MSD Street Fire, MSD 8910, tach connection
 

Hi.

Not having any success with my tach working on my 1970 911t. Quick question, does the black and purple tach wire stay connected to the distributor and then splice in the grey wire from the MSD?

I have a white wire from the MSD on the distributor now, and the white wire from the 8910 spliced into that.

No mention of the tach wire in the MSD instructions.

Cheers

The 1970 Tach needs a high-voltage pulse to operate. With the original 1970 wiring, this was derived by a filter network (intermediate/ballast unit), a small round can on the electrical panel. One side of that network connected to coil-positive and the other side went to the tach (black/violet wire). From what you describe, you have the black/violet connected to the distributor. That doesn't sound right, so maybe there has been some prior modifications.

The MSD has a specific tach signal output and unfortunately, it is a low-level (12V) square wave. So direct connecting that wire to the black/violet will not work. You need a device that will convert that 12V square wave signal to a high-voltage pulse. I'm not sure the 8910 will do that for you .. I believe it is supposed to, but I get repeated complaints that it does not always work properly. What does work is the TACH-ADAPT unit that I manufacture and is sold by our host. Pelican part number PEL-TA-V3. The output of the TACH-ADAPT unit is designed to directly drive your tach and therefore the intermediate/ballast unit (if still present on the vehicle) is not used.

Apologies for self-promotion, but if you have no luck with the current setup, and in the interest of getting your problem solved please consider the product.

Thanks Bob, if this doesn't work I'll give yours a go. For now though, and with too much money spent already I'll persevere.

I guess my question relates to the grey wire from the MSD which is not connected to anything at present, the instructions say this should go to tach. If the tach is the black with purple stripe where and how does this connect to the grey of the MSD? The end of the black and purple stripe is also not connected to anything since the conversion.

Car starts and runs well but no tacho.

Cheers

There are two sets of instructions that come with the 8910. One for triggering with the white wire and one for triggering with the magnetic pickup. You are using the white wire, so the black and violet tach wire connects to the spliced white wires as stated in the note on the diagram in these instructions. The gray msd wire is not connected.

http://static.summitracing.com/global/images/instructions/msd-8910_frm28850.pdf

Bob is correct but there is also another factor in play. The older tachos are very particular about duty cycle, requiring the mark (on time) to be no more than around 5% of the space (off time).

Provided the duty is correct, you can drive an old tacho with a 12 volt signal. Believe it or not, you just disconnect the 12V feed from the back of the tacho and connect the signal wire to the 12V terminal instead. That's right, just GND and signal going to the 12V terminal. No connection on the I terminal.

Thanks JonnyH. Forgive my inexperience but when you say duty cycle, are you referring to the dwell?

I spliced in the black and violet to the already spliced whites of the MSD and tach adapter without any tach movement.

Will set dwell and timing this morning. I kept the points as my experience with Pertronix has not been positive.

RyanH, yes in effect it is the dwell. If the MSD a is outputting a conventional square wave, it won't drive the early tacho. It needs a very short on time (5%) and a long off time (95%). I would imagine Bob's box does something similar.

Have you done the install yourself? How much time have you spent on it so far?

I fitted the MSD and tach adapter, then had an auto electrician wire it up. He tried both sets of instructions with and without the diode and it didn't work. It was only on my own inspection when I got home from work I noticed the black and violet tach wire was not connected to anything, so how could it possibly work?

I'll set up the dwell, going by your explanation would the dwell still be set at the original degrees; without checking I think it's 37 +- 5.

Thanks for your help...

I just realised you have an early car. In your '70, the tach is driven straight from the from the dizzy. This is the black and purple wire. It should connect at the distributor on a double spade. The other end goes to the 'speed sensor' (round silver Bosch can) then on to the tacho. I can't see any reason to have disconnected it when fitting the ignition.

Thanks Jonny. I thought it should still have the two connections on the distributor, trust a professional eh! Do you know what colour the black and purple tach cable changes to when it exits the electrics panel heading for the tacho dial? It isn't the same black and purple stripe. I'm guessing this is where it gets spliced to the white MSD and Tach adaptor, ie before the tach wire enters the electrics panel and Bosch cylinder/barrel thing? Thanks Jonny I appreciate it.

Ryan H, it should still be black/purple all the way to the Tacho.

It's confusing because it goes through the 14 pin connector twice. The numbers in bracket refer to the diagram.

Distributor(5) ->connector(84) ->speed switch (11)

Speed switch (11) ->connector(84) -> tacho (off page label 43 and 67 on part 1 of the diagram)

The two black/purple wires must be joined at the speed switch.

Here's part 2 of the diagram which shows the routing. It is for a '71 but I'm pretty sure the '70 is the same.

http://www.pelicanparts.com/911/911_...971_Part-2.jpg

I'm happy to help you out.

[I will just get a bit of promotion in here though. Although our CDI+ has a higher ticket price, it is designed to work with all the original Porsche components including the tacho straight out of the box. If you value your time or pay a garage for install, our units can actually work out cheaper.]

Great, thank you Jonny. Thinking about it, there should be no need to trace it back through the pin connectors should there? It was working fine before. I can splice the white and the black/purple as it leaves the distributor so that the black/purple carries on to the Bosch canister and the tacho, and the white carries on to the MSD and Tach adaptor?

I'll give it a go what's the worst that can happen!

If the tacho is being driven by the dizzy as per the original fitment then surely you don't need an adapter?

If, however, the tacho is being driven by a signal from the MSD and it has a 'normal' square wave output then you would need an adapter.

So, I have car running and starting first time; thanks to all for your input. Still have no tacho working though, I've been through many online instructions about the Street Fire and MSD tach adaptor and what bugs me is the grey wire which is the tach wire. At the moment this isn't attached to anything. Should this splice into the purple and violet tach wire attached to the distributor? Some say the grey is only for a magnetic pick up but I can't find this info anywhere, they list another two colours, which I can't think of right now!

Somebody must have had this before?

Thanks...:confused:

Yep, a lot of people have this problem,.
Here's your options and I've given you three more.

1/ The MSD adaptor. pro if it works great, con doesn't always work.

2/ Autometer Monster Tach. pro fits hole exactly, con no turn signal and high beam indicator lights, not "correct", mo' money.

3/ TACH-ADAPT. pro it works, con mo' money.

4/ Rebuild you tach with modern internals, "how to" link below. pro cool DIY mod, if you into this kind of thing it's a cool project, con mo' money and time, takes a bit of skills and soldering talent.

914World.com - The fastest growing online 914 community!

5/ BTW The guy who wrote the above will also do this for you. pro great guy, con mo' money.

6/ North Hollywood speedo. pro nice, con way-way mo' money!

Damn hard to get past the mo' money bit..... ;)

SmileWavy

Still struggling 10 days later?

I still don't see why you need the MSD to drive the tacho. Just wire the tacho as it was originally and forget connecting it to the MSD.

Alternatively, fit a unit that has been designed to work with the 911. ;)

This ^^^ Re-connect the black/purple wire in the engine harness to the points connection at the distributor as it was originally. They should still fire the tach if nothing in the rear fuse panel harness was messed with.

Hi again Jonny!! I've been driving without a tacho for a while and have decided to delve into this topic again. On further inspection the purple and black goes from the dizzy through the 14 pin connecter and on to the rpm transducer. From the tach end it goes through the 14 pin connector and into the rpm sensor (Bosch can). So the two don't even meet. Advice? Should I connect the rpm transducer to the Bosch can so it flows through both, or bypass the transducer and connect at the Bosch can? With the MSD do I need the transducer? Thanks Jonny, anyone also lease free to advise.

Cheers...

Ryan,

I'd just put the tacho connections back to how it was originally in the diagram I posted. I can't see any advantage in using the MSD tach output to drive the tacho.

Hey Jonny. There are no connections from the MSD except power to the dizzy. The purple and black wires I talk of above were like that originally however, the original black from the dizzy would have gone to the transducer according to the diagram. This is not needed now due to the MSD. if I go by the diagram I should just bypass the transducer and connect the black and purple from the tacho end, to the black and purple at the dizzy end, at the Bosch can. This would create a flow that bypasses the transducer. Can you see a problem bypassing the transducer?