2.4 MFI idle intermitant prob

[jeff m]

I actually caught the car with the fault "on"
I posted this problem before, I thought it was fuel, and was about to "get into" the MFI pump, but due to some house guests and fear of what I mght find I delayed it.

Today the fault was on, I got a neighbor to start the engine, I kept it running from the rack and disconnected the wire from the micro switch. The engine idled ok.

Now I want to check my understanding of the Speed switch and micro switch.

Under nornal conditions car on idle there is no shut off
On over run (throttle closed) the micro switch ops and tells speed speed switch, which cuts fuel at pump, fuel is then restored after revs drop below a certain rev count prob around 1800.

So it would seem, that my micro switch is ok, but my speed switch is working in one direction only.

Is my logic correct?

I haven't got the test lamp out yet, but will do so tomorrow.

I imagine the speed switch must be a complicated little blighter (because of what it does) any links appreciated

Thanks
Jeff
'72 911T

[Early_S_Man]

Jeff,

There were several threads last year pertaining to the Speed Switch, explaining the operation, troubleshooting, and repair ... and several Pelicanheads have repaired their unit successfully.

The two revised drawings replace earlier versions I posted last year ... Mike Gillies of Brisbane (who provuded Pelican with some of the MFI documents on the 'Tips' page) drafted the schematic, complete with a transribed error in the value of R10 I made when re-copying the original sketch, which was caught a couple of weeks ago by a Pelicanhead ... and corrected by hand, today.

WARNING - All '69-on MFI Owners!!! Preventable Failure of $283 Speed Switch module!

http://www.pelicanparts.com/ultimate/Forum3/HTML/009064.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009138.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009170.html

http://www.pelicanparts.com/ultimate/Forum3/HTML/009218.html

[jeff m]

Warren
Thanks again,

[jeff m]

Results of testing.
After reading the information, I realised that my "logic" was a little off.
I thought that the 12v was supplied to the speed switch via the micro switch.
Actually the speed swich is "hot" all the time and the 12v output is dependent on revs and "sits" on the micro switch waiting for it to close.
Took out the speed switch, opened it up to find a transister floating loose.
Transister replaced, with a new one. Replaced speed switch, no output. Tested continuity from terminal 1 to thro' mirco switch to fuel sol. from term 4 to distributer, ground on 3 and 12v on 2, all ok.
I suspect my speed switch has never worked,and this latest problem was caused by the floating loose component making random contact inside the unit hence the intermittant nature.
My neighbor has a carbed 72, I shall try to see if he has a speed switch still in place that I can borrow for a conclusive test.
Reason for this test is that I do have a 100 ohm resister that sits across C of permatune which of course is the same as 4 on speed switch.



Jeff
72 911T

[Early_S_Man]

OK, just to bring the thread up to date ... from a PM exchange ...

1. The 100 Ohm resistor isn't a problem, since it is present in both Bosch CDI and Permatune units ...

2. The transistor that was rattling around was D1, the Zener diode ... and my recommendation is to check R12 to be sure it is still 150 Ohms (within 5 %) and use an 8.2 Volt 400 mW or 1 Watt Zener diode. When I can't get 2% Zeners, I usually buy 10-50 (depending on how many I will eventually need) and test all of them ... and select the ones for critical applications that are within 1% to 2%. Off-the -shelf Zeners are usually 10%, so it is hard to match operating characteristics in a timing-critical analog circuit without doing some selection and matching.

3. After Zener replacement has been done, the Voltage on the
regulated bus needs to be checked and verified ... from the 'north' end of R12 to ground or Terminal #3 with 11.0 Volts to 14.2 Volts applied to terminals #2 & #3. Ideally, it should be in the 8.0 Volt to 8.4 Volt range. A 3-terminal 8.0 Volt regulator IC could also be used, but it would require a small 'adapter board' be used inside the case, and insulated and mechanically attached and stabilized. there are suitable regulators are available in TO-92, and 8-pin DIP and SOT-8 (surface-mount) packages ... for under $1. Digi-Key, Mouser, and Allied Electronics all carry suitable Zeners and Voltage regulator ICs.

3. The actual timing/operating rpm values for the Speed Switch are not terribly critical, since the Porsche 'Blue Book' of MFI actually suggests using the carburated version of the Speed Switch (the one with paint dot on the case) for 2.4 MFI engines!

Hopefully, none of the other transistors are on the verge of rusting thru their leads ... but I wouldn't hold my breath! Exposure to the full +14 Volt supply shouldn't have damaged any other components.

Your friend's 2.4 carb engine should have the speed switch, but it may need repair, also!

[jeff m]

Warren et al
Before putting in the zener diode the bus voltage was high at 9.5v (from a bench supply of 12.45)
After the diode the bus was 8.19v, BUT there was an output at term 1!
During testing the output dissapeard, I tried to get the fault to reappear but couldn't. (by tapping components, pushing components gently)
Left it for two hours set up with meter set on "high hold" next to a vibrating shell cleaner. The false output did not return.
Replaced it in the car, no output on Term 1 at req rpm.

I haven't given up, will keep trying.
jeff

[michel richard]

I can speak a foreign language too, you know. (Well, it's not foreign to me)

LOL

seriously, keep these posts coming, guys, I have MFI too and read all of these religiously, this one will need a little studying.

Michel Richard

[pwd72s]

It's a foreign language to me as well...nevertheless, it's printed. To be filed in a notebook of downloads I keep. Sort of like a personal repair manual?

[Matt Smith]

Hey guys- foreign languages can be learned!!

Print this info off, work it out or pass it on to a friendly local expert/friend. Whatever.

This is great information. Thanks again Warren

[Early_S_Man]

Jeff,

That is a great idea about the vibratory cleaner ... I guess a 'stress' test could be done by clamping the tab on the aluminum case to the lid wing nut of my Lyman Turbo 600 shell case tumbler while doing my next batch of .50 AE cases ...

You may already have a pulse source for testing the Speed Switch, but for any guys lacking same ... below is another diagram drawn up by Mike Gillies for what I call my 'junk bin' tach. calibrator ... my first one was built 33 years ago from 'recycled' parts on hand! Radio Shack generally has all of the needed parts in stock ... for those without an electronic parts version of the 'Recycle Bin.' Without even planning for it ... when used on 60 Hz power, it provides a test signal equivalent to 1200 rpm and 2400 rpm for a six-cylinder engine (1800 rpm and 3600 rpm for a four-cylinder engine) ... that just happens too be perfect for testing the Speed Switch!

Thoughts of being strapped to something vibrating for test purposes remided me of a presentation I saw in 1994 by Dr. James E. Tomayko on the subject of NASA computers ... back in 1965-66 there used to be an inside joke among Gemini astronauts about a porpoising motion in the pitch plane that was demonstrated by the Titan II booster (originally designed to be an ICBM) during the powered launch phase of flights. The astronauts used to joke that the Titan II guidance computer was having trouble deciding whether to go ahead and put the Gemini 'capsule' in orbit ... or find the right target in Russia to make a big hole in the ground!

[pwd72s]

Warren, completely off the topic, but during your NASA days, did you ever work with a Scott Simpson? If so, he's now the mayor of this little burg in Orygun...drives an S-10 pickemup with a Camaro mill..sounds like you were both working there around the same time.

[jeff m]

I put together Warren's tester, ($18 for bits and used a transformer from landscaping lights for 12v ac souce, modern lights are ac now not dc)
Was able to repair the speed switch, to be honest I never really located the fault, but as I replaced parts that I broke during testing it became ok.
The speed switch now works perfectly on the tester.
But only once on the car.
Removed it from car back to tester........still good 10 /10.
Points are adjusted, 0 ohms between dist, permatune and speed switch. Grounds also good. All spade terminals cleaned and checked.
Car is running smoothly and tach works properly.
One difference is that my test battery is 12.5v
Voltage on the car is 14v. But the zener diode should take care of that. ???
Help
If I put a meter looking for a ground on the out put of the tester at 60htz I get a low readind (pulsing ground)
If I put it on the output at 120Htz I get a much higher reading.
The meter is trying to read a ground that is pulsing.
When I do this on the car at the speed switch socket I get a constant reading when I rev the engine. (but tach is good)
This is a crude sort of test, the meter is trying to average the reading.

On the good side because of all the cleaning and checking of this and the fuel system the car has never run so well.

Jeff
911T Will try raising my test voltage to 14.

[jeff m]

YES YES YES
It's fixed, I set up the speed switch remotely from the car with jumper leads so I could switch between the car and Warrens tester.
The remaining problem was one that was caused by finding the problem on the circuit board.
The socket of the speed switch had become loose due to the number of times it was pulled in and out.
Retentioned the socket, well just bent the terminalsa bit.
It is now 100%
Thanks Warren for all your assistance

Jeff
911T

If any one needs to borrow the tester just shout.

[pwd72s]

Jeff? I've always loved a story with a happy ending.

[304065]

Just looked at this old thread. . . our departed friend Warren was a fan of .50 Action Express Hardball! RIP Buddy.