No start. Tach showing different revs while cranking

[chapron67]

I took my ‘78 911SC on a 1,200 mile rally in British Columbia and it worked flawlessly.

I haven’t driven it since (about a month) and it won’t start. It fired a couple times but nothing now after the last few tries when I’ve left it for a few hours in between.

Weird thing is that I noticed the tach was showing 4,000’ish while I was cranking it. Next time it showed 1,000; and this past time it didn’t show anything!

Could this be somehow related?

PS a friend told me to take the air box cover off and gently hold the flap up for 5 seconds after turning the ignition ON. This didn’t rectify anything.

[pampadori]

The tach will do that as the battery starts to put out less and less voltage during cranking. It needs to be charged up after you get it started.

The flapper in the air filter box is the fuel pump safety cutoff so that the CIS doesn't flood/hydrolock the motor when you have key on hut engine off.
With the key on, if you lift the flap, you would hear fuel pump activate. If not, it check the fuel pump and for relay next.

[chapron67]

I'm back to getting this figured out! So, I squirted a bit of starting fluid 'under' the flap and the car started right up but then died. So, I guess it's fuel. I've copied the blurb here on what to do and have questions:

3. You can jumper a hot wire to the fuel pump and listen to it whir to prove that it's working.

WHEN I TURN THE KEY I HEAR A HIGH PITCHED BUZZ FROM THE LEFT REAR OF THE ENGINE. IS THIS A 'WHIR'?

Run a short wire from the fuel pump fuse (#6) to an adjascent fuse (#5 or 7) at the bottom of the fuse terminal, the pump should whir.

HOW DO I DO THIS? TURN TO THE KEY TO 'ON', THEN RUN A SHORT WIRE FROM THE FUEL PUMP FUSE TO.. WHERE? AND TO THE TERMINAL ON THE TOP OR BOTTOM?

4. If the fuel pump whirs (when jumpered), try starting the car this way (this overrides the DME control of the fuel pump). If it runs, it probably indicates some kind of DME problem (the DME is not sending power to the fuel pump). If still doesn't run, it's probably not a DME problem.

5. If it is a DME problem, replace the DME relay, which sends power to the fuel pump. This is another one of those parts that is known to be flaky. The relay is under the driver's seat next to the DME computer.
HOW DO I FIND THIS IN A 1978 911SC COUPE? IS THIS 'PROBABLY' THE PROBLEM (CAR RAN FLAWLESSLY AND THEN I DIDN'T USE IT FOR A FEW WEEKS AND IT WOULDN'T START. I THOUGHT I SMELLED A LOT GAS WHEN TRYING TO START IT SO DIDN'T THINK IT WAS FUEL. AND WHEN I WENT BACK TO THE CAR A FEW WEEKS AFTER TRYING TO START IT THE BATTERY WAS TOTALLY DEAD. DOES THIS HELP TROUBLESHOOT THIS?

6. If the relay doesn't fix it, it could be the DME computer itself, faulty sensors (flysheel speed and position), or faulty grounds. Try disconnecting the DME computer and cleaning the contacts.

THANKS!

[chapron67]

BTW I looked under the seat of my '78 and no DME (whatever that is!) or Relay! What now? Thanks!

[boyt911sc]

Originally Posted by chapron67

BTW I looked under the seat of my '78 and no DME (whatever that is!) or Relay! What now? Thanks!

Chapron,

Your ‘78 SC does not have DME nor ECU. Search and read DKLever48’s thread about FP pump relay & FP relay socket tests. Everything you need is well discussed in this thread. You seem to be unfamiliar with the electrical and testing procedure for the SC fuel pump. Do not do any electrical troubleshooting until you read several times and fully understand the procedures.

If you have some questions or doubt, that’s OK. Just ask. There is no such thing as STUPUID questions, only STUPID answers. Someone recently ignored this advice and almost burned his car. We all make mistakes and I have committed some too. Better to err on the side of caution than be sorry later.

There are plenty of help available in this forum. Keep us posted.

Tony

[chapron67]

On it! Thanks!

[Brian Cameron]

That high pitched squeal at the left (drivers side) is the normal sound from your CDI box (capacitative discharge ignition). You want to hear that... if you didn’t it may mean a CDI failure, assuming your battery and wiring are good. So be glad that’s not your issue.

I would troubleshoot fuel pump as Tony advises, based on the starter fluid getting the engine to run briefly it sounds like you’re not getting fuel. Also, the bouncing tach can signal a low battery per pampadori; when the car sat for a month was it on a maintenance charger? If not try giving the battery a good charge or swap for a known good battery.

[chapron67]

From DKLever48’s thread about FP pump relay & FP relay socket tests

1). Pull out the FP relay and do the test (ignition switch @ OFF position) for the individual terminal at the socket: A simple 12-volt self-powered test light will work well for this test or a regular non-powered test light, or a volt meter.

87...........no power
87a.........no power
86...........no power (87a 86 are bridged)
85...........no power (should be grounded at this point)
30...........no power (has ground contact?)

GREG: YES

2). Turn ignition switch @ ON position (not start):

87............no power
87a..........has power
86...........has power
85...........no power, presence of ground contact
30...........no power, presence of ground contact

GREG: YES

For test#3: FP relay installed, ignition SW @ RUN.
87...........no power
87a.........has power
86...........has power
85...........should have ground
30...........no power

GREG: YES

Test I. Remove the air filter and lift the AFS plate to activate the FP for a second or two (no longer). Ignition SW @ RUN.

GREG: NOTHING

Test II. Remove FP relay (ignition SW @ RUN): Bridge terminal 87a and 30............similar test as test I to activate the FP manually.

GREG: NOTHING

So, this means the fuel pump is toast, or ?? Thanks, Greg

[timmy2]

Fuel Pump is up front under car mounted behind fuel tank under a cover plate attached to steering cross member. That is where you want to listen for the pump.

Final test to try. Be careful...
With Ignition off, run a #12 AWG jumper wire (with an in line 15 amp fuse installed) from the empty relay socket terminal 30 (the one off by itself) directly to the battery positive, and listen for the pump. No other noises will come from the car. This test isolates the fuel pump from any other circuits. Direct test of the pump only.

BTW, when was the fuel filter last changed? Lots of gas in the tank?

[chapron67]

Thanks!

- what is a AWG jumper wire?
- no idea when the fuel filter was last changed: bought the car a year ago.
- gauge shows above R and the light is not on.
- shouldn't I hear the pump trying to pump even without gas?

[timmy2]

AWG is American wire gauge.
#12 AWG is a size of wire diameter measured using the AWG standard.
“Jumper wire” is a wire used to bridge the space between 2 electrical points that are not normally connected.
Yes, you should hear the pump run.

[chapron67]

Got it, thanks.

OK, when I do that there's a soft clunk sound that last for 1/4 second (fast). No sound like it's pumping or anything. But the sound seems to be coming from where you described the pump as being installed.

[timmy2]

That thump Sounds like a seized fuel pump to me.
Look up threads on draining the fuel tank, and changing the fuel pump.

[timmy2]

https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/33-Fuel_Pump/33-Fuel_Pump.htm

[chapron67]

I don't understand the reason to drain the tank if I can just use vice grips to stop the flow. Tank is probably a 1/6 full. Is it critical to empty the gas tank?

I will order a new fuel pump, thanks. Greg

[timmy2]

Not critical to drain the tank, Clamp works fine if fuel line is in decent condition.

Yours is likely only 41 years old, what could possibly go wrong...

Maybe have a large container that fits under the car handy?

[chapron67]

Good idea! Thanks. Greg

[jwmwu]

MY 83 SC would not start but only slow crank when I jumped the battery. The car was stored over a 6wk period with an aftermarket alarm that probably drained the battery. Now i am in a full debug mode learning from some of the PO experience here.

Here's my situation:

1. Connected Battery at ready to jump start position. BTW, Battery was new less than 1yr old.
2. All lights and alarm triggered. I have to shut the alarm system off with the remote.
3. Turned ignition switch to START position. Gear in neutral.
4. Heard whirring sound from CDI box in the engine trunk (Good news i guess)
5. Heard micro clicking from the FP relay the first time. 2nd time its gone.
6. Tried to start the engine but no crank at all.
7. Pulled the FP relay out from the fuse box to check pin condition and reconnect back.
8. Waited for 5mins to allow the battery to get charged up
9. This time it cranks but very show crank lasted for about 3 secs.
10. Turned off ignition switch. No apparent smell of fuel... but i did hear some swishing sound in the front but unsure.

Next step is to measure voltage drop from Battery to FP relay to CDI box to Coil.
I have trouble finding all my grounds in the frunk as well as the Grounding strap under the car...

I'd appreciate if anyone could post a link or provide photos of the actual location of those ground contacts.

I also posted some photos of the FP relay, CDI box, Dist cap, rotor and alternator since everything was working fine before the battery got drained.

joe