Tach accuracy question - Page 2

Tony Radford · #21 (**permalink**) 10-28-2003, 11:25 AM

It sounds to me like the Tach Adapter was designed to address the majority of the issues listed on this thread. My problem was that I'm using an MSD distributor to begin with. If I was using a non MSD unit, I would put up the $45 and install the adapter. I rewired my engine compartment over the weekend, which included moving the MSD ground. My problem seems to have been resolved with the new ground location.

REDSC400 · #22 (**permalink**) 11-07-2003, 07:29 AM

I too "suffer" from this SW tach issue which I and the previous owner called "Float" or "Bobble". In my case, this symptom seems to be a "feature" only when the engine is COLD. The darn thing just flips from 1K to 2K higher, usually never lower RPMs.

When the engine warms up it settles down to read right but is still very sensitive and not "buffered" it appears.

I have the UniLite Malloy distributior.
Mallory Power Conditioner.
Ballast Resistor
Mallory Pro Coil.
MSD 6AL ignition box

One thing I do not have is that tach Adaptor. This is the second 6AL box I have been running. The sympton was the same with each so I think we can deduce it is not a feature of a specific 6A box.

I wonder if there is some sort of inductance caused by cross talk of wires. My electrical training (Ham Radio) tells me that voltages can transfer from one wire to another if they are in very close proximity to each other. Ususally, this can be cured by separating the wires or coiling the wires or by the use of Ferrite Cores (little magnets at the end of each wire, you see these alot on Video Cables from you Monitor to the PC).

Does anyone know the part number of this Tach Adaptor? Is it an MSD part or an Mallory Part?

All Ears Here... Cheers

Tony Radford · #23 (**permalink**) 11-07-2003, 10:06 AM

Art, I believe the MSD part number is 8920. I posted it somewhere earlier in the thread. You can find it on the MSD website. I was quoted a price of $44 from a local shop here in Atlanta. My system stabilized a bit when I moved the ground connection, but it is still a bit unstable at times. I may ultimately purchase the Tach Adapter in hopes of getting a clean reading.

REDSC400 · #24 (**permalink**) 11-07-2003, 10:19 AM

Tony:
Yah, I think that is a good way to go. I did find that 8920 installation document on their web site. Thanks for that info.

However, I just received an email from MSD Tech Support in answer to my question (I gave them my particular configuration). They said that I should get the 8910 Tach Adaptor and not the 8920? I will confirm that. But then perhaps because you have the MSD distributor and I have the UniLite Mallory, that is the reason for the variance??

In any case, this sounds like a good winter project for me to undertake (adding to the list of things to do to the Cobra). I will get one of those on order (once I get confirmation) and we can compare notes.

Thanks

Len Hurlburt · #25 (**permalink**) 11-07-2003, 05:25 PM

OK Guys - At the request of SW (which by the way is in El Paso TX now) I removed my tach and sent it to them for inspection/repair. A copy of this thread was also sent to them which really woke someone up at SW. According to them, their tach should function OK with any combination of ignition parts listed here. Right now, they don't know what may be the problem but it seems based on the information here, the tachs work OK for a time and then start to act up. In my case it was about a year. I'll get whatever information I can from SW, and share with all after my tach is sent back to me. In the meantime, you can email SW using the address a few paragraphs up. Len

mgreene · #26 (**permalink**) 11-24-2003, 11:05 AM

Just started having the same problem with my SW tach in My Unique - 351C running a Davis Unified ignition system. Worked fine up until about a week ago, then started reading about 3000rpm too high and does not go back to zero when switch is off. Pulled the tach out of the dash and checked connections, stuck it back in and it seems to read OK for now. Seems like SW should do a recall/replacement...

Len Hurlburt · #27 (**permalink**) 11-24-2003, 10:52 PM

Guys - Here's a phone number for Ed Mantunas, the tech specialist at SW - 925-593-1965 x138. I'm sure he would love to hear your comments on your SW tach problems. He's the guy looking at my tach right now. Len

REDSC400 · #28 (**permalink**) 11-26-2003, 11:35 AM

Some preliminary good news. Having the day off today, I heated up the barn and got to work on installing the MSD 8910 Tach Adaptor unit. I separated all the tach cables from the bundle of wires that run to and from the MSD. So all the tach wires (2) were rerouted away from the power wires and such. Spliced in the MSD 8910 and mounted it up in the nose, in front of the radiator cowling. It apparently likes a cool environment.

Started up the engine and the SW tach is a smooth as can be. No jitter or jumping. These are preliminary findings. Now, since I did change the wire route as well as installing the 8910, I have changed two variables. Sadly, I am not sure which of these changes did the "CURE".

Just to firm up everyones perspective... I have the Mallory Unilite electronic eye distributor with 605 unit in the distributor, the MSD6-AL box, the Mallory Power Conditioner and a ballast resistor on the red power wire 12v small.

As I get some time on this configuration, I will let you all know how this works.

With fingers crossed...

Len Hurlburt · #29 (**permalink**) 12-28-2003, 07:09 PM

....well sort of. Without saying exactly what the problem is (if any), SW sent me a new tach (same model, just updated they say). During testing, they didn't find anything conclusive with my old tach. However, they are aware there are a significant number of us having similar problems with SW tachs. They maintain SW tachs should be compatible with all electronic ignitions including MSD. At least it's nice to know they will stand behind their product. Has anyone else heard anything from SW?

Cobra20646 · #30 (**permalink**) 12-29-2003, 07:32 AM

Len,

No.... probably because I've been working on other issues with my car and have put the SW thing on the back burner.
Nice to see that SW is treating you good...... and I guess time will tell if the new tach works and lives longer than the 'un-improved' version.

REDSC400 · #31 (**permalink**) 12-29-2003, 08:52 AM

Well guys, the Tach adaptor from MSD has indeed done the good deed for me. That combined with running the tach wires from the MSD box to the passenger's side of the engine compartment seems to have solved the problem.

The wire separation perhaps stops or eliminates something called "inductance" or in laymans terms, stops the wires from acting like a radio antenna picking up the ignition pulses.

The MSD tach adaptor went in easy and is mounted up near the MSD box. A rather simple install.

The tack is now steady and reads with accuracy. I did not change out the tach.

Cheers..

MAXVELO · #32 (**permalink**) 12-29-2003, 12:38 PM

sorry

G.R. · #33 (**permalink**) 12-30-2003, 11:57 PM

Quote:

Originally posted by Tony Radford

It sounds to me like the Tach Adapter was designed to address the majority of the issues listed on this thread. My problem was that I'm using an MSD distributor to begin with. If I was using a non MSD unit, I would put up the $45 and install the adapter. I rewired my engine compartment over the weekend, which included moving the MSD ground. My problem seems to have been resolved with the new ground location.

Tony where did you ground your MSD?--I have a 6al, MSD distributor, a Blaster coil--a SW tach that is erratic--it rarely "zeroes" after shutdown, works ok just cruising but when I put my foot into it, it will jump to 7K rpm +/- , I know the engine isn't turning over that fast . I thouht I had a good ground but now your post has made me rethink.

Tony Radford · #34 (**permalink**) 12-31-2003, 03:55 AM

Gary, I have a couple of grounds sharing a common crimp lug connected to a valve cover stud. In addition to moving the terminal, I replaced the previous lug as it was for a much larger than 1/4" stud, which left very little lug surface contacting the valve cover. I do occasionally get a little eratic behavior with regards to tach movement, it just hasn't been enough for me to justify ripping out my engine compartment wiring and adding the adapter. From the posts in this thread, I'm sure the adapter would improve the display. I have a lot of conduit and dress work under the hood and can't get excited about redoing it.

REDSC400 · #35 (**permalink**) 12-31-2003, 06:03 AM

Quote:

Originally posted by G.R.

SW tach that is erratic--it rarely "zeroes" after shutdown,

HI GR:
The SW tach is not a return to zero tach. This none zero reading after shutdown is "normal" behavior.

G.R. · #36 (**permalink**) 12-31-2003, 09:57 PM

Thanks Art and Tony

Going to re check/redo my MSD ground , then see what's up--may have to get one of the MSD tach thingies--one thing though my SW tach in my SPF always seemed to "zero" out when the ignition was shutdown

rbray · #37 (**permalink**) 04-24-2006, 04:49 PM

Hi guys,
Did you all ever find a resolution to this problem?

Silk · #38 (**permalink**) 04-28-2006, 12:50 PM

Some of us did.

I finally remedied my problem (Unique + SW tach + MSD Pro-Billet dizzy an no MSD box). I added a Pertronix rev limiter w/ tach output. Running the tach directly off the rev limiter apparently cleaned up the signal and and the tach works perfectly now. Go figure. One of the many mysteries of electricity.

CobraEd · #39 (**permalink**) 04-28-2006, 01:04 PM

Gary, Good for you guy. At long last, . . . and it only took 6 years

Ed

Silk · #40 (**permalink**) 05-01-2006, 08:58 AM

Ed,

You never want to rush into these things - - -