JSFFive

Problem:
* Cold Start - won't start after engine has been off for more than 3 hours
* Low boost - only 1.2 max at 3000 RPM
* Excessive fuel consumption - 18 MPG average around town, 20 MPG at 65 MPH
* Sluggish and uneven running when cold
* Requires three minute warm-up

Prelude:

I have a 1988 5000 CS Turbo Quatro. Originally I was trying to use the wiring diagram in the Haynes manual, but as many of you may know, the Haynes manual only covers the "S" models, and doesn't include anything for the "Quatro" models, and the wiring harnesses across different models ARE different, in both physical lengths, and in color coding.

It's absolutely imperative to have the correct wiring diagram, if only to get the wire colors straight.

Analysis:

After getting the Bentley manual, I was able to figure out that I had a short to ground that was effectively killing five different devices on the engine... Looking back on it now, I'm amazed that I was able to get the thing to run at all before fixing the issue.

The devices that were disabled were the cold start valve, waste-gate solenoid, deceleration valve, charcoal canister, and heating element for the oxygen sensor.

The fuel pump relay supplies two legs of +12vdc power to various devices on the car. The first leg of the relay basically supplies the fuel pump relay, and is a separate wire altogether running from the relay panel, back to the gas tank where the fuel pump resides. This leg didn't appear to be affected, because when I would turn on the ignition key, I could read 12 vdc on the multi-meter.

The second leg is a single blue wire with a black stripe coming from underneath the relay box, into the driver's side foot-well, over the transmission hump, where it then disappears BACK through the fire-wall, into the engine-well, where it appears about a foot underneath the window washer reservoir on the passenger side. At this point, what WAS one 18 gauge wire coming from the relay box, is now FIVE smaller 22 gauge blue-black wires that continue on to the cold start valve, waste-gate solenoid, deceleration valve, charcoal canister, and heating element for the oxygen sensor.

I suppose that these blue-black wires are somehow mated inside the wiring harness to the single wire coming from the relay box. This second leg (the Blue-Black wire) was shorted to ground somewhere between the relay box and where the harness leaves the passenger compartment on the passenger-side foot-well. This caused all five components on that leg to fail.

Note, the only way the car could possibly start BEFORE fixing the problem, would be if the first leg of the relay (the one that supplies power to the fuel pump), were somehow isolated from the second leg of the relay because, INSIDE THE RELAY, leg one and leg two are electrically the same when the relay is switched ON, which is to say, they are both physically connected. If leg two is somehow shorting its 12 volts to ground, then leg one must ALSO be shorting to ground.

It really confused the heck out me when I looked at the wiring diagram, and couldn't understand how I could read 12 volts on leg one, and yet I was reading 0 volts on leg two (fuel pump). I was seeing on the diagram on the side of the relay that they were electrically the same when the relay is switched ON, and it took about two weeks before it finally occured to me to open the relay up and take a look at it.

After opening the relay, a close inspection revealed that the solder run going to the second leg on the printed circuit board looked to be cut and peeled back. There didn't appear to be any burn marks on the solder run, but there was a slight trace of burning on the other side of the board, on one of the contacts that plugs into the relay board.

I suspect that, before I acquired the car, someone had troubleshot this shorted wire, and they either decided against fishing a new wire through the wiring harness, or it didn't occur to them to fish a new wire, and in order to get the fuel pump to operate, they must have physically opened the fuel pump relay, and cut that solder run.

Ironically, if I had tried to replace the fuel pump relay BEFORE fixing the wiring problem, I would've most likely BURNED OUT the new relay, and
then I would have really been confused. I had ran the output test on the fuel pump, and it was running ok, but if I had installed a new relay, it would have most likely burned out, and then the fuel pump would've stopped running too.

Lesson: "shot-gun troubleshooting" or, just replacing components without thinking it through, can cause more problems than it solves.

Solution:

Even though I'm a little pissed at the mechanic who didn't fix this problem properly in the first place, I can empathize. There was no way I was going to dive into my wiring harness and try to find out where all five of those blue-black wires were shorting to ground, since I suspect it's somewhere behind the stereo system. Also, running the new wire required lifting the relay box up enough to get my hands on the blue-black wire where it comes out from the bottom. THAT was scary.

So I cut the five blue-black wires, right where they come into the passenger compartment underneath the glove box, made sure that none of THOSE wires were shorted to ground, and then I ran a new wire from the relay box to that point. Then, I just joined the five wires with the NEW wire coming from the relay box, and soldered them all together and re-wrapped them in electrical tape. Then I just zip-tied the NEW wire up into the nether regions of the dashboard, away from the brake and clutch pedals. This was to prevent future wire-chafe issues.

Then, instead of trying to fix the solder run on the old fuel pump relay, I just replaced it.

Now:
* Cold Start problem - gone
* Boost (WAS only 1.2 max at 3000 RPM) - now it's 1.3
* Fuel consumption - 27.6 MPG average around town, 33.1 MPG at 65 MPH)
* Was sluggish and uneven running when cold - now the engine runs smoothly
* Used to require a three-minute warm-up - now I start and go.

Will Ng

Sometimes I can't help wonder what their electrical engineers were thinking with some of the the car's wiring schemes.
:-)

This post is worth archiving.

Adam Mowafy

I'd have to say that's the best "Newbie" thread I've ever read.

TonyOldSchool

to haunt us Type 44 nuts. Without these postings, I would have given up long ago. Still can't figure why they use so many relays w/ little timers etc in them, and I don't know about you guys, but I found some to be interrelated (pull one and a couple other things go out)! Thanks

nb::

Sounds like it was quickied a few times in the past. Thanks for taking time to write up and post -- a keeper!

Luckily, my car is in pretty original condition and the wiring is in overall good shape. However, the PO "installed" a hands-free wireless kit and some stereo gear himself. The work is pretty $hitty... Makes me a little queasy, really. But, I'm letting sleeping dogs lie for now, since it's at least sturdy work and not mission critical...

GREAT MPG, by the way...

later :: nick

kamig1

Howdy JSFFIVE,

Are you still around so I can ask you some questions pertaining to my cold start issue?

I have an 89 Audi 200 Quattro Turbo...and the results you achieved from your work is what I need but I have someone who will be doing the work for me.

I would like to ask you a couple of questions to compare symptoms and verify that I have the same problem as you described in your post.

Give me a shout if you are still around. It's been over three years since your post.

Thanks,

-John