Speedo Saga

[phatcat]

Having some speedometer issues, a bit of history here:

1. When I first got the car, the speedo generally worked fine, but the needle was a little wobblier than it should have been.
2. I went on a few track days and at some point in the day the speedo would go bonkers and then stop reading, but after a bit of road driving it would go back to its normal, minor wobbles.
3. The speedo sort of died a couple of weeks ago (the LCD display stopped working on the way to get an MOT, but the speed was still reading ok and I got my MOT ok).
4. So I fitted a brand new speedo, and it was reading rock-solid as per GPS, no needle wobbles. Perfect, I thought, all was well.
5. I go out for a more spirited drive today, and it's suddenly all over the place.
6. I read up on forums and they say to run a new earth lead from the sensor. I guess I must have messed this up, because now, although I get a flashing LCD on the sensor at the wheel, I now get no speed reading at all on the speedo (the speedo LCD is fine).

So now I'm wondering what to do?
It seems unlikely I've blown a fuse as the sensor LCD and speedo LCD still work, but I can check tomorrow as I don't know which fuse it is so I'd have to go through all of them I guess.

I'll have another fiddle at improving the earth too, but I'm wondering if, given that the LCD on the wheel won't flash without an earth, if it's an earth issue at all?

Maybe it's just a bad sensor. The sensor gap must be ok as I haven't adjusted it since it was working. But, if it's a bad sensor, why does it flash when I move the wheel?

I guess my earth is just worse than the original, as it's gone from mental to no reading at all. Or maybe it's the non-earth connections, but they still click together with the econoseal connector so that seems unlikely unless it's the connector itself.

When running my new earth, I just pulled out the black wire from the sensor, crimped another cable to that, and ran it to the brake pipe/de-dion connection, so altogether its about 2.5 feet of wire from earth point to sensor. The sensor is no longer connected to the original earth.

Any thoughts on the best way to go about determining the cause? How do I know if my earth is good enough, or whether there is a problem with the other connections? I'm guessing the answer involves a multimeter but I wouldn't know what readings I was looking for? I'm pretty clueless with electrics. Wild guess - is it stick one terminal on the chassis, and expect +12v on the green, 0 on the black, and yellow should be reading a low variable output from the sensor when you turn the wheel?

Thanks for any help. 

[Jonathan Kay]

1. You've already checked the sensor by watching the little light.
2. Have you got a copy of the Workshop Notice on improving the wiring? Is what you've done the same as that?

Jonathan

[Tim S]

A working speedo is not a requirement for an MoT.

[phatcat]

Thanks Jonathan.

I've sent John Vine a BM asking for "WN572 - Speedometer sensor groundwire installation", so I'll take a look when I get that.

Seems the problem is common enough for Caterham to have listed a part for it:
https://caterhamparts.co.uk/senders/5685-wheel-speed-sensor.html

For the moment, my earth point is at the same location as the bottom-right picture here:
https://fernlahone.wordpress.com/tag/caterham-7-speedo-faults/

I haven't attached my ground wire through the connector though, like his other pic shows, but rather pulled out the black sensor wire from the connector and directly attached my ground cable to that, hoping this would give a better connection. My earth cable is currently crimped to the black sensor cable, but I might try soldering it to see if that helps.

I did try the earth wire at various points on the chassis, and very few lit the sensor LED. The brake pipe T did, which is why I used it, but then so did the original connection. I wonder if the workshop notice has a different earth point? Hopefully I'll be able to check later.

I'll report back if/when I get it working.

[Jim 123]

You mention that you have not touched the speedo sensor gap.  I think the spec is 1 mm.  Is it worth checking this and making sure the LED head is clean? 

I am probably completely wrong on the following and have never had to prove this, but I was told that the Caterham mod for the speedo ground wire is to achieve an earth to the chassis using the shortest length of cable run from the sensor to the chassis.  The reason for throwing this comment into the ideas pool is that, from memory, the modified cable didn't look to me like it would reach as far as the T piece where you currently have the earth.

[phatcat]

I've assumed the sensor gap must be ok, because the speedo was working perfectly as per GPS until it decided to go mental again. Though I guess I should check if it has just become loosened. 

Cable length could indeed be an issue. I intend to reduce the length once I get my hands on the workshop notice about where the official earth point is. I'm itching to get out there and have another go, but I'm trying to avoid fiddling with it until I get that info... 

 

[Jim 123]

You might be able to get the Workshop notice using the attached link to a PDF, and going through the registration process.

http://californiacaterhamclub.com/chat/showthread.php?2278-Intermittant-Speedo-Fix

 

[phatcat]

Thanks to Jonathan, I now have the workshop notice, will read it through and then heading back to the garage... 

[phatcat]

Here's what I did today:
1. As per the Caterham work notice, I drilled a hole in the chassis tube at the point indicated by the WN. I then ran a new, shortened, earth lead in case the old one was no good, crimped the wire with an eye connector, tapped the hole, and screwed in a bolt to secure the lead to the new earth point. I then crimped the new earth lead direct to the black lead on the sensor, as before.
2. Double-checked the LED flashing on the sensor went on/off as the wheel turned. I tried 1mm with a feeler gauge as suggested, but that just made the light stay on all the time, and I had to back it out to ~1.3mm.
3. Refit the old speedo in case the new speedo was the issue.
4. Checked the wires in the econoseal connector were not loose, then disconnected and reconnected it.
5. Checked all the fuses.

But, still no speed showing 

At this point I'd got bored of taking the car off the axle stands, out for a 1 min drive to confirm no speed reading, then jacking back up again. So I thought - maybe I can check with a multimeter if there is a pulse signal being received by the speedo? So, I took the speedo out of the dash and pulled the connection out of the back of the speedo. Then I hung the connector out of the dash, and stuck a pin in each wired hole to get the voltage readings, and got the following results:

Green - 11.8v
Red/white wire - zero
Black - zero
Yellow - 11.9v

I then turned the rear wheel for the sensor, which flashed away merrily, and watched the voltages of each of the above connections - they didn't vary at all. 
I then tried swiching the multimeter to amps, but again no change when I span the wheel.
But I really have no clue what I am doing with a multimeter, so I gave up at this point.

So I know it's not the fuses or the speedo, and I've tried 3 different earths.

I'm starting to get suspicious of the sensor and/or the wiring loom.

To eliminate the wiring loom as the issue, I'm thinking I should kill the power via the battery master switch, connect pins at both ends of the loom to the multimeter, switch it to the continuity test setting and see if it buzzes. This is turning into a lot of effort! 

[Jonathan Kay]

Then I hung the connector out of the dash, and stuck a pin in each wired hole to get the voltage readings, and got the following results:

Green - 11.8v
Red/white wire - zero
Black - zero
Yellow - 11.9v

Where was the other lead of the meter connected for each of those?

... connect pins at both ends of the loom to the multimeter, switch it to the continuity test setting and see if it buzzes.

For each circuit: if it shows continuity also measure the resistance.

Jonathan

[phatcat]

> Where was the other lead of the meter connected for each of those?

To a fastener screw on the body of the car. (A door fastener).

I think the biggest problem in trying to fix this fault is not knowing if you've fixed it without taking the car out and all the jacking up/down to get at the stuff behind the wheel. I expect an auto-electrician would know the source of the fault in minutes, maybe I should phone one...

[Jim 123]

This is pure guesswork..............  From the pictures in your blog of the rear multiplug connector, my failing eyesight sees there is a green wire, a yellow wire, and a black (earth) wire, but no red/white which is giving a zero reading at the speedo clock connector. It might be worth a look see as to where the red/white goes to from the Manual wiring diagram?  Re the black wire at the speedo clock, should you try a continuity check to make sure it is finding an earth since you relocated the sensor earth?

[phatcat]

Not my blog as it goes! Looking at the sensor sub-loom on the caterham website, it's not an uninterrupted wire that connects from the sensor connector all the way to the speedo connector as the caterham part has an econoseal connector at both ends. So I expect the yellow wire becomes the red/white wire, but I guess I'll confirm that when I do some tests tomorrow (assuming it passes the continuity test). I'll report back here if/when I find out more.

https://caterhamparts.co.uk/looms/2150-sub-loom-wheelspeed-sensor.html

 

[John Vine]

When running my new earth, I just pulled out the black wire from the sensor, crimped another cable to that, and ran it to the brake pipe/de-dion connection, so altogether its about 2.5 feet of wire from earth point to sensor. The sensor is no longer connected to the original earth.

I think you've pulled out the wrong sensor wire.  On my R400D (2008) the sensor-loom connections are:

This matches WN572:

It appears you have earthed the sensor signal wire.

JV

[phatcat]

Yes. You're absolutely right! 
I was wondering if anyone would have picked up on that before I got back!
I was staring at the WN this morning and shaking my head feeling like a proper idiot, then pulled myself together and ran out to the garage.
(It's odd that the sensor LED still worked given it was wired wrong, but no doubt that all makes sense to sparkies).
I can confirm it's all fixed.
Thanks to all for help.

on the sensor:
brown = +ve
blue = -ve (earth)
black = signal

on the loom:
green = +ve
black = -ve (earth)
yellow = signal

Something vaguely useful I learned though to share:
Rather than jumping in the car for a drive to test the wiring and sensor for each attempted fix, here's what you do (assuming you have a relatively recent 7 model):

1. loosen off the nuts on rear wheel on drivers side.
2. jack the car up, stick it on axle stands, and take the rear wheel off.
3  turn car power on (key turned to first position so you can hear the fuel pump prime).
4. rotate the brake disc and the wheel sensor LED should flash as it passes the grooved teeth on the disc behind.
5. forget a feeler gauge (the correct gap seems to vary anyway), just slowly turn the brake disc, and make sure you get on/off for each tooth all the way around, and if not, adjust the sensor in/out as needed via the nuts. note that the toothed wheel may not be perfectly round, so its important to do a full wheel rotation to check for any dead spots. also note that you need to do this test again after you have tightened the sensor in place.
6. leave the car jacked up, and kill the power via battery master switch.
7. pull out speedo from dash via releasing thumbscrews at back and disconnect it. (a bracket and a couple of wrinkly washers will probably fall into the footwell - try to catch them as you undo the thumbscrews).
8. put multimeter probe into yellow wire socket in the speedo connector that is now hanging out of the dash.
9. earth other mulitmeter probe on chassis (I used a metal door fastener as it's convenient).
10. set multimeter to read DC voltage, e.g. 20v DC and turn on multimeter.
11. turn on power, multimeter should either read 12v or 0v.
12. slowly turn the rear brake disc, the voltage should now alternate between 12v and 0v as you turn. (If not, I guess you may have a bad earth - see the WN, or perhaps some other wiring/connection issue, perhaps try continuity tests).
13. having confirmed all is well, put the rear wheel back on, get car back on the ground and tighten up the wheel.
14. reconnect the the speedo and calibrate it if it's a new speedo (see chart on other posts).
15. go for a blat and check the speedo vs GPS.
16. if all is well, properly reattach the speedo to its bracket behind the dash.

Interestingly after all this, the old speedo now has a working LED again (I guess it likes the new earth). 
But it still wobbles its needle and overreads the speed (possibly due to damage from having a bad earth for 3490 miles, who knows?)
I fitted the new speedo again, and no wobble - rock solid and speed matches GPS.
Hopefully it will stay that way this time...

[Jonathan Kay]

Well done, and great spot, John.

12. slowly turn the rear brake disc, the voltage should now alternate between 12v and 0v as you turn. 

That's very helpful to know.

Jonathan

[John Vine]

Pleased to hear it's now fixed.

And #12 is a great tip - thanks for that.

JV