Intermittant Tach Short?

[PeteD]

Okay, I've put a year into this problem and think I might finally be on the right track.

First, my 75/1.8 is carbed, 205 dizzy and regular points, MSD ignition system. So no fuel injection sensor answers to this problem please...

Car starts cold just great. Idles well. Runs great, no awesome, for several miles, then all of a sudden the tach goes to zero and the engine dies or sputters. Sometimes it comes right back, sometimes it takes a while. Switching over to standard (coil/points) ignition does not change anything. The folks at MSD very kindly replaced the MSD unit thinking there was an intermittant short in it. Coil and condensor have been replaced. The 12V power to the ignition system stays at 12V just fine. That is not the problem...

Finally today when I could not get the car to re-start (this does not happen cold, only warm), I disconnected the tach wire at the ignition end and VROOM, started right up and idled just fine for an hour. Now I suspect the tach. Has anyone ever seen a tach have an intermittant ground problem? Could there be an intermittant ground short on the relay board (the tach wire runs through the relay board)?

PS: I read the string where the problem ended up being a bad connection on a pertronix unit. That does not apply here.

MANY THANKS!

Pete

[John Rogers]

Did you try one of the transformaers for the tach that MSD sells? I think the tach wire runs up the tunnel from the coil connection to the tach w/o any other connections, etc. You have two possibilities, either a bad tach or bad wire to the tach. If you do not have a spare tach try running a new wire up the tunnel and use new connectors and see if the problem goes away, if so then you are in good shape. If it still happens, then you will have to replace the tach. Good luck.

[Conrad W Peden]

My car had the same problem. Hasn't done it since I replaced the tail-light assemblies.

Only did it under braking, mostly harder braking.

Your guess is a good as mine.

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CWP/VIR
72 914 L20E in rusto.
73 914 L20E 2.0L in resto.
http://members.rennlist.com/a914lover

[Dave at Pelican Parts]

Yes, it could be a problem inside the tach. It could also be a problem with the relay board, or some of the wiring from the coil to the board or the board to the tach.

Try unplugging the tach itself (just the "signal" wire) and taping it up so it can't ground on anything. If that works, you have just verified the wiring and the relay board. If it gives you problems, you are pretty sure that the tach is not the problem.

You can keep narrowing it down after that.

--DD

[PeteD]

Dave,

MSD told me yesterday that grounding the MSD-6A tach output will not kill the
engine. (I might test it for myself). If they are right, I am barking up the wrong tree.
Before I pull the dizzy (again), do you know if the 205AA dizzy has a ground strap
(that might be loose)? And if so, where should it be?

Thanks,

Pete

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Pete Dubler
See my 914 at http://dublerfamily.com

[Dave at Pelican Parts]

[Already answered in direct e-mail, but repeated here for the benefit of everyone else.]

The ground braid runs from the points plate to the inside of the distributor body. I think that one of the screws that holds the cap retaining clips goes through into the end of the ground braid. The braid should be pretty obvious after you take the cap and rotor (and dust cover, if any) off.

--DD

[PeteD]

Dave, et al.

It finally warmed up a bit again and I got to work on and drive the car. What I have concluded:

1- The problem is not the tach wire. The problem still occurs with the tach disconnected.

2- The problem is not the 12V wire to the coil. It reads around 12.4V all the time.

The problem is still intermittant. I can't seem to figure out what makes it come and go. I fiddle around for a while after it happens and then it magically goes away.

I am thinking that the problem must be in the dizzy (a 205AA). The ground braid is in tact. What should I check in the dizzy? And why would it come and go? (could the dizzy be disengaging from the drive temporarilly? The spring btwn the drive and the dizzy shaft is in place (last I checked). The dizzy clamp is tight and seated all the way up on the dizzy. What else could it be? Sticking advance plate? Any ideas? Remember, it just misses a bit then dies from 2000 or 3000 rpm or even idle, then won't restart for a while. The engine oil was not even up to 120 deg when it happened today.

Thanks again for the help.

Pete

[John Rogers]

When the engine is cold measure the coil resistance, pri to sec and then measure it when the engine dies. If out of spec that could be the problem. Check the resistance of the rotor button as they can change when hot too. And if you have a condenser on the side of the distributor, check and/or replace it. Finally you did not say in your original post if you had spark when the engine dies?

[codex]

I have a .050 with Crane XR-700 and I would have everything go out at 75 m.p.h. on I-25 north of Denver. It was just like turning off the ignition. Thought it might be the ignition switch since it was probably original and a bit sloppy. Replaced it and it still happened. I had learned by this time to stay in the far right lane. I pulled all of my ignition connectors, cleaned them, put on electolite grease and since then have had no trouble with that problem. It may not be that simple in your case but I know it is very frustrating. It was interesting that as soon as I stopped and turned the ignition switch off that it would immediately start again and run for another 25 miles.

Dave Chamberlin

[PeteD]

John,

Thanks for writing.

1- I have had this problem with three different coils. Now I am running with a new MSD Blaster coil.

2- I have replaced the condensor. No change.

3- I can generate a spark by manually openning and closing the points. It is a very powerful spark and will jump right across the cap from the center point to one of the spark plug points when the cap is off the distributor. The spark is so strong that you can hear it quite easily.

4- If I run a 12V test light (ground side) to the green wire (condensor/points), with the green wire off the coil, and roll the car in gear, I can see that the points are openning and closing.

5- Put a new rotor in today. That did not "instantly" fix the problem, but the car did start up eventually and ran great. (I had to get home to take the kids to the dentist, so I have not gotten stranded again...3 times today was enough).

On my list of things to check:

Theory A: dizzy temporarilly disengaging from drive (not supported by observation 4 above, but easy to check)
- check that spring under dizzy is in place
- check that hold down clamp is actually all the way on the dizzy

Theory B: something is sticking in the dizzy
- lube pivot points, oil pad, holddown ball, etc.

Theory C: Cap is the problem (I'm fishing...)
-Inspect cap carefully for scoring.
-Drill small holes in cap to allow air to not become ionized inside cap, per MSD manual.
-maybe change cap if I have another one.

Theory D: 12V supply line is not providing enough current once hot. (again, not supported by #4 above)
-Check resistance from T14 pin 8 to T12 pin 7 on relay board, when failure occurs.

Other theories welcome!


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Pete Dubler
See my 914 at http://dublerfamily.com

[This message has been edited by PeteD (edited 03-01-2001).]

[PeteD]

David C.

I have already completely rewired just about everything when I put in the new MSD system, so I don't think it is as easy as a poor connection, but I will double check that again. Thanks,

Pete

[PeteD]

Update:

-Dwell: 47deg (okay)
-Gap: .014" (okay)
-Lubed dizzy (new)
-Checked that clamp in on correctly (okay)
-Spring on top of dizzy drive is in place (okay)
-Dizzy will not come disengaged from drive. (okay)
-New Bosch Cap and Rotor (new)
-Drilled holes in rotor per MSD advice (new)

Drove my 12 mile test loop, on which I experienced three cut-outs earlier today, without incidence this time. Runs like buttered silk on a bed of graphite with silicone on top. BUT, I'm not gonna celebrate yet. Can't see anything wrong with cap. Will drive a lot more tomorrow is weather cooperates. We'll see if this fixed it or not...

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Pete Dubler
See my 914 at http://dublerfamily.com