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haroldk · Joined: 24 Nov 2008 Posts: 171 Location: St Paul, MN, USA

I had my 1980 924 running last year and then over the winter I pulled out the battery to plug up the big hole under it. I grounded the welder to the stud in the battery box and I made sure no wiring was harmed in the process, but now it won't start.

The fuel pumps don't prime all the time like they should, so I first suspected the relay or the switch, but the switch is fine (I bench tested that sucker until I was blue in the face) and the relay works. If I bypass the relay, the pumps run. They don't run with the key on and the air meter plate off the seat (reaching through the air box and pushing it up manually).

The only thing I can do to get the pumps to prime consistently is to, get this, adjust the mirrors. I can't figure out any connection between the mirror electrics and the fuel pumps.

With the key on, I'm also only getting a little over 6V to the coil. The odd thing is that on the other side of the coil, I'm getting about 1.5V to ground. That kind of tells me there's current running through it when there shouldn't be. Does a 1980 have a ballast resistor, or is it all contained in the box under the steering column?

I had the positive cable well and truly protected while I was welding and I really wouldn't expect there to have been any issues, but what are the chances that a bad ignition box could cause this odd behavior?
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1980 924 (for sale)
1984 944 (for project)

Ozzie · Posted: Fri Mar 09, 2012 1:36 pm Post subject:

'80 electrics- not sure but think the pump relay runs off the coil pulse not the air valve flap.

So- if you are not sparking you get nada.
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Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

924RACR · Posted: Fri Mar 09, 2012 11:00 pm Post subject:

You should always unplug all electronics when welding on a car.

That said - I'd expect you have bad grounds, at least check that first.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
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haroldk · Joined: 24 Nov 2008 Posts: 171 Location: St Paul, MN, USA

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

The prime circuit for the pump with the ignition on is feed by the 30 (battery) circuit. Check the battery feed line(s) to the fuse relay board for corrosion. Also check the pump fuse and relay for corrosion.

The 80 MY has measured resistance wires instead of resistors. Your 6 volts at the coil with the ignition on is correct. The voltage during cranking should be a little higher.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

haroldk · Joined: 24 Nov 2008 Posts: 171 Location: St Paul, MN, USA

musicalannette · Joined: 21 Feb 2012 Posts: 413 Location: UK

i know you have checked you grounds, but it is either getting a ground or feed from the mirrors. why it should, i don't know, but as it effects it and you have checked the grounds i would start there, i would also look to see if any welding has effected the electrical components and fuse box and relay board below, if you have applied a bit too much heat or a bit of weld has spattered through it might have have effected there. to be honest, this is where most of my problems were, and they joke about jaguar electrics!! the german stuff can be just as bad, poor contacts were to blame, most of them on plugs under there. but if a bit of weld has created a short or melted the odd wire, you might have the answer. hope this helps
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I KNEW white wall tyres were invented by Americans .....just not at Boeing.... to be fitted on the 737.....

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

On the 79 and later US models there is a circuit in the relay that runs the pump for a few seconds when the ignition switch is turned to the on position. When cranking, there is no 15 circuit activated so the pump shouldn't run. Once the engine starts and the key is moved to the on position the relay gets the ignition pulse and starts the pump. Also, during cranking the ignition is fed by the 16 circuit from the starter. This is through a reduced resistance wire to the starter which gives the coil a hotter spark. At no time should there be 12 volts at the coil.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

D Hook · Joined: 02 Nov 2002 Posts: 3158 Location: Omaha, NE

My '80 does the same trick with the mirrors cycling the fuel pump. Very handy when having fuel pump problems because you can turn the key on, hit the mirror switch and hear the pump come on for a few seconds and confirm it has power.

I always thought it was a feature built into the car from the factory. If it wasn't, it should've been.

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'80 924 n/a SOLD

Ozzie · Posted: Sat Mar 10, 2012 6:56 pm Post subject:

The pump will run when cranking.
15 supplies power to the TIU and pump relay.
without it you can crank all you like and it wont start.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Ozzie · Posted: Sun Mar 11, 2012 9:14 am Post subject:

then what powers the TIU during cranking?
We use the same ign switch.
check fig 13.33 and 15 is energised as soon as the key is turned to any position- on or cranking.
I have the same Haynes wiring diagrams as you.
15 powers the oil lamp as well and the tacho.

so you are saying none of them work during cranking for US models???
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

Ozzie · Posted: Sun Mar 11, 2012 8:41 pm Post subject:

DPW928
I dont know why you dont believe 15 has power when cranking.

PET show the same ign switch across all models.

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Going by this diagram 15 is activated when the key on and cranking. Otherwise it would look like the contacts for X.

15 powers the TIU, FPR, tacho, generator, oil lamp instrument cluster and several other items.

Without the TIU the coil will not work, no spark, no go. (16 on the TIU is the coil control) Crank all you like.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance

dpw928 · Joined: 02 Nov 2002 Posts: 1860 Location: owasso, ok 74055

Ozzie,

If the 15 circuit was activated during cranking you would power on both lines going to the coil rather than just the 16/50 circuit. Also, the tach receives its signal from the low voltage contact of the coil, so it operates regardless of the 15 or 16 power source. the oil pressure gauge acts like an ohn meter and receives the reading from the sender. i.e. no power (15 or otherwise) s required. There is a 15 circuit to each of the gauges but a close look will indicate that is for the lighting only.

Concerning the switch, if you've ever taken one apart, you will see the rotating contact only activates one circuit at a time.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

haroldk · Joined: 24 Nov 2008 Posts: 171 Location: St Paul, MN, USA