• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Electronic pulse frequency modification (vehicle speed sensor)

Status
Not open for further replies.

Apple_Eater

New Member
Hello,
Firstly, allow me to apologize in advance if my post is not accurate or in the wrong area. I have a very limited knowledge of electronics in general but am interested in learning. With the disclaimer out of the way, here is my issue:

I have recently switched the tire sizes on my 2001 LandRover Discovery. This has caused a discrepency in the speedometer on the vehicle vs. the actual speed being traveled. I am looking for an electronic, rather than mechanical, fix to this issue.

According to the repair documentation of my vehicle, speed is picked up by the speed sensors above the wheels, which goes straight to the ECU. After reaching the ECU, some magic is done, then the speed is transmitted to the instrument cluster in the following manner:

The Self Levelling and Anti-Lock Brake System (SLABS) ECU provides the signal input for the road speed. The signal is at 8000 pulses per mile (1.6 kilometres).

My assumption is that the ECU uses the speed sensors above the wheels to calculate the actual distance being traveled (for odometer and speedometer operation) and will pulse whenever it determines the vehicle has moved 1/8000th of a mile. As my speedometer accuracy discrepancy is around 10% (I will obviously do more testing to be more precise when the time is right) it will in actuality be pulsing for every 1/7200th of a mile.

Here is my idea/question: Is there some sort of electrical device or easily-obtainable (read: cheap) chip that can take an input frequency in the 8-800khz range (to account for all speeds from 1-100 MPH) and reduce the frequency by 10%, then output the result on the same wire?

If there is such a device, will it be able to run without any external power source (using the voltage in the wire it is modifying) or will I need to power it somehow?

Also, if my idea is impossible/impractical, is there some other electronic method I could use to cause the vehicle to report the correct speed?

For the curious, I am interested in doing this electronically so that it may be modified in the future if I again change tire sizes for some unforseen reason. I do realize that this fix will only fix the speedometer, not the odometer, but I am not terribly concerned with that.

I am looking forward to your responses and to learning a bit more about electronics during this project, and any help/advice is greatly appreciated!
 

Sceadwian

Banned
You should contact a shop that replaces tires, there is either going to be a way to simply and easily reprogram the ECU or even possibly dip switches to allow for different tire sizes.
This is not something you should start off as an early project as there is already a system in place to do it normally. You're not the first person to get larger or smaller tires on a car and need their ECU and odometer readjusted. Don't make work for yourself that a proper solution already exists for.
 
Last edited:

Apple_Eater

New Member
Sceadwian,
Thanks for your response. Unfortunately, my car is a few years too old to allow for easy adjustment of the speedometer. I would have to pull out my transfer case and replace the gears that drive my speedometer with another gear of the correct size -- a job I likely couldn't accomplish on my own and would require a few hundred $$$ to be done at any of the shops in my area, thus my interest in a DIY solution.
If it is unlikely that I will be able to accomplish some sort of electrical fix to my problem, I'll likely just leave my speedometer in it's current state and just learn to live with it, but this is not an idea solution (especially if I sell the car at some point). As I am a college student, I do not have the spare funds to pour into fixing such a minor problem, but I do have an interest in learning about electronic circuits (I am a CS major, so some knowledge would be beneficial) I thought this would be a good project to start with.

If it is likely to cost more to accomplish this fix electronically, or if it would be outside my abilities, I will just leave the issue uncorrected.

Thanks for your response.

EDIT: The apparent difficulty in recalibrating my speedometer is because of its "gear-driven" nature. The owner of a particular landrover shop in my area has the same issue with his truck and he just "lives with it" due to the complexity of repair.
 
Last edited:

Apple_Eater

New Member
I have finally been directed to an after-market product produced by SuperLift that does exactly what I was planning to do, albeit more simply and reliably. Thanks for your help.
 

mneary

New Member
For your future reference, your math is all wrong.

The frequency is not 8-800 kilohertz for 1-100 MPH. It is 2.2 to 222 Hertz.

8000 ticks/mile * 1 mile/hour / 3600 seconds/hour = 2.222... ticks/second.
 

crutschow

Well-Known Member
Most Helpful Member
As a DIY electronics project you could use a digital binary rate multiplier chip such as the CD4089B to reduce the pulse rate for less than the cost of the SuperLift device. But using one of those requires a fair understanding of digital electronics.
 

Mr RB

Well-Known Member
In post #3 he stated it was a mechanical driven speedo, these are harder to correct. Some of the instrument repair shops now sell tiny "gearboxes" with replaceable gears to give a mechanical ratio correction, they connect in-line in the rotating speedo cable. I believe the Superlift product he mentioned was that type of corrector.
 

tcmtech

Banned
Most Helpful Member
If its a mechanical speedo your far better off just pulling the speed cable and gear out of the side of the transmission and putting the correct one on. All of the mechanical speedo drives have either one or two screws, a snap ring or clip retention system on the actual piece that the speedo cable attached to. In any case its usually a five minute job and about $10 to swap out the right drive gear for your tire size.

Getting the vehicle jacked up and cleaned out underneath so you can find the speedo cable takes longer to do and is usually most of the work involved.
Talk to the local dealer and they will tell you the specifics to that you are working with exactly.
 
Status
Not open for further replies.

EE World Online Articles

Loading
Top