Electric Speedometer Help

[scannon]

My Caterham has an electric speedometer. There is a two wire connector to the sensor on the transmission. One side is hot and if I make a connection across the two terminals, I can see the speedometer needle make a small jump. Nathan at RMSCI tells me that the Caterham sensor is supposed to produce 8 pulses per revolution.

 

The electric sensor in my Miata transmission also makes 8 pulses per revolution, however, it is a reed switch which I can watch make and break with a continuity checker while I turn the gear. The Caterham sensor is not an off and on switch and when I connected the Miata reed switch to the Caterham speedometer wiring, I got nothing in the way of needle movement while driving.

 

Can anyone educate me about how the Caterham sensor works and what I might do to make the Miata sensor work with the Caterham speedometer?

 

Another problem is the idiot light for the alternator. It stays on whether the engine is running or not. I have measured the voltage at 14.4 volts while the engine is running and about 12 volts when the engine is off so I think it is safe to say the charging system is working. Any suggestions to make the idiot light function? I connected the wire in the Miata harness for the idiot light to the wire in the Caterham harness for its idiot light.

 

Thanks,

 

Skip

[bsimon]

Skip,

 

I had some trouble posting to this thread yesterday as my router was untraining just about the time I was ready to post. See the related thread in the General section.

 

As to the speedo situation;

 

The stock Caterham speedo sensor is a two wire Hall effect device. I'm only guessing here, but the input to the speedo is probably pulled high ether in the speedo or in the wiring loom in order to provide current to the sensor. If you can figure out which wire is the actual speedo input, pulling it to ground through the reed switch should provide the signal you need. If this doesn't work, it might need to be pulled high through the reed switch.

 

Maybe a simpler explanation would be; you won't be using both wires from the reed switch going to the speedo, only one. the other wire to the reed switch will go to ground or power depending on how Caerbont has configured the Caterham speedo.

 

Since the Caterham speedo uses a hall effect sensor, you can buy one from Automation Direct and mount it to the front upright. The signal is then picked up off the disk bolts for a more accurate solution.(no wheel spin errors) The speedo will have to be recalibrated with the odo push button.

 

 

 

-Bob

[scannon]

Bob,

 

Thanks for the info on the speedo. I have an extra Miata sensor so I can do some experimenting. Do you have a guess whether it will be the hot wire through the reed switch to ground or the non-hot wire through the switch to ground. It seems like right now I have the hot wire through the reed switch and back to the speedo which doesn't work.

 

How do you calibrate the speedo using the odo push button? Is this documented somewhere I can access?

 

Thanks,

 

Skip

[scannon]

Bob inspired me to do some late night experimenting in the shop. I made a couple of discoveries.

 

1. Neither of the wires when run through the Miata sensor to ground make the speedo work or even quiver. Shorting between the two wires going to the speedo produces a jump in the speedo needle as seen before.

 

2. Directly grounding one of the wires produces nothing, grounding the other one directly produces a blown fuse. No suprise there.

 

3. Running a continuity check with my multimeter produces 8 beeps per revolution.

 

4. There are 16 distinct detents felt when rotating the sensor gear through one revolution. Checking the resistance while rotating the sensor gear produces a momentary resistance of either 500 ohm +- a bit or 50 ohm +- quite a bit. This happens during the transit between detents. When the gear stops in any of the 16 detents, it reads 225 ohms every time. It looks like this sensor is not a simple off and on switch.

 

Any suggestions? As you can tell, electronics is not my strongest skill.

 

While preparing to raise the car on stands to get at the connection I noticed that my oil pan seemed much closer to the floor than I remembered. The last time I measured it was 3.75". When I checked it tonight it was down to 2.25"!!! I have added parts and fluids since the last time it was measured and I assume the springs have settled a bit as well but I wasn't expecting to lose 1.5" of clearance. I imagine it drops even lower when I am in the car.

 

 

[bsimon]

Skip,

 

these are the instructions for the Caerbont speedo that Caterham uses.

 

here

 

Unfortunately, There may be some specific alterations made to the speedo just for Caterham's sensor. The programing instructions are the same. Looking at the Caterham wiring diagram, I see they are using pin 8 on the speedo plug. Caerbont calls this lead "not used"

 

What you see with the multimeter hooked to the Mazda sensor has me puzzled. I would agree that it's not a simple reed switch. It may be a variable reluctance sensor that produces an alternating sine wave like a crank position sensor. As far as I know, Caerbont does not work with VR sensors. Maybe Keith knows what Mazda uses for a speedo sensor.

 

1. Neither of the wires when run through the Miata sensor to ground make the speedo work or even quiver. Shorting between the two wires going to the speedo produces a jump in the speedo needle as seen before.

 

If you repeatedly contact the speedometer input wires together at a constant rate can you get the gauge to begin reading speed? You may have to use the programing button to set the pulses/mile to a low enough setting to do this test. If you see speed, you at least know the speedo will work with a reed switch or other solid state switch. This does not answer the question of what the Miata sensor is however...

 

Are there any aftermarket speedometers that work directly with the Mazda sensor? If we could find instructions for such we may be able to determine what kind of sensor Mazda uses.

 

 

[scannon]

 

If you repeatedly contact the speedometer input wires together at a constant rate can you get the gauge to begin reading speed? You may have to use the programing button to set the pulses/mile to a low enough setting to do this test. If you see speed, you at least know the speedo will work with a reed switch or other solid state switch. This does not answer the question of what the Miata sensor is however...

 

Are there any aftermarket speedometers that work directly with the Mazda sensor? If we could find instructions for such we may be able to determine what kind of sensor Mazda uses.

 

Bob,

 

Thanks again for the new info.

 

I tried repeated touching of the two wires. The speedo jumped up to about 40 mph immediately. I wonder if I am feeding too much current to the speedo doing this.

 

I'm not aware of any other speedometers working with the Miata sensor but I will see if I can find a service manual for the '00 Miata and see if there is any info in there on the signal structure.

 

I'm leaving in the AM to pickup my new trailer in Nebraska and won't get a chance to pursue this until the weekend. I will take some ohm readings on the Caterham sensor to see if we can determine what it is.

[scannon]

I had a chat with Keith this AM and he checked the manual. Apparently the sensor should be providing a 5 volt pulse. From Keith's later email:

 

"Looks as if it's not that tough to convert voltage pulses into the sort of signal you need. You've got half of a "hall effect switch", and it looks like a switching output transitor will finish things off. In other words, a moderate electronics geek should have no trouble sorting this out with parts from Radio Shack."

 

Any electronic geeks out there willing to figure this out for me?

 

Thanks,

 

Skip

[bsimon]

Skip,

 

Pin 7 (oddly enough) is the low voltage (5 vdc TTL) input on Caerbont speedometers. You may be able to move the contact in the connector shell from pin 6 to pin 8.

 

Unfortunately, I found the wiring diagram for a Miata here. It clearly shows the speedo sender as a variable reluctance AC sensor. Look at page 20 of the year 2000 diagrams.

 

I have to call APT Instruments today to find out when my new OP and temp gauges are shipping. I'll question them about wiring up a Miata.

 

 

 

 

[Al N.]

Skip-

 

I don't know if this makes a difference whenever you figure the sensor thing out, but I do know that the dip switches on the back of the speedo make a difference in terms of how the speedo reads. There's a certain combination depending on your final drive and wheel/tire combo.

 

Be sure to sort that out down the road using a GPS, mile markers, or pacing a known accurate vehicle....mine read on the optimistic side of things until I got it sorted (which probably means my odo is on the high side of reality too).

[bsimon]

Caterhams built after 2003 (I think this is the year) use the front button programmable speedometer. There are no DIP switches in the back.

 

 

[scannon]

Skip,

 

Pin 7 (oddly enough) is the low voltage (5 vdc TTL) input on Caerbont speedometers. You may be able to move the contact in the connector shell from pin 6 to pin 8.

 

Unfortunately, I found the wiring diagram for a Miata here. It clearly shows the speedo sender as a variable reluctance AC sensor. Look at page 20 of the year 2000 diagrams.

 

I have to call APT Instruments today to find out when my new OP and temp gauges are shipping. I'll question them about wiring up a Miata.

 

 

 

Bob,

 

Oh that it should be so simple as moving a pin on the connector! Please let me know what you find out after you talk to APT.

 

Thanks,

 

Skip

[bsimon]

Skip,

 

I finally heard back from APT gauge.

 

Their expertise is somewhat like mine on the subject of inputs for the Caerbont gauges. "try this, if it doesn't work, try something else." "everything will interface to the gauge, eventually"

 

The good part is they do know that AC sine wave signals (VR sensors) work with their speedometers. The input needs to go to pin 7. Since the VR sensor generates it's own tiny AC voltage, only one side of the sensor goes to pin 7, the other to ground. :thumbs:

 

Since the VR sensor produces such a small voltage(1-2vac), electrical gnomes might be able to get inside the signal cable and create havoc with the digital circuitry inside the speedo. The sensor leads may have to be screened(shielded) from the sensor end. Probably not.

 

-Bob

 

 

 

 

 

 

 

 

 

 

 

 

[scannon]

Bob,

 

Thanks for the info. I went out to the shop and pulled the speedometer and had a look at the connector. Although the pins/sockets are not marked, there is an empty space between the two wires coming from the sensor which I believe someone said are 6 & 8.

 

I tried stuffing a wire in the empty socket hole and connecting it to one side of the sensor and the other side to ground. No joy when spinning the sensor. There are two other empty sockets so I tried both of them in the spirit of "if it doesn't work, try something else". No joy there either.

 

I can't be sure my jumper is actually contacting the pin in the speedo side of the connector so I tried to remove the non-hot wire (pin 8) from the connector and move it over to pin 7. I could open the little protective door across the sockets but could not release the socket from the connector. They appear to be similar in construction to the ones on a Miata only smaller.

 

The Miata sockets can be removed by inserting a small pick or allen wrench along side the wire to release the clip holding the socket in place. The hole is smaller in the Caterham connector and I couldn't find anything small enough to slip in there yet still rigid enough to do the job.

 

Do you or anyone else know if this is the correct way to remove the socket from the connector or is there another trick to it?