Any advice re my Rev counter stopped working???

Julian West · 09-18-07, 06:17 AM

The rev counter in my GTD was working fine for 30-40 miles & all of a sudden the needle just dropped to zero.

I'm using a standard GTD revcounter, with and MSD 6AL ignition and an MSD tach converter.

I've done a quick visual check of all the connections & everything seems fine. I'm wondering what part of the system has failed & how do I go about testing to find the fault??

Any help would be gratefully received!

Malcolm · 09-18-07, 06:40 AM

Julian,

Did it fail when you were getting to about 4000 rpm, ie does it work at all below that rpm level? Reason is that dependant on what type of tacho you have they either work at all rpm or at low rpm only! The differences in them is whether they are volt driven or amp driven. I had to change my tacho to the other sort but being so long ago I can't remember which way round it was and is now. I have a standard GTD tacho and same MSD 6AL but I do not beleive I have a tacho converter.

Not a lot of help but all I can give. Paul Thompson is the electrical wizard so you could try him.

Julian West · 09-18-07, 04:17 PM

Thanks Malc,

It certainly worked fine up to 4000 rpm, can't remember if I revved it much higher than that.....

Maybe Paul will chime in???

regds,

Paul Thompson 'Hooligan' · 09-18-07, 06:53 PM

Funny things the Tach's in GTDs, I've seen both current and voltage triggered types and as I recall, I've used 8910/8920 tach adapters before to cure problems, but the real question is how long has the tacho been working? did it fail after a 40 mile trip? or has the car/tacho only done 40 miles? i.e. has it worked ok for a long time and just recently failed?

MSD Tach Adapter info attached:

Julian West · 09-19-07, 06:27 AM

Thanks Paul,

The rev counter worked fine for 3000 odd miles before i fitted the MSD ignition. Once I fitted the 6al the rev counter didn't work at all, so I fitted the MSD 8920 tach adapter.

From the diagram on the back of the rev counter I deduced the rev counter was 'Current triggered' & wired the MSD tach adapter accordingly. The rev counter worked fine for a couple of short trips (40 odd mile in total).

I did a close inspection of the wiring last night & it looks like the red wire from the back of the tach (which goes to green in the GTD loom????) has shorted.... however with the ignition on, both the green & white/grey wires show 12V.

I'm guessing that due to the short, something has fried... question is, is the tach adapter fried, or the tach??

I greatly appreciate any advice or opinions!

Paul Thompson 'Hooligan' · 09-19-07, 04:44 PM

Hi again

When you day there is a short, how did you measure it? - resistance to earth? It cannot be a true short to earth as it goes to +12v when the ign is switched on?

I have not come across an 8910/20 that has failed thus far, but have a few tachos. Additionally, I have had several 6ALs where they operate ok but their tach outputs do not... Just a thought..

Do you have access to a scope?

Julian West · 09-19-07, 06:52 PM

Hi Paul,

I guessed it was a short due to the insulating sleeve on the green wire being slightly melted.... I'm not sure if you can see this in the picture above.....

I don't have access to a scope. I have (since the last post) tested the MSD tach adapter accoring to MSD instructions & it appears to be working correctly.

I guess then that the tach itself is fried... Is there anywhere in the UK that can repair these, or is it a case of replacement??

Thanks for your help...

Gary Kadrmas · 09-19-07, 07:53 PM

Before you go the step of replacement, I think you should confirm what caused the current tach to take a crap. Are you sure it was the insulation on the green wire?

Have you tried to hook the tach to come other vehicle without MSD?

Paul Thompson 'Hooligan' · 09-20-07, 02:47 AM

As the tach is out of the car at present, a test on another vehicle would be a good idea, preferably an 'older simpler' model having having just points and coil...

Also, the slightly melted insulation on the green wire could have been caused by a slightly loose connector, which in turn would be a bit resistive and then get warm/hot when current is drawn across it. So check for tightness of connections there too.

Julian West · 09-20-07, 05:04 AM

Gentlemen,

Thank you for taking the time to post your opinions. I will try & find a vehicle with 'old fashioned' ignition, great idea.

From the information I received from MSD it looks like the tach adapter is working correctly. It's looking like a new tach is going to be the answer....

Hmmm.... lots of choice here! Anyone got any experience of Elliot tachs??

Elliot Design tachometers and rev counters: Products - Classic

Regards,

Malcolm · 09-20-07, 06:31 AM

Sorry no experience of Elliots but as an alternative and if budget permits, Stack do a good historic looking tacho (have one in Wendy's Lotus) that also has built in shift light and tell tale. It is designed to compliment Smiths instruments. However need to check diameter of any make you go for as GTD item is an unusal diameter!

Julian West · 09-21-07, 05:27 AM

Malc,

Good tip re the size, I thought it was 100mm??

Regds,

Malcolm · 09-21-07, 06:30 AM

I think it is just under? Best to check....

Julian West · 11-14-07, 12:01 PM

Just to finish this thread, I decided to go with the elliot tacho rather than mess around with the old GTD unit.

I ordered direct from the manufacturer & they were a pleasure to deal with. I would happily reccomend them, great service, great price & a really nice tach, details at Elliot Design

the unit fitted right in the dash, no problem & is wired straight to the MSD 6al, thus not needing the tach converter.