924Board.org

horsleyflyer · Joined: 29 Sep 2005 Posts: 10 Location: guildford

hi folks,
I have a 1984 2.0l 924. It starts and runs fine until I put any heavy electrical load on it [ ie switching on the headlights - it's fine on side lights]. I've recently had trouble with the number 2 fuse on the additional fuse board [ fuse overheating ] which i corrected by putting a modern blade fuse and fuse holder in to replace the old one. Any ideas please?

Tigger937 · Posted: Fri Sep 30, 2005 2:09 am Post subject:

Yeah, I've been having electrical problems too. Hot F2 fuse on the additional fuse board (fuel pump fuse), hot fan relay (Relay I) and fuel pump relay problems (Relay II). Haven't had any problems with headlights though. I removed the fuse/relay panel and additional fuse panel and found several wires that had evidence of overheating near the panels..............ie., browned insulation, hardened wires, some corrosion, etc. I decided it was time to look at replacing the fuse/panel, additional fuse board and overheated sections of wire. I was afraid the fuse panels were going to be horribly expensive. I called Sunset Porsche in Oregon, who advertises OEM parts at cost + 15% and to my surprise, the main fuse/relay panel was only $83 usd and the additional fuse panel only $8.50 usd................not too bad! So they're on order. Also, found that EagleDay.com (someone here on this board suggested them) has the OEM electrical connectors and wires too.

Now the question becomes, what caused the overheating of the wires. Was it just corrosion over time, or was it overloading in the circuit caused by deteriorating components? I am suspecting both since it is well known the fuel pumps and cooling fans can have excessive current draw and yet appear to be functioning OK, ie., they don't just fail completely, at least not initially, they can gradually get worse over time. Quite possibly, this is related to accumulation of dirt/contaminants. As the current draw increases, heating in the wiring, relays and fuse/relay panels goes up leading to increased resistance and components not performing to full specification. It's kind of a cascading effect. At least, this is what I think appears to be happening. One must keep in mind these cars are 20+ years old now.
_________________
1981 931 (Concours)
1982 931 (Daily Driver)

"Think outside the box"

Paul · Posted: Fri Sep 30, 2005 9:21 am Post subject:

Bad grounds add resistance to the circuits which increases the amps that the wires must carry.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Tigger937 · Posted: Fri Sep 30, 2005 3:29 pm Post subject:

At first Paul, when I read your post, I thought NO WAY!!! For a given voltage, a higher resistance always results in LOWER current. But after thinking about it for a while, you may be on to something. With a bad ground in one circuit, the return current for that circuit could easily get diverted through the return of another circuit thereby increasing the current in that circuit, even to the point of overloading it. The return current will always seek out the lowest resistance path back to it's source, the battery neg. connection, even if it has to go through another circuit to get there. It appears this may open up the possibility of another failure mode. Hmmmmmmmmm.
_________________
1981 931 (Concours)
1982 931 (Daily Driver)

"Think outside the box"

horsleyflyer · Joined: 29 Sep 2005 Posts: 10 Location: guildford

Hi and thanks for gettiong back to me! I spoke to my mechanic who advised me that the old stlye fuses build up resistance between the fuse and holder as the fuse holder becomes weaker. He lent me a blade fuseholder with two wires and crocodile clips on the end advising me to bridge the original fuse with this set-up, hey presto the fuse didn't heat up! I've now replaced the original fuse and holder with blade types and this has cured the heat problem- the engine still cuts out when the headlights come on but I guess the fuel pump relay must have been fried when the fuse was arcing and I've ordered the new one. I'd better come clean and tell you that I live in the Uk but WILL order parts from the US if they are easier to obatian. Once again many thanks for your help and will advise about the headlights/relay prob.!

Paul · Posted: Sat Oct 01, 2005 10:21 am Post subject:

Think of it this way:

Bad grounds add resistance to the circuit, thus increasing the total resistance of the circuit, thus more amps and heat.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Paul · Posted: Sat Oct 01, 2005 10:31 am Post subject:

BTW electrons travel from the negative post of a battery to the positive post.

http://www.pbs.org/wgbh/amex/edison/sfeature/acdc_insidebattery.html
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

Paul · Posted: Sat Oct 01, 2005 11:01 am Post subject:

The British had it right in the 60's, they grounded the positive cable!
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Tigger937 · Posted: Sat Oct 01, 2005 11:59 am Post subject:

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

The 924 needs at least 10 volts in the primary circuit to see 5 volts at the coil. If it drops below 5 volts your engine will cut out or quit. It appears the voltage is dropping when the alternator has a higher load with the headlights. This is usually a symptom of one or more bad diodes in the alternator. Another possibility is the battery is shorting at a high load. Most autoparts stores can test for these problems.

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

Tigger937 · Posted: Sun Oct 02, 2005 3:33 am Post subject:

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

The current comes from the 15 post on the ignition switch, runs through a resisted wire and on to the coil. Without at least 5 volts the coil won't charge up enough to generate a spark.

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

Paul · Posted: Sun Oct 02, 2005 5:47 am Post subject:

Paul · Posted: Sun Oct 02, 2005 5:51 am Post subject:

Another:

The direction of electron flow is from a point of negative potential to a point of positive potential. The direction of positive charges, or holes, is in the opposite direction of electron flow. This flow of positive charges is known as conventional flow. All of the electrical effects of electron flow from negative to positive, or from a high potential to a lower potential, are the same as those that would be created by flow of positive charges in the opposite direction; therefore, it is important to realize that both conventions are in use, and they are essentially equivalent. In this manual, the electron flow convention is used.