player? I filled them at the Hess station the other day and only realized yesterday that they were all slightly over inflated (between 32 and 34 psi). I brought them all back down to within spec, but have not had a chance to test this theory yet.
 

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The traction control issue you are experiencing should be expected with a front wheel speed sensor removed. I assume you get an amber abs fault light with the sensor removed? You get the traction hit turning left because with the right front sensor removed the front wheel reference speed cannot account for the turn and the traction system correctly believes the rear wheels are slipping. As far as your original issue were the events at low speed? You mentioned one event while turning, is it consistent with turn direction? Do you have any play in a bearing? It looks like the wheel speed sensor is integrated into the bearing so unless there is a sensor harness issue this may be due to bearing play. Are you running stock size tires? ABS and traction control isn't smart enough to tell when tire rolling circumference changes.
So, if one wants to disable ABS for track days to avoid the possibility of "ice mode" then pulling the ABS fuse is best?? Presumably this would leave traction control with the correct reference signals if one wants to play with TC. Just following a train of logic here and would appreciate confirmation or correction from someone with real knowledge.

Wmangm, I keep hearing that ABS (most makes, not just Lotus) can't handle tire rolling circumference changes. I don't disbelieve you, but it seemingly wouldn't require much more than simple logic and a wheel speed sensor check side to side/front to rear. Why isn't this done?
 

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Pulling the ABS fuse gets rid of the following:

*Speedometer
*Odometer
*ABS
*Traction Control
*Ice Mode

On my car, pulling either of the front wheel speed sensors would panic the TC and cause the TC to aggressively engage when turning in one direction. YMMV
 

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Disabling ABS on the track is a personal choice and very dependent on the way you brake. On the street you don't know what you will encounter. Painted lines and gravel road shoulders can teach you a lot about split coefficient braking.

As far as dissimilar tire sizes, even stock sizes can be mismatched depending on the actual build. Tire rack publishes revs per kilometer on many of their tires which is a better measure than a calculated circumference based on tire profile. A radial tire actually can be considered similar to a belt or track, rolling radius is not directly relayed to the true circumference. ABS and associated chassis controls typically work off of calculated tire slip relative to a calculated vehicle reference velocity(s) and usually some measure of a rate of change of tire slip. Depending on the complexity of the control algorithms and the desired character of the vehicle the sensitivity of the ABS and other chassis controls can vary significantly. Likewise the sensitivity to a perceived change in tire slip can be significant based on the needed or desired reaction. I am speaking in generalities say the difference in an elise and a malibu.

As with any control system changes could be made to allow alternate tire sizes but it takes more than just changing some inputs or a couple algorithm values to do it well. I don't know enough about our cars yet to know what would be involved.


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I am still suspicious of a wheel speed sensor. The connectors and wires at the wheel ends don't look melted or chafed? Does the car lead in a direction when this occurs? If a front brake is releasing the car should lead opposite the releasing brake first if you can tell. Left brake releases and car starts to turn right. I don't have a service manual yet but there is often a sensor output check that can be done with a multimeter.

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Discussion Starter #13
I am still suspicious of a wheel speed sensor. The connectors and wires at the wheel ends don't look melted or chafed? Does the car lead in a direction when this occurs? If a front brake is releasing the car should lead opposite the releasing brake first if you can tell. Left brake releases and car starts to turn right. I don't have a service manual yet but there is often a sensor output check that can be done with a multimeter.

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Wow, tons of good info in this thread! Thanks for your input! to answer your question, the car pulls to the right when this happens, then to the left and then back to the right under prolonged braking until it sort of gets a handle on things and then brakes straight but still entirely too slowly. After doing some research on ice mode, it has occurred to me that my symptoms somewhat resemble being in ice mode permanently. After inspecting all 4 wheel speed sensors, none appear to be damaged. If there is some way to check sensor outputs with my multimeter, this will be my next course of action.

Also, is there some way of disabling the abs for track use without panicking the traction control?
 

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Right, i mean pull a wheel speed sensor for the track and shut off the traction control.

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Right, i mean pull a wheel speed sensor for the track and shut off the traction control
Supposedly this should work. However, my traction control has turned itself back on several times (or perhaps it really is not off when I turn it off). With several "ice modes" experienced at Waterford Hills, I don't want ABS on there nor flakey TC so it is pull a fuse. On the other hand, I haven't experienced "ice mode" at Grattan and on two turns appreciate traction control. There, so far, I have kept ABS functionality but don't really push the braking into turn #1 and the "toilet bowl" just in case IM decides to make an unwelcome appearance.
 

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Discussion Starter #18
Right, i mean pull a wheel speed sensor for the track and shut off the traction control.

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It doesn't seem to matter that the traction control is off (think i may have mentioned that in one of my first posts). Even with the traction control turned off, it goes absolutely nuts when you make a left turn if the front right wheel speed sensor is unplugged.
 
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