MFI help required: cold start & microswitch

[2.4te]

Thanks to all for your suggestions. With regard to the speed switch, someone (Tim) in the UK on DDK has kindly offered to try to repair mine and Lightweight Andy on DDK has sent me a working speed switch. My micro switch is working and correctly adjusted now.


With regard to the cold start, I'm stuck. I've run more tests and found:
11.9V @ terminal 30 with ignition on
10.3V at yellow wire of thermo-time switch when cranking
10.3V at starter take off at cranking

Should the last figure be 12V? My batteries are in excellent condition and quite new.

If I feed a live to the yellow wire side of the thermo-time switch (leaving the red/black wire connected), then the cold start solenoid operates, but the thermo-time switch doesn't shut-off, so neither does the solenoid.

I'm sure I can wire up a remote switch to operate the cold start solenoid, which would solve my cold start problems, but I would rather have it working as Porsche intended.

Can anyone give me more advice please?

Thanks.

Dave

[2.4te]

I ran another test suggested by Grady that I found in a past thread: I grounded the red/black wire to the thermo-time switch and cranked the engine. I got a dribble of fuel. Tried again and there was the briefest of splurges of fuel. The next time cranking and thereafter, no more fuel (I've got a long hose connected to my cold start fuel nozzle emptying in to a glass jar). Does this mean my thermo-time switch needs replacing?

Thank you.

Dave

[Grady Clay]

Dave,

The cranking voltage looks a bit low. I don’t hear any complaints about slow cranking. I assume you have the 1.5 hp starter. As a safety measure (and is regular maintenance), it would be useful to clean the ground straps from the batteries to chassis, clean and tighten the B+ terminals (including where the wire is clamped) and the ground strap between the chassis and the transmission.

There are two useful tests for starter wiring integrity. First is to connect your multi-meter (set at 12 VDC) between the chassis and the starter frame. Crank the engine. Normal is only a very few mV.

Next is to connect the meter between your right battery B+ battery terminal and the threaded starter B+ terminal and measure the voltage while cranking. Normal is only a few mV.

It sounds like the Thermo Time Switch is working. There is a heating element inside that warms as you crank. This prevents you from ‘flooding’ the engine if it doesn’t start. This is why it quit flowing on your 3rd try. Somewhere I wrote a test procedure for a Thermo Time Switch out of the car. I’ll see if I can find the link.

While cranking the heating element in the Thermo Time Switch takes some serious current. The yellow wire connection at the switch can have resistance that inhibits its use. There is a ‘tee’ connection in the engine wiring harness. It is useful to check the voltage at pin #1 of both 14-pin connectors while cranking.

I don’t think there is a Porsche specification for fuel flow from the Cold Start Solenoid Valve but the flow is very substantial. I’ll guess 300 ml in one minute. You can test this by applying external power to the solenoid and measuring the flow (calibrated container and stopwatch). Another important aspect of the solenoid valve is that it shuts off completely – no drops or dribble.

Where do we stand with the overrun system and RPM Transducer?

Best,
Grady

[2.4te]

Dear Grady,

Thank you for your suggestions. Here goes:

"The cranking voltage looks a bit low. I don’t hear any complaints about slow cranking. I assume you have the 1.5 hp starter. As a safety measure (and is regular maintenance), it would be useful to clean the ground straps from the batteries to chassis, clean and tighten the B+ terminals (including where the wire is clamped) and the ground strap between the chassis and the transmission."

My ground straps to both batteries were in a woeful state, so I replaced them. All terminals cleaned. The transmission to ground strap is quite new and I cleaned it too. Cranking wasn't and still isn't slow; the starter was new (rebuilt) about 3 years ago (less than 5000 miles).

"There are two useful tests for starter wiring integrity. First is to connect your multi-meter (set at 12 VDC) between the chassis and the starter frame. Crank the engine. Normal is only a very few mV."

I measured 0.34V

"Next is to connect the meter between your right battery B+ battery terminal and the threaded starter B+ terminal and measure the voltage while cranking. Normal is only a few mV."

I measured -0.44V (connecting one wire of my multimeter to the battery via a long leader and the other wire of my meter to the threaded starter terminal).

"While cranking the heating element in the Thermo Time Switch takes some serious current. The yellow wire connection at the switch can have resistance that inhibits its use. There is a ‘tee’ connection in the engine wiring harness. It is useful to check the voltage at pin #1 of both 14-pin connectors while cranking."

Grady, this might be where it gets interesting:

The 14-pin connector of the two that is toward the front of the car (nearest the fuel filter cannister): I disconnected it and had someone turn the key as if to start the car. It didn't crank, but the voltage at the socket of pin #1 read 12.7V.

Then, the 14-pin connector of the two that is toward the rear of the car: I disconnected it and had someone turn the key as if to start the car. Again, it didn't crank, but the voltage at pin #1 read 0.1V and at the socket of pin #1 read 0V.

I double checked the yellow wire at the thermo-time switch (still connected) it read 10.6V while cranking.

The threaded terminal of the battery reads 12.7V at rest and drops to 10.9V when cranking. The yellow wire at the other connection on the starter reads 10.6V when cranking.

"I don’t think there is a Porsche specification for fuel flow from the Cold Start Solenoid Valve but the flow is very substantial. I’ll guess 300 ml in one minute. You can test this by applying external power to the solenoid and measuring the flow (calibrated container and stopwatch). Another important aspect of the solenoid valve is that it shuts off completely – no drops or dribble."

When I supply a live feed to the terminal on the fuel solenoid it gushes fuel- I still haven't measured the volume over time, but expect it's flowing in the region of the rate you suggest (300ml/min).

"Where do we stand with the overrun system and RPM Transducer?"

I haven't done anything more on this yet Grady. It's so difficult to start now because I've put back the washers in the thermostat I took out to get it started, so I thought I'd wait until I can get it running. (As a reminder, I had the car set up on the rolling road last summer, when Bob Watson set up the MFI for me and this revealed the inoperative cold start system. The car had evidently been running rich and therefore started reasonably well from cold; after the rolling road session, I removed some washers from the thermostat so I could start it from cold, but as I said, I've now replaced them).

So, does it sound to you like the tee in the engine wiring harness is faulty? If so, where do I find it? I'd rather dismantle as little as possible of the wiring harness.

Many thanks for your help Grady.

Dave

PS, The wiring diagrams for our early cars can be found at Nick Moss' Early 911 web site (Thanks to Tim on DDK pointed this out to me):

http://www.early911.co.uk/html/technical.html

[Grady Clay]

Dave,

Just like all usual MFI issues, there are a variety of simultaneous things to deal with. The general order is to get the engine running correctly under normal operating conditions first. Second is the cold running (thermostat) and third the cold start.

You can address the cold start with a simple wire from the Cold Start Solenoid Valve to a switch in the cockpit. It sounds like this circuit is working but you might use this technique to get the car to run. (You can even give a quick ‘squirt’ during warm-up if it doesn’t run properly.)

The cold running Thermostat effects the normal running mixture if it doesn’t fully turn off. Usually this is because of rusted heat exchanger or defective air hoses. It can also be incorrectly set thermostat (wrong washers, etc.) The key is to MAKE SURE the thermostat turns off for normal running. There has been significant Pelican discussion on this subject.


The #1 issue is to get proper running at normal operating temperatures. All else is ancillary. You can ‘jerry-rig’ the other systems and concentrate on this. If you can start it and get it through the warm-up phase, the only issue is the running & mixture. Next we can address the overrun system.

On to starting.

The best way to measure the voltage at the #1 pin of the 14-pin connectors is to pop off the cap to the connector while connected. Be careful to not allow any of the pins to get loose. You can then clip your meter to the #1 pin. This gives an “in situ” test with current flowing through the terminal.

You seem to have 0.78V drop through the starter wiring during cranking. That is not excessive but I suspect it can be improved. Less voltage drop is always better.

I would do some more diagnosis before invading the engine wiring harness. I don’t remember where the ‘tee’ is but I can find it for you if necessary so you don’t have to take apart your entire harness. I don’t think you have a serious problem here.

My recommendation would be to re-clean all the battery, starter and ground strap connections. The connection between the battery terminal part and the actual battery wires is commonly neglected. You can get your total voltage drop below 0.100 V. I have seen 0.020 V with a large displacement, high compression engine and 1.5 hp starter. This is good maintenance but probably not related to your issues.

(As an aside, Porsche used a larger battery cable (25 mm) in the chassis starting in ’74. The ’72 with the new 1.5 hp starter was the first year with any current-draw issue, exacerbated by the fact we were putting 2808 cc, 10.5:1 CR engines in these cars.)


I’m very impressed with your diagnostic skills. That makes e-diagnosis possible. Please don’t sell yourself short on your electrical skills. One important function of threads like this is to educate someone many years from now.

Best,
Grady

[2.4te]

Dear Grady,

OK, thanks for your reply, your time and your compliment! Progress might be slow until next weekend (it's Sunday evening here), but I'll go through cleaning contacts again whenever I can escape into the garage. What is the best way to clean the connections? Are there any handy hints? I've been using fine wet/dry sandpaper followed by cellulose thinners to degrease. I find the male spade connectors very difficult. For the 14-pin connector 'sockets', I wrap sandpaper round a small allen wrench to enable me to apply pressure.

Kind regards,

Dave

[2.4te]

Hello all,

My cold start is fixed and so is the speed switch! I've spent hours on the car and would like to thank everyone, particularly Grady, for their input, which certainly boosted my confidence in dealing with electrical issues that I'm usually so unsure of.

The speed switch needed repair and in the UK, Tim Bennett generously did that for me and Lightweight Andy sent me a working unit to help me out. These guys post on the maverick early Porsche site DDK (http://www.ddk-online.com/). I ran the tests in CMA (after eventually getting the car running- see below) and all appears to be in order.

The cold start problem I believe was due to a faulty ignition switch and perhaps a defective thermo-timne switch. However, since I replaced both with new items at the same time (both still available from Porsche), there is the possibility that the thermo-time switch was not defective. I've spent so many hours on these issues that I don't have the energy at present to go back and diagnose more specifically.

What led me to the ignition switch was that I noticed late one night when checking for a spark at the HT lead (because I was at a loss as to why the car still wouldn't start after cleaning, re-cleaning and re-re-cleaning all the connections- all grounds, battery straps, 14-pin connectors, etc) that the intensity of the spark was much higher when the ignition key was released as the engine cranking was 'running down', with the satrter motor not being actuated. When the starter was cranking, the spark was very, very poor. In part, I was lucky that I was doing this at night when I could see the spark intensity more clearly in lower light levels.

I removed the ignition switch (thanks again to encouragement from Tim on DDK) and interestingly found that someone had been inside it before- it had been butchered open and crudely taped back together. This observation gave me the confidence to bite the bullet and I ordered a new one, together with a new thermo-time switch.

In the meantime, waiting for the parts to arrive, I wired up a remote switch in the cockpit to activate the cold start solenoid. I like to think I tried hard to make a neat job of it, but this care caused my next problem: when the new switches arrived, I fitted both (ignition and thermo-time), leaving my remote switch in place because I was uncertain if the new thermo-time was really the solution. I cranked over after activating the cold start with my switch. I could hear it activate, but the engine wouldn't start. I checked for spark- brilliant blue, but now no fuel flow! Aaaaagh!! First fuel no spark, now spark no fuel! One step forward two steps back.

Eventually I traced the no fuel issue to the fact that in wiring in my remote switch, I must have disturbed the single-pin connection in the feed to the electric fuel pump (that connection lies towards the front (ie- front of car) of the engine bay, beyond the fuel filter). I sorted this problem, which was minor but took hours to find, decided to replace the orginal lead that activates the cold start solenoid instead of using my remote switch, tested fuel flow- hey presto! Reconnected the fuel lines, cranked and the engine fired almost immediately.

It's so gratifying to have cured these problems myself, with all your help. Not only do I have the satisfaction of doing it myself, but more importantly I've learned much more about my car. Although we've been together 10 years almost to the day, I'm still learning to live with her. Ha ha!!!

Thanks again,

Dave

[Grady Clay]

Dave,

Great that these issues are resolved. Thank you for posting many of the details. Please post the DDK link with these issues and solutions.

I too am a DDK member (my alu card is #1345), I just don’t have time to monitor all the forums I would like.

Best,
Grady

[impmad2000]

It is always good to reach a positive conclusion by your self (With a little help is fine too !)
Well done Dave for sticking with it.
Cheers
Tim

[domtoni]

Great job Dave, and sorry I have not spoken to you lately. Just got back from the states and Germany.

Best
Dom