MSD Street Fire, MSD 8910, tach connection

[Mark Henry]

The instructions from MSD says it has a tach output on the box, or am I not reading you posts right?
Which MSD is it you say "Street Fire" is that a 6, 6al, 6al digital, 7, etc...?
What coil are you using?
How are you triggering the MSD? Points, Pertronix, other aftermarket trigger?


Wiring the Tach Adapter PN 8910 EIS

[RyanH]

MSD Streetfire does have a tach output Henry however, as I am on points I was advised this was not used and the white wire is used.

The MSD is operating fine, the tacho isn't, and didn't with the MSD tach adaptor so I am going by the writing diagram in this post and attempting to connect to the tacho from the original wiring. As explained the wiring from the tacho and the wiring from the distributor do not connect, one goes to the rpm sensor and the other to the rpm transducer. I guess my question is could I bypass the transducer and just use the sensor to connect the wiring? Does the transducer do anything with an MSD installed?

[Mark Henry]

Did you try the grey wire off the MSD box?

The instructions say that is the tach output. 1st Diagram.

http://documents.msdperformance.com/5520.pdf

[Jonny H]

Originally Posted by RyanH

MSD Streetfire does have a tach output Henry however, as I am on points I was advised this was not used and the white wire is used.

The MSD is operating fine, the tacho isn't, and didn't with the MSD tach adaptor so I am going by the writing diagram in this post and attempting to connect to the tacho from the original wiring. As explained the wiring from the tacho and the wiring from the distributor do not connect, one goes to the rpm sensor and the other to the rpm transducer. I guess my question is could I bypass the transducer and just use the sensor to connect the wiring? Does the transducer do anything with an MSD installed?

Please clarify what you mean by RPM Transducer. Is this an MFI car? If so, you mean the box that controls the fuel cut off, yes? If so, then the purple/black would connect to it as well as the Bosch can.

As the tacho used to work before you installed the MSD, I don't understand why you can't return the wiring to exactly how it was.? Unless of course, someone else made the changes.

Does your car backfire on the overrun?

[RyanH]

Originally Posted by Jonny H

Please clarify what you mean by RPM Transducer. Is this an MFI car? If so, you mean the box that controls the fuel cut off, yes? If so, then the purple/black would connect to it as well as the Bosch can.

As the tacho used to work before you installed the MSD, I don't understand why you can't return the wiring to exactly how it was.? Unless of course, someone else made the changes.

Does your car backfire on the overrun?

Car is not MFI....

[Jonny H]

Originally Posted by RyanH

Car is not MFI....

So what is the RPM transducer? Porsche part or MSD part?

[RyanH]

Originally Posted by Jonny H

So what is the RPM transducer? Porsche part or MSD part?

Porsche part, in between the can and the rear window defrost. I just bit the bullet and connected the two black and purple and connected at the can. No tacho. Now for the blue and yellow wire from the can which I realised is not connected to anything either?

[Trackrash]

RPM sensor, red arrow. These are on the 69 to 70 cars. Something about using the old style tac with the CDI. I have never read an explanation of what they actually do.

[timmy2]

Blue/yellow comes from the coil in the 70 engine harness to the transducer along with the black/purple wire from the points., there is also a ground to it.
I believe it sends the correct type of voltage pulse to the tach for that model year of tach.
Not used from 71 and newer.
Two wires on the '70 coil positive post. One from the CDI and one to the engine harness.

[Trackrash]

Sorry the poor scan. '69 diagram.

[RyanH]

This is what I have. Haynes manual states the centre silver box is the transducer, can is sensor. Next photo shows wiring at the back...

[RyanH]

Wrong way around! Last photo is wiring this is the set up.

As you can see the blue and yellow comes from the can, this is not connected to anything at the other end. I joined the purple and black and connected to can, this now runs straight to tacho.

Cheers...

[Trackrash]

FWIW, I rewired my engine compartment, eliminating a bunch of unneeded components.
See post #28

[Jonny H]

Ryan,

As far as I am aware only MFI cars have RPM Transducers (they are used to shut off the MFI pump) but you said you don't have MFI. I guess this is a non standard setup then. In this case, the RPM Transducer has no function so it does not need to be wired.

Just connect the Bosch can as per the colour wiring diagram that I originally posted and it should all work.

Confusing name by Porsche for the two components causes no end of confusion:


RPM Transducer also known as Speed Sensor (rectangular alloy box). Shuts off the MFI pump at certain RPM.


Speed Switch (round alloy can). Allows a CDI system to drive an early tacho by adapting the signal.

[timmy2]

1970 factory print SL35 photo for you. Sorry about the quality, it loses it getting uploaded.....

[Jonny H]

^. Non MFI I guess?

I read somewhere that in the US, a 'T' had MFI but everywhere else it had carbs. I reckon Ryan's 'T' has been converted to carbs but still has the MFI wiring, hence the redundant RPM Transducer.

[RyanH]

Guys, thanks for all your assistance it is really really appreciated. It would make sense the car historically being MFI, it was shipped from the US originally by the PO. Jonny, if I could ask you what 15 and 13 are on the diagram you first posted, there is a wire to them from the Bosch can.

I like the idea of stripping out all the excess wiring and de-cluttering the fuse fuse/electric board to simplify things.

Very confusing these wiring issues, I can fix most mechanical faults but electrics

Thanks again everyone for sharing your time and knowledge...

[Trackrash]

Sorry, I meant RPM Sensor. I will edit my original post. Thanks, Jonny.

[Jonny H]

Originally Posted by RyanH

if I could ask you what 15 and 13 are on the diagram you first posted, there is a wire to them from the Bosch can.

This sort of confirms the hunch, 13 and 15 are listed as "USA T only" (as European T didn't have MFI)

Legend:

[timmy2]

Hopefully a better photo of US SL 35
1970 only schematic. Email me for the readable original if you like.

EDIT: cannot get the same quality as original photo to upload... Grrrr.