924Board.org

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

OK, so I went out and bought a light, and here's what's going on:
1. The washer pump leads get 12+ volts when disconnected from the used pump I bought. The test light lights right up, so it seems that I'm getting good current. As soon as I hook up the pump .... Nothing >

. No light no voltage. But when I hook the pump straight to the battery it works. Any ideas???

2. I finally figured out that the defroster lead was unplugged from the relay, so now I'm pretty confident I have it plugged in to the right place. When I disconnect the + and - leads from the hatch struts, I get 12+ volts across the leads. As soon as I connect the test lamp in this situation, the voltage reading goes to zero, and the test lamp will NOT light. Any ideas??? Bad relay? Crispy wiring?

3. I'm hooking up my new sound system, and I think I've located the original factory radio wiring harness. It has four leads, which I believe are: ground, battery, ignition, and illumination (dimmer). Two questions on this one: when I meter the voltage for the dimmer lead, it starts at -1.6V and goes to -2.0V when I move the dimmer switch. Why the negative reading? Secondly, the ignition lead is "hot" as soon as the key is inserted...I was expecting it to only be hot when I turned the key to ACC position. Is this normal behavior for factory Porsche sound systems?

Here's a pic of the new sound system prior to installation. I'll post finished results later:

Note: sometimes these pics don't display in the board properly...just right click on the broken image graphic and do a "view image".
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erstwhile owner of just about every 924 variant ever made

Smoothie · Posted: Sun Oct 31, 2004 1:02 am Post subject:

1. Has to be still not enough current. Enough to light the light, but not enough for the pump. Are you testing the pump with both (+) and (-) to the battery? Try (+) to the battery and (-) to the (-) of the connector, Then try (-) to the battery and (+) to (+) of the connector. That might isolate the problem.

2. Again, a current carrying bottleneck somewhere. On my car I found the problem was with a frayed wire - the ~10 gauge wire where it connects to the left hatch strut at the body end.

3. Don't know why on the neg volts or if that's normal. Bulbs work with the flow in either direction, but LEDs need a certain flow direction. ACC is activated by just inserting the key on these cars. It's not one of the positions that the key is turned to.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

On the rear defogger, is it possible that the relay is bad? Or should I just go after the wire? It's a straight run from the relay directly back to the hatch strut. The wire looks good, but who knows...
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erstwhile owner of just about every 924 variant ever made

924RACR · Posted: Sun Oct 31, 2004 6:22 am Post subject:

It's quite likely that the relay is bad or incorrect.

You really need a multimeter for these debugging sessions, not a test light. They can be had for less than $20 at Home Depot, etc., and are far more useful. A test light will never tell you if you don't have a ground, or if you have a shorted motor, if a fuse is bad, if a coil is bad, etc.

#1 sounds like a poor connection at the pump.

#2, again, poor connections, unless the struts are swapped left/right or upsidedown.

Regarding the sound system - I wouldn't really bother hooking up to the stock system.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Sleykin · Posted: Sun Oct 31, 2004 12:27 pm Post subject:

Well Vaughan,
A test light can tell you things a voltmeter can't too. You just have to use the test light properly. Same goes for the voltmeter. If used correctly and interpeted corectly it will tell you a lot ... it will lie to you too

A test light will find a blown fuse faster than a voltmeter too. If it lights across a fuse it is bad if it lights on only one side of the fuse it is bad. The thing with the test light is it draws a significant current compared to a voltmeter and many troubles that will show 12VDC on a meter will not light the light. Put the light in series and you have an idea of how much current is going through the circuit. In parrallel you can tell if voltage is there but not being used by the device under test.
Both are very useful troubleshooting tools but each has it's place.
_________________
Glenn Neff
Medford, OR
87' 924S

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

On the defogger, I'm bypassing the struts altogether. I'm taking the multi-meter from the positive lead to the negative lead, and get 12+ V when the car runs and the switch is on, but it won't light the test light. I think I'm going to replace the relay next and see what happens.

Still need to do some further testing on the washer pump. Stay tuned.

New sound system is in and sounds great!
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erstwhile owner of just about every 924 variant ever made

Sleykin · Posted: Mon Nov 01, 2004 3:06 am Post subject:

If you are measuring across the defogger grid with the test light it won't light normally as the defogger grid is drawing all the current. Put it in series with the grid and if it lights your defogger is probably working ok. The rear defogger takes longer to clear than the mirrors. There also are 2 fuses for the defogger circuit. One runs the relay and mirrors and the other runs the rear defogger grid.
You measure 12VDC across the grid because it is the nly resistance in the circuit and is dropping all the load.
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Glenn Neff
Medford, OR
87' 924S

Smoothie · Posted: Mon Nov 01, 2004 5:28 am Post subject:

Sleykin · Posted: Mon Nov 01, 2004 6:14 am Post subject:

Yup you are mistaken. In a series circuit the current is the same in all devices and limited by the device with the most resistance. In this case the grid has the least resistance (if it is intact) and the lamp will limit the current to just what it takes to light it up. The voltage is divided in a series circuit with the most voltage being dropped across the greatest resistance. (IR drops) This is what makes the test light so useful for checking bad grounds and what makes the voltmeter lie. A voltmeter will measure the same voltage across an open grid as a good grid and the voltage to ground will be the same with a good ground or a poor ground.
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Glenn Neff
Medford, OR
87' 924S

Smoothie · Posted: Mon Nov 01, 2004 7:26 am Post subject:

Thanks - good info.

I dun some learnin' in electronics close to 20 years ago, but didn't use and practice what I learnt, so I guess them brain cells died off and some info went with'em.

_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

kaffine · Joined: 13 Jun 2003 Posts: 644 Location: Las Vegas

I really wish they would ban test lights. I have done electrical repairs on cars and trucks for close to 4 years now and don't have a test light. If you know how to use a volt meter it will tell you alot more than a test light ever will you just need to know what you are doing with the meter.

If you have power for the test light with the componet not connected but when you connect the componet and it doesn't light then there is excessive resistance in the power circuit to the componet.

Using a volt meter put the positive lead an the B+ terminal of the battery. Take the negative lead and with the circuit connected and being powered take voltage readings at the connectors starting at the componet and working back towards the source when you have a more than .5 volt diffrence between to points there is high resistance between those points (bad contacts, corrosion, ect) repair and start again at the componet working back towards the source. You want to start back at the componet each time because you will have more current flow through the circuit after removing resistance from the wiring and it may cause another connection that was previously tested good to have to much of a voltage drop.

If you insist on using a test light with one side connected to ground and the componet connected and powered, start at the componet and work back towards the power source when the light comes on the problem is between that point and the previuos point.

Is there a section in the FAQ or tech that talks about doing a voltage drop test?
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80 924
80 931

The best desciption of an atom boils down to something unknown is doing we don't know what.
Sir Arthur Eddington

Sleykin · Posted: Mon Nov 01, 2004 4:29 pm Post subject:

Tell me which is easier to read when you are upside down under the dash

I never said a test light would replace a voltmeter. I said in effect that it augments a voltmeter. They each have their place and both can tell you lies if you don't use/interpet them properly. I have been using both for 40 years right along with vectorvoltmeters spectrum analizers, wheatstone bridges, fluted lines, meggers, milliohmmeters and scopes out the wazoo.
The proper tool properly used makes life easier.
I will defer all electrical problems and solutions to you from now on and keep my mouth shut.

Bye

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Glenn Neff
Medford, OR
87' 924S

Joes924Racer · Posted: Mon Nov 01, 2004 4:41 pm Post subject:

Im 5' 11" 187lbs I get upside under my dash. Ive used my cordless up under there upside down to screw stuff to the firewall. I fit elecrtics are tough.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

kaffine · Joined: 13 Jun 2003 Posts: 644 Location: Las Vegas

Sorry if I came accross strong in my opoins of test lights. I need to remeber we are working on cars from the 80s not brand new 05 model year cars.

When I worked on Dodges I replaced sevral air bags, seat belt pre-tensioners and computer modules because someone went poking around with a test light.

I've never had a problem using a voltmeter under a dashboard.
_________________
80 924
80 931

The best desciption of an atom boils down to something unknown is doing we don't know what.
Sir Arthur Eddington

924RACR · Posted: Wed Nov 03, 2004 12:46 am Post subject:

I never said voltmeter. I said multi-meter. They usually include an ammeter. They also usually have an ohmmeter, so you can test resistance to ground, or coil or motor resistances (even rear defrost grids!), and even check diodes, such as those in the headlight motor.

Yes, you can accomplish some of these with a test light.

Agreed about the defrost grid time - it's pretty slow. I'm even wondering if I can speed mine up, may try reworking some of the wiring to see if it'll work faster - can be very slow sometimes.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype