Lotus Elan

Tacho Repair

PostPost by: Chrispy » Fri Jan 19, 2018 6:35 am

Well my speeod has been playing up since I got the car. Feels like it reads reasonably well when cold, but as soon as the car warms up it feels like it reads nearly double what it's doing! I decided to give it a go of repairing myself. It's a early +2 but converted to negative earth with a Smiths RVI 2402/04 tacho.

I found this excellent write up http://www.sunbeamalpine.org/downloads/alpine_tach_repair_rev5.pdf showing what to do. I pulled my tach apart and it looked reasonably clean but some of the components looked a bit unhappy - especially the thermistor which had split it's casing. I do note that there is for a different part number tacho - but the internals look identical. We shall see!

I went to order all of the parts from Digi-key as the author had them all listed, but several were now out of stock or no longer available. I went through and found suitable replacements, the only one I couldn't find a like for like for was the thermistor.

The thermistor recommended was a Panasonic 300ohm item with a beta factor of 3900 in conjunction with a 34ohm resistor. The closest I could find from Digi-Key was a Amphenol 300ohm item with a beta factor of 3468. I threw together a quick spreadsheet to see how far out it would be. It was pretty close and needed a 37ohm resistor to bring it back.

This shows the difference in resistance vs temperature with the inline corrector resistors for both thermistors.
thermistor-comparison.jpg and



Here's the updated component list from Digi-Key with available items. All together they cost less than AUD$5 + post. The green ones are the items still available from the original article.
part-numbers.jpg and


I'm still out at work, but I hope the little box from the States will be in my hot little hands when I fly home next week. I'll put up another update when I've actually done some real work.

PS, I'm a mechanical bloke - so no guarantees that I'm doing the right thing :lol:
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PostPost by: pharriso » Fri Jan 19, 2018 11:17 am

Thanks for posting, most interesting.

Alternatively you can replace the electrical circuit completely with a module from Spiyda - https://www.spiyda.com/M/smiths-rvi-rvc-conversion-board.html
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PostPost by: RichC » Fri Jan 19, 2018 5:25 pm

Agreed !
very interesting reading . It's what makes this forum GREAT
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PostPost by: Chrispy » Sat Jan 20, 2018 12:22 am

pharriso wrote:Thanks for posting, most interesting.

Alternatively you can replace the electrical circuit completely with a module from Spiyda - https://www.spiyda.com/M/smiths-rvi-rvc-conversion-board.html


I didn't know that was available, surprised I hadn't stumbled across it in my searching on how to repair.

I find this site very helpful, so would like to give back with my successes (along with the failures) if it can help others :)
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PostPost by: elanfan1 » Sat Jan 20, 2018 6:46 am

I don't know if they would sell them to you or nit but Speedy Cables repair these things so must have suitable components available.
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PostPost by: 2cams70 » Sat Jan 20, 2018 2:52 pm

If you are in Australia you can probably find the same electronic items from either element 14 or RS components for a much lower postage cost. RS components is free postage within Australia and element 14 is free after after a certain value threshold which is pretty low - $AUD15 to $20 from memory. USA based Mouser or Digikey charge quite high postage rates to Australia in comparison.
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PostPost by: MarkDa » Sat Jan 20, 2018 9:13 pm

I fitted a Spiyda conversion after installing Aldon electronic ignition.
It calibrated ok and seems ok up to 4000 rpm but doesn't move any further.
I've not pulled it out to check whether it was down to my soldering or calibration nor have I got round to talking to Spiyda about it.
The idea is great and I suspect that my issue is down to me rather than the kit.
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PostPost by: mbell » Sat Jan 20, 2018 9:30 pm

MarkDa wrote:I fitted a Spiyda conversion after installing Aldon electronic ignition.
It calibrated ok and seems ok up to 4000 rpm but doesn't move any further.
I've not pulled it out to check whether it was down to my soldering or calibration nor have I got round to talking to Spiyda about it.
The idea is great and I suspect that my issue is down to me rather than the kit.


I am having similar issues. Calibrated ok ish couldn't get it to 1500 and 3000rpm on the same calibration setting but probably close enough.

When fed with low voltage signal from my rev limiter it would show idle, 1500 or 3000 rpm only. Now wired direct to coil and not getting the stepped behavior but it's stalling at higher revs.

Need to spend some time charactising the issue thou.

Like you I was thinking that it was most likely my fault but maybe it's a bad batch or something. Either that or they just don't like people called Mark...
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PostPost by: MarkDa » Sat Jan 20, 2018 9:41 pm

That's interesting, maybe I'll give him a ring and see what he says.
He was helpfully enough during initial set up.
It may well he knows what the problem is and knowing it's not a complete one off will help.
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PostPost by: Chrispy » Sat Jan 20, 2018 11:17 pm

2cams70 wrote:If you are in Australia you can probably find the same electronic items from either element 14 or RS components for a much lower postage cost. RS components is free postage within Australia and element 14 is free after after a certain value threshold which is pretty low - $AUD15 to $20 from memory. USA based Mouser or Digikey charge quite high postage rates to Australia in comparison.


Yes, the postage cost was a little eye watering for such a little package. I ended up getting over the minimum spend for free postage by getting some Deutsch connectors I'd been wanting for a while.

Still better that the postage quote I got for a new headlight bezel clip from the US. $1.90 for the part, $102 for the postage :shock: :lol:
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PostPost by: mbell » Sun Jan 21, 2018 2:20 am

MarkDa wrote:That's interesting, maybe I'll give him a ring and see what he says.
He was helpfully enough during initial set up.
It may well he knows what the problem is and knowing it's not a complete one off will help.


Please let me know what you find out. Will see if I can find chance to do some testing on mine too confirm the behavior.

Thanks.
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PostPost by: EPA » Sun Jan 21, 2018 12:18 pm

When I fitted the Spiyda kit to my tachometer it calibrated fine off the car but when fitted to the car it would register whilst starting the engine but return to zero as soon as it was fully started. After discussing this with the very helpful guy at Spiyda I fitted a 200k variable resistor in series with the wire from the coil and adjusted it upwards until the tachometer worked. It would appear that the signal from the electronic ignition was taking the circuit out of its operating window which resulted in the tachometer not working.
It?s worked fine ever since
One other point I would make is that you should Test the tachometer with all the ranges available on the web site.
I initially tried the kit in a spare tachometer that I aquired with the car and I just calibrated it at 3000 and 6000 rpm which worked ok but when I tested it on the car it appeared to add 1000 to 1500rpm throughout the Rev range. I later ran the 1000rpm calibration off the car and discovered that it was the tachometer itself causing the problem
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PostPost by: jeff jackson » Sun Jan 21, 2018 12:35 pm

I would endorse the Spyder kit. Piece of cake to fit, and works fine. Has done since I fitted it three years ago.

Also appreciate that some circuits might need a bit of adjustment, but playing around with things like this makes it fun.
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PostPost by: Chrispy » Thu Jan 25, 2018 5:26 am

I flew home last night from work and had a nice box of tricks on the kitchen table when I walked in the door.

Got cracking with it this morning and it was all pretty easy, biggest surprise was the size of the old components compared with the new ones. I don't have anything to add to the original article I linked in my first post. I have a plunger type solder sucker which helped.

new-bits.jpg and


Showing the difference between the old and new capacitor. Huge difference in size. Only just enough length on the legs to get them in the correct spot.
oldnewcap.jpg and


All the removed components. The damage to the termistor can be seen and the cap has a crack in its casing.
oldbits.jpg and


All done. Reasonably neat.
done.jpg and



Put it all back together and into the car. Calibrated using the engine and a optical tack with a mark on the water pump pulley. May not be perfect, but I'll take a little bit of tolerance over what I was getting with tootling down the street and it reading 7000rpm. Fired it up and it read 1200rpm and by the time I worked out what I was doing the idle was ~900rpm. Adjusted the trim pot in the back of the unit until correct and that was it.

On a little test drive it 'felt' quite accurate and lined up with the speedo for what I thought it should be. I'm not sure if the temperature correction is entirely correct when I got home it was ~100rpm over, but well and truly accurate enough for me! It is a hot day and probably 40degC+ under the dash.

I'm a happy little camper now. Next things to tackle are the leaking exhaust header, rattly parcel shelf and deteriorated fuel tank breathers.
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PostPost by: 2cams70 » Thu Jan 25, 2018 5:41 am

Well done. I like those kinds of fixes. I note that there's still an old blue axial electrolytic capacitor in place. Might be worth changing that out too at some point. Electrolytics have a limited lifespan.
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