Non-working Tach
 

My car is a 71 911T and I have a tachometer that is from a period after CDI's came into play but before Porsche had VDO change the input signal to the tach. Here is a thread from Early S Reg that talks about the Intermediate Unit that is required to convert the signal for the tach. Post 5 by 304065 is the relevant one.


http://www.early911sregistry.org/for...ing-Tachometer



I figured this out when I had a 2.2 liter motor in the car and had the tach working fine. I just finished installing an 82 3.0L motor with PMO's and MSD 6AL. It's long story with almost a year of waiting for a couple critical parts. Meanwhile, I forgot about this little detail, so I didn't tell the member (timmy2) that made my engine harness to include another wire in the harness to allow for this. timmy2 made an awesome harness, btw. I highly recommend him!


I remembered about the Intermediate unit and confirmed I had wired the blue/yellow wire from it into pin 10 of the 14 pin connector on the male side. Once I confirmed that pin 10 was unused by the harness he made, I connected a blue wire on the female side and ran it to the + terminal on the coil.


I confirmed continuity from the + side of the coil to the male connector on the bottom of the Intermediate Unit where the blue/yellow was plugged in. The tach still didn't work, hmm?


I confirmed continuity from the black/purple at the base of the intermediate unit to the black purple wire on the back of the tach. Seems like it should work. I would think the next step would be to check the voltage at the tach with the engine running, but was sure what a big square wave voltage would do to my multi meter as the 304065 in the link above says its several hundred volts, not 24v.


I thought maybe someone here could throw some light on the subject.

OK, I just heard back from timmy2, he suggested that because I no longer have points, the coil signal through the Intermediate Unit won't work. He suggested a jumper between the blue/yellow and the black/purple at the Intermediate Unit should produce a tach signal that will work. I'll try this today at lunch.

We have continuity from MSD to tach on one wire now. Still no go.
Tach adapter must be what is needed.

Nowadays you can purchase a digital pocket sized oscilloscope for under $100. Hardly laboratory grade, but super useful for the frequencies and voltages (a 10X reducing probe is good to have along with a regular one). The 6 pin CDI box sends a really nice square wave to the tach at about 12VDC. The CDI has circuitry inside just for the purpose of creating this, unlike the 3 pin predecessor, which just forwarded, so to speak, the signal from the points to the tach.

I have a surplus tach adaptor. BTW, Timmy2 fixed me up with the correct connector to put an MSD in my SC.
Drop me a PM if you are interested in the tach adaptor.

If you are using an MSD, the intermediate adapter that was powered from the coil won't work for you anyway because of the multi-spark of the MSD. Also the MSD might not be compatible with that old adapter for other reasons. The older tach requires the high voltage kick of the Kettering. The MSD tach output is a 12V square wave. Bob Ashlock makes a tach adapter that will convert that 12V square wave signal from the MSD tach output and give it the high voltage component necessary for the tach to register. Although I do not own a 911, I have a 911 tach (early like yours) and Bob's adapter works like a charm with it. Fred

Fred, thanks for the input. I found an MSD 8920 used for a good price. Do you know if it will work?

There is another trick you can try and it costs nothing. Remove the 12V line from the back of the tacho, keep the GND. Do not use the intermediate unit at all. Take the tacho drive wire from the spark box and connect it to the 12V pin on the tacho.

And before you ask...

Yes, it sounds crazy. Yes, it only has two connections. Yes, it has no power connection.

This has worked on a number of '69 cars we have seen.

Edit, I have done what you are suggesting other than pull the 12V line from the back of the tach. We have the tach drive wire (grey wire on the MSD) going directly through to the purple/black with in Intermediate Unit in the path. The tach didn't move. I'll pull the 12V as you suggest and see if that makes a difference.

I'll give it a try.

Thanks

Yes the 8920 will probably work. If you follow Jonny's suggestion, you will have to remove the usual 12V feed to the tach otherwise the 12V square wave from the tach output will be swallowed up in the 12V already being supplied, IE no pulse. Doing what Jonny has suggested is very similar to the way AC tachometers used on GM products worked in the early sixties. They had only two wire connections, one to ground, and the other to the coil negative. That coil negative connection both powered and provided the impulses to the tach. Sounds like the VDO tach might perform the same way, although hardly designed for it. I will have to test it on my tach. Thanks Jonny! Fred

Went by the house over lunch, pulled the 12V wire from the back of the tach and fired it up. No luck. Tach doesn't budge. I've got someone sending me an MSD 8920 for good price to try.

I tried Jonny's suggestion on my 1969 911 tach driven from the points trigger on one of my CDIs. It worked but only registered slightly over 1/2 the rpm it should have registered, but the signal was clean down to 8V. I didn't try it with an MSD tach output. Still, it's worth trying before going out and buying an adapter. I've found that particular VDO tach quite sensitive and will register from a few turns of wire around the high tension lead from the distributor to the coil, although that method would not work with a multi-strike unit like the MSD with the spark events so far apart. Might work with Jonny's though as each spark is close enough together to simulate a single spark. Fred

The 71 tach (like earlier ones) still needed a narrow high-voltage pulse to trigger. With the original Bosch CDI, the 'intermediate unit' (a filter network or 'ballast' as some call it) was used to condition the coil-positive discharge signal to make the tach happy. With an MSD, the discrete tach output signal is only 12V rectangular. There are numerous 'tricks', (such as MSDs own adapter) sort of emulates this. The little product I make (TACH-ADAPT) and sold by our host is commonly used with the MSD to solve this problem. If at some point in the troubleshooting process you tried hooking the tach direct to the coil-positive terminal WITHOUT the ballast in-between, it may have been damaged. You can't really expect to use the coil-positive signal from the MSD because there are multiple pulses occuring at lower rpms which of course would make the tach read all whacky.

Thanks for your input Bob! When I tried hooking it to the + side of the coil I had the IU in between. If the used MSD doesn't work, I'll be buying your adapter.

adapter installed, no start
 

Got back in town after a few days away and installed the tach adapter. Because the IU was connected to the + side of the coil, I followed the current driven instructions for the MSD 8920 tach adapter. That didn't work so I connected it per the voltage driven instructions and it worked! I only ran it for a few seconds, in fact it's been a little hard to start since I leaned out the idle mixture by turning in the mixture screws.

So, I installed the tach adapter in a little neater and tucked all the wires away, etc. Went to start it and it just popped a few times and then spun but like it had no spark. Cranked it for a long time and even jumped it as the battery started wearing down. Spent some time trying to determine if I had spark and found the MSD instructions for jumping the connections in the mag signal (green) wire and grounding the coil wire and looking for a spark when I break the jumper contact. I have spark, hmmm.

This morning I pulled cyl 1 spark plug and it was very wet and smelled like gas. Here's a pic. It had dried out by the time I took the pic:
[IMG]http://i592.photobucket.com/albums/t...psbghwjwed.jpg[/IMG]

All six of them looked the same and they were all wet. I left them out and headed to church and lunch with the family. I'm currently running NGK BP7ES plugs, but after reading some posts about flooding, I picked up some BP5ES plugs as that seemed to be the recommendation for a PMO carbed 3.0. Should I wait a couple days and let the cylinders dry out? I need to change out the full pump anyway as my Faucet is putting out just over 2.8 PSI and I bought a Mallory 7 PSI pump to replace it. Am I on the right track with thinking it's just flooded?

Confirmed with Richard at PMO that my 3.0L with 46 PMO's should do well with BP5ES plugs and timed at 10 degrees btdc to start with.

From the looks of those plugs, maybe any new plugs would be better. Though if that is carbon fouling, a hotter plug like you have gone to can't hurt, can it?

Flooding usually clears up pretty quickly with the plugs out - just crank the engine some, that will dry it out. With no plugs, no new fuel should enter the cylinders.

Did you pull a plug, attach its wire to it, ground the plug (often just lying it on bare metal on a grounded part of the car will do), and try to start the engine while looking at the plug (a helper is useful) to see if there is a spark is a good test of the whole system.

I'd not suspect your 2.8 psi pump of being the culprit for the flooding, though. Fuel bowl levels low enough?

Walt,

Believe it or not, those were new plugs when I started this engine in the car for the first time a couple weeks ago. I've taken 1 drive in it of about 10 miles. The rest has been at idle setting timing and trying to get the idle fuel/air mix leaner.

I don't have a handy assistant, but I will ground the plugs to see if I'm getting spark.

Thank you for the feedback!

Tom

One of those portable remote starter button setups is really handy for those without a handy assistant. On my two race cars I have contemplating permanently wiring in a starter switch in the engine bay for just such things.