Tachometer Doesn't Work

[jhale356]

Recently installed a 70 2.2s motor in a 69T. Since the swith I have not been able to get a tach to work. Have a 7300 RPM S tach which has been bench tested and found to be in good working order. A digital multimeter tach will work at the dash. Voltage to the tach and good ground. Wire direct from the distributor to the dash makes no difference. Any ideas? Have checked the other threads but can't come up with an answer.

Thanks, John
'69 T
'80 SC

[Early_S_Man]

John,

Do you know what year model the tach is from or its' date code on the back? Some versions around '69-'71 needed a ballast unit, and did not hook to the points, but, rather, the coil primary through the ballast unit, essentially a Voltage divder network. The 'S' tach was different than the 'T' tach in '69 because the 'S' engine had a CDI-unit, and the 'T' engine didn't! The 'T' models didn't have the ballast unit, but the 'E' and 'S' models did, and that may be the key to getting it to work.

[CamB]

From my sample of 2...

My '69T's tach was triggered by the coil directly (no CDI box) (thus I needed an MSD tach adaptor to make the tach work with the msd unit. Note, this little unit appears to be some sort of transformer which ups the voltage from the points trigger). Trigger was through a black and purple wire.

Matt Smith's 72T's tach was triggered by (I THINK, but am not certain), the trigger wire from the distributor.

This makes me think they have different voltage requirements (or something like that).

I think the RPM gauge you measured with is designed for coil type voltage. I think the S tach might work from points type voltage.

This ought to be enough empirical information for someone to figure out the correct answer.

BTW, are you running the CDI box from the S motor as well? If not, you may find plug fouling a problem (although I would expect the tach would work from the coil, so I'm guessing you have a CDI box).

I had some luck having a dig through the wiring diagrams in the Haynes manual. Pelican also has a couple:

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1965_911S.jpg

4 is distributor, 5 is coil (both bottom right), 26 is tach (middle right). This is how my '69 was connected. Follow the black/purple wire.

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1973_CIS_Part1.jpg

This is trickier - a '73T (with CIS). Tach is number 67. Black/purple wire goes to #43.... which connects to:

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_1973_CIS_Part2.jpg

The points trigger - bottom right - dizzy is number 5.

Which is what I remember from when I looked at Matt's car.

Unfortunately this exhuasts my knowedge...

Cam

[jhale356]

Warren,

I'll check the date on the back of the tach and let you know. Thre original from the 69 does not work.

Cam,

Replaced the ignition with CDI from a later model Although some suggested running the T ignition, I was also concerned about the plug fouling.


Thanks, again. I'll keep you posted.

[jhale356]

The blue date mark on the back of the tach is smudged, but appears to be 7-68 or 7-69. With my luck, it is within the date range of a 69 that does not work directly from the distributor. Since the tach from the '69 ran through the dashpot or whatever the device, can I now run the black/purple from the speedswitch through the components from the '69? If not, how can I make it work?

Thanks, John
'69 T
'80 SC

[CamB]

Based on my limited knowledge, two things might work:

1. connect it to the coil (negative side I think, BUT WAIT UNTIL SOMEONE CAN VERIFY THIS WON'T TOAST THE TACH!).

2. use an MSD tach adaptor on the distributor trigger wire (also hooks into a 12v switched source). This results in a small (but quite ugly) red box in your engine compartment.

Someone might know...

Cam

[jhale356]

The tachometer is now working. The key was identified by Warren as a ballast. The ballast turns out to be a series resistor as shown in the parts diagrams. The only one still available is P/N 901 602 853 00 and is about $28 through the dealers.

John
'69 T
'80 SC