SW Tach "wandering needle"

Dangerous Doug · #1 (**permalink**) 06-08-2009, 01:18 PM

A couple of times now I've noticed my SW tach needle wandering incongruously to my engine revs.

Once I looked at it and it was happily reading 2400 rpm, when I'm thinking, "I've got to be turning close to 4000 rpms..." When I was shutting down this morning, it read 3000 rpms when I was at an idle. It wanders, then settles, and seems to work okay.

Don't tell me: Check the ground connection...

Anyone else have this with their tach?

I cant' think of anything that would have loosened any ground wires...

DD

Pete Munroe · #2 (**permalink**) 06-08-2009, 08:16 PM

DD,

I had the same problem, wandering SW tach.

It would be ok for a while then take off on its own, stay in one place...

The best price I found on the internet 3 or 4 years ago was through XK's Unlimited. I think it was about $90+...

http://www.xks.com/

The new tach has DIP switches to set the number cylinder, the old tach had a rotary switch to set "4-6-8".

The down side on the new tach is the face of the looks like the rpm increments were drawn by hand...but, it works.

Pete

strictlypersonl · #3 (**permalink**) 06-09-2009, 04:19 AM

Are you driving the tach through an MSD tach output?

Bobcat · #4 (**permalink**) 06-09-2009, 05:33 AM

You didn`t say whether you are using an MSD system or not , but I ended up having to use MSD`s tach adapter to get my SW tach to read correctly consistently . The other thing you might check if you are using and MSD system is your wire routing . The tach signal wires must run away from the power wires . I believe ERA shows the routing in their manual .

Bob

Dangerous Doug · #5 (**permalink**) 06-09-2009, 07:24 AM

I have an MSD distributor, but no box.

It just started wandering a couple of days ago. I thought it might be a loose ground, but that tends to make the needle go "off" or down. I see it wander upwards, too...

DD

Pete Munroe · #6 (**permalink**) 06-09-2009, 09:57 AM

Guys,

In my ERA FIA #2027 I have a 6AL MSD, a tach adapter was NOT necessary for this setup with the SW tach.

( unfortunately I have a NEW MSD tach adapter I bought before I discovered the SW tach works fine with the MSD 6AL in the ERA application.... if someone wants to buy the adapter, send me a e-mail )

Check the back of the tach to see if it an early one, rotary switch, or a newer one with the "computer like" DIP switches...if its the earlier one I would guess that is the problem.

Give Doug a call at ERA, he has had some experience with the problem

Pete

Dangerous Doug · #7 (**permalink**) 06-09-2009, 01:22 PM

I seem to recall DIP switches.

Working fine---today...

DD

Dangerous Doug · #8 (**permalink**) 09-22-2009, 05:39 PM

Well, I'm back to the wandering tach needle problem.

At Doug@ERA's suggestion, I jumped the ballast resistor and then tested the tach. It actually seemed to help, but didn't rectify the problem. As I climb in rev's the needle stops going up at about 3000 rpm, even though I'm obviously spinning the motor faster.

I did buy a new tach---which I haven't yet installed. I think tonight I'll put the ballast resister back in line and then wire the tach directly and see if it reads differently than the installed tach. I don't recall if the current tach as the rotary switches or the DIP, but the new one definitely has DIP switches to select the number of cylinders.

DD

Argess · #9 (**permalink**) 09-22-2009, 05:47 PM

MIne has been "wandering" for the past couple years. But as it was fine for years before that, I'm sure the problem is inside the tach. For the record, I did try a tach adapter, and I htought it seemed fine for a bit, but then back to the normal "wandering".

I strongly suspect a bad solder connection or leaky capacitor inside the tach, however the tach doesn't come apart as the front bezel is swaged over the case.

I havn't bought a new one as the pics I've seen on line don't seem to match the old unit (cosmetically,, ie.e the face and needle).

So, I am thinking of taking the tach,, cutting the case around the middle, repairing the circuitry, and then fabricating a band to hold the case back together. Probably an internal band with countersunk screws through the case so the unit will slide into the dash easily.

And with the $100 I save, I can.........hmmm, what can a person do to his Cobra for $100?.........

Dangerous Doug · #10 (**permalink**) 09-22-2009, 05:51 PM

Quote:

Originally Posted by Argess

And with the $100 I save, I can.........hmmm, what can a person do to his Cobra for $100?.........

Uh, put gas in it...

I found the "Stewart Warner Deluxe Series" on SummitRacing.com, which matches what I have in my ERA. My Cobra was purchased about 4 years ago.

I MAY have an extra, faulty one (tachometer), assuming I find the problem to be the tach itself. I have a matching speedometer with a broken odometer, in case you're interested.

DD

Argess · #11 (**permalink**) 09-24-2009, 02:33 PM

OK...here's some news. Today I changed spark plugs. All good but 1, and it was a mess (whole 'nother story, but engine is fine). Put new plugs in it and tach works perfectly. Perhaps it is co-incidence as it doesn't make sense. Input signal to tach from coil (-) terminal should be the same no matter what is going on at the High Voltasge side, but possibly it isn't.

So, perhaps new plugs and wires might fix your tach?

ps: if mine starts wandering again, I'll report back here that I was on a false lead with this

Dangerous Doug · #12 (**permalink**) 09-24-2009, 02:58 PM

Interesting. Excessive electrical noise from the wires and plugs?

I tried to read the pulses with my DMM. Too much electrical noise to pick up the signal. Hhmmmm...

DD

Argess · #13 (**permalink**) 09-24-2009, 03:08 PM

You need an oscilliscope to watch the "pulses"......one might see 7 pulses and then an 8th that looks different. I'm sort of lazy, but I should hook mine up and have a look.

I might be on the wrong path, but my tach hasn't worked this smooth for quite a while, although there have been better days than others.

ZOERA-SC7XX · #14 (**permalink**) 09-24-2009, 03:10 PM

You must ground it to a dedicated ground wire on the harness, not just bare metal. That did the trick for me.

Dangerous Doug · #15 (**permalink**) 09-24-2009, 09:00 PM

I am using a harness ground. Admittedly, when I first wired it six months ago I hadn't grounded it correctly (neg needs to be tied to ground). I think with the floating ground it may have damaged the circuitry and now reads incorrectly.

I've already purchased a new tach, so I'll put that in, test it. I'll check the ground again in the process, as well as the +12V. Beyond that, I guess I may be playing with better shielded spark plug wires...

DD

Rick Parker · #16 (**permalink**) 09-24-2009, 09:32 PM

There are some internals that are unique to SW that typically fail with these results, its not unusual. There are shops that can put updated VDO type internals in the SW case.

ZOERA-SC7XX · #17 (**permalink**) 09-25-2009, 03:04 PM

My tach was going crazy. once I found the correct ground wire, it worked perfectly. Don't give up. It's probably not the tach itself.

PANAVIA · #18 (**permalink**) 09-26-2009, 06:23 PM

99/100 it is a ground wire that is getting some sort of voltage feedback. --

Argess · #19 (**permalink**) 09-30-2009, 11:45 AM

Crap....after about 40 miles, tach started acting up again, so the plug change didn't affect it after all.

Mine's coming apart this winter. If I ruin it, I have to buy one. If I don't try to fix it, I still have to buy one. If I fix it, then I won't have to buy one, will be happy, plus I'll post what I find in case someone has a similiar problem.

But for right now, I'm forgetting about the tach and will try to get as much driving in as I can, before putting the car away for winter.