tach and dash wiring problem on mk3 sprite

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tach and dash wiring problem on mk3 sprite

Postby newoldspriteowner » Tue Aug 19, 2008 11:51 pm

Hi all still working to get car on the road
Mechanics done at last
Now to electrics a lot of the electrics on the dash had been modified why? sorry I do not know
all instrument dash lights now functioning
Turn indicator had been modified to a toggle on the dash basically the car was missing the steering column cowl and therefore no way to install the arm type turn switch. Well I found the cowl and an original boxed Lucas switch never used. So that has been fixed
horn switch had been displaced and fixed under dash. Refitted to steering wheel location.

The problems (2) I do have left are the following
1 Tachometer is irratic and I noticed a spot on the white wire leading to tach where the wire has overheated. Since tach receives electrical impulses could this be the reason why the tach is irratic and even at times intermittent? Also what could be the cause of the wire overheating? If I want to replace that white wire and trace it to it's source where will it lead me to
2 Same overheating of the red wire that seems to bring the current to dash board instrument lighting Again the same question for tracing the source of this red wire in order to replace.
The rest of the wiring seems OK I would like to fix these problems without changng the harness. Can someone out there help?
I am a little concerned about insulation overheating and thinking of installing an electrics Kill switch Is this a good idea?

I only have the Haynes diagram as reference I find it complex and also the color of wiring do not match 100%

I heard of some chap that creates colour diagrams of very good quality that are very explicative as well can somebody point me to the source.
Thanks Regards
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Re: tach and dash wiring problem on mk3 sprite

Postby spritenut » Thu Aug 21, 2008 10:20 pm

The tach will be much better if it has a clean new ground, that knurled nut where the ground wire goes is OK for the dash light but I have found a good ground to the tach housing makes a world of difference.
The white wire comes from the key, makes a loop around the impulse unit on the back of the tach and continues to the coil.
The red wire is the parking light wire. It comes from the head light switch and goes to all four corners for the parking/tail lamps and it meets the red/white wires at the dash lamp switch for the dash lamps.
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Re: tach and dash wiring problem on mk3 sprite

Postby newoldspriteowner » Thu Aug 21, 2008 11:15 pm

Thanks for the info Frank.
Very explicative
My worry is that both the white tach wire and the red show each one spot where there seems to have been some overheating of the shielding maybe something happened to the car in the past. All electrics work OK Dash lights, parking lights, turn signals, headlights, heater, wipers and horn, the car does not have a radio those signs of overheating still worry my mechanic and I. Could there be a fire just waiting to happen, so for now we intend on installing a remote battery kill switch, as a safety measure as for that slight overheating sign on the white tach wire can that be the cause of the tach's irratic behaviour since it receives elcetric impulses from that wire?
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Re: tach and dash wiring problem on mk3 sprite

Postby hoggie » Sat Aug 23, 2008 1:20 pm

I believe the MKIII Sprite is a positive ground electrical system. The white wire that goes in the tach just loops through a coil pickup and comes back out. It does not terminate on anything in the tach. The white wire becomes hot (neg 12V for positive ground wiring) when the ignition switch turns on. (It is not fused) After it loops through the tach, it terminates on the SW (SWitch) post of the ignition coil. The SW post on the ignition coil connects to the primary winding that is open and closed by the points connected to the CB side of the coil. As the points open and close the circuit, the pickup coil in the tach detects the rate of the current changes and moves the tach needle. The tach sensor is powered from the Green wire stacked on the B post of the flasher unit that continues to the fuse box. If the tach white wire shows signs heat damage. I guess it could be possible that the ignition coil shorted out, or the SW post on the ignition coil was accidentally shorted to ground while the ignition switch was on. Most likely someone may have tried to convert the electrical system to Negative ground and accidentally shorted something out in the process. Or maybe someone got too close to the wiring with a solder iron. Anyway, here is how I would proceed. First, is the electrical system still a Positive ground system? Is the Positive battery post connected to the chassis? If not, then the electrical system has been converted to negative ground. You will then need to check to make sure it was done correctly. There are many documents on the web and on this forum related to negative ground conversions. Otherwise, you could check to make sure all the connections are good and the tach has a good ground connection.

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