Wanted ballast resistor relay unit for MFI 911

[gridgway]

In my conversion from a carbed engine to an MFI one, it seems that I need the ballast resistor which is hard to find, it's this one apparently one of two on ebay that I can see in the US...

https://www.ebay.co.uk/itm/284723810882 ... SwAyViPQ9h

Does anyone have one in the UK that is surplus to requirements please?

Thanks
Graham

[inaglasshouse]

Hi Graham,

As you probably know, it's not the MFI which requires the ballast. The ballast resistor's purpose is to modify the signal from the CDI box, thus allowing the use of an older style tacho (originally designed to work with conventional ignition). It's a 1969-1970 thing, while they were using up the old tachos....

Having stated the obvious, questions are: what year is your tacho, and are you using CDI? If CDI, is it a standard box, or a Classic Retrofit Jonny CDI+ replacement? If CDI+, it can drive the tacho direct.

... which is no help if what you really want is to buy one....!

best, Richard.

[Jonny Hart]

^ What he said.

[gridgway]

Thanks both. The car is originally a 69 T from the US.

It was previously running a 2.0 engine on carbs with a CDI box. So from that I must assume it's a later tacho as it works ok. It's it possible to verify this by how the tacho looks?

I'll talk to the guy doing the work again today to get to the bottom of it. He's a very good guy who does car resto and runs my race cars, but isn't a specific Porsche specialist.

[inaglasshouse]

It was previously running a 2.0 engine on carbs with a CDI box. So from that I must assume it's a later tacho as it works ok.

Seems likely. Either that or the car previously had a ballast resistor or other solution fitted.
Anyway, if nothing has changed in the CDI -> Tacho wiring run, should all be fine...

Tachos are date stamped
I think (but this is only from fallible memory) that Porsche had already properly fixed the tacho problem by the time they started doing US "T"s on carbs with CDI.

[gridgway]

Thanks very much, makes sense!

Looks like I don't need a ballast resistor. As you were everyone.

Cheers
Graham

[Lightweight_911]

.

This shot shows the difference between '69 & '70-on tachos (though they're for an E model):



1969 tacho (on right) didn't have the main beam symbol above the blue warning light (& was held into dash with clamps on threaded studs on the rear).

1970 tacho looks the same as the '71-on tacho from the front except the part no. (visible at the top of the dial right under the bezel) ends in '00' whereas the '71-'73 version ends in '01' ...

.

[gridgway]

Thanks Andy, super info

[gridgway]

So here's what it looks like, but not sure I'm any the wiser!
911 tacho by graham Ridgway, on Flickr

[Lightweight_911]

.

That looks like the original '69 911T tacho ...

.

[Jonny Hart]

In the early CDI cars, the tacho is driven direct from the coil 'A' terminal. Giveaway is the extra wire attached - from memory it's either blue/yellow or red/white.

[gridgway]

In the early CDI cars, the tacho is driven direct from the coil 'A' terminal. Giveaway is the extra wire attached - from memory it's either blue/yellow or red/white.

Thanks both. We swapped the (later) new complete motor in and the old motor out including coil and wiring, so that wiring won't be there I imagine. I assume we can just add it and the tacho will work ok?

[gridgway]

In a picture I have of the old engine install, there is a black/purple wire attached to a coil terminal. In wiring diagram L57, that colour wire goes from the coil to the tacho. But in diagram L57, there is no CDI box. Hmmm

[Lightweight_911]

.

Was the previous engine the original engine to the car ?

If it was, maybe the CDI unit failed so, as a cheaper option (to getting it repaired or forking out for a replacement), a previous owner reverted to the earlier 'Kettering' ignition system.

If you have access to the previous engine, see what coil is fitted.


.

[gridgway]

.

Was the previous engine the original engine to the car ?

If it was, maybe the CDI unit failed so, as a cheaper option (to getting it repaired or forking out for a replacement), a previous owner reverted to the earlier 'Kettering' ignition system.

If you have access to the previous engine, see what coil is fitted.


.

It wasn't the original motor. I don't have access to it any more, but have this pic. Does it help identify the coil?
It is certainly possible that it was running on a kettering ignition with the CDI box there, but not being used.
IMG_20200711_112607