zetecr

First, hi to all - I'm new to this forum. Bought my FC3S Turbo a few days ago and loving it!

One thing I want to fix, though, is the non-functioning boost gauge. Been browsing and searching for the last few hours to try and find out the signal type and levels expected at the pressure sensor terminals under various conditions (eg ignition on, idle and boosting). Doesn't seem to be covered in the service manual, either. Any help would be appreciated...

SureShot

The dash gauge is slow to respond and a bit unreliable at best.
I got mine to work better by pulling the dash & cleaning the harness-to-dash connectors.
I never could get it to work perfectly, so like most others I added a cheap mechanical boost gauge.
I found a sunrpo for $15 but the a-pillar mount cost $25!

SureShot

I found the picture..

J-Rat

iTrader: (2)

Well, that gauge not working could be indicative of a bit more serious problem (no boost signal)..

SureShot

Welcome to the forum. (I just noticed your post count.)

What year? Any check codes?

I have a copy of the factory manuals (US ver) at the link in my sig.
Download whatever you might need..
-Bill

HAILERS

Like Mr Rat says, the signal from the boost sensor goes to the ECU and the boost gauge. Same output wire, it just splits the signal off a splice in the output wire. IF the gauge has NO response then it's most likely the pin 2B of the ECu is not seeing the output of the boost sensor also. Not good since the ECU uses that signal for fuel cut and timing to some extent. Check out pin 2B per the FSM, FUEL SECTION sub section CONTROL UNIT.

SureShot

J-rat & Hailers both really know there stuff.

Testing the pressure sensor output is an excellent recommendation.
If you have signal there, then it’s just the gauge connection.

zetecr

Thanks for the reply guys. I'm honoured to have the likes of j-rat and HAILERS reply to my initial post! I've read some of your posts and I can safely say I've never seen anyone more knowledgeable AND helpful in any kind of internet forums.

What you mention was my concern too. However, the engine seems to run and perform flawlessly - in fact, more than flawlessly. I wouldn't have even worried about the gauge not working except the car seems to be way too fast for a stock setup, which is what I was told by the guy I bought it from - Thus my curiosity about manifold pressure. What kind of symptoms would you expect from a non-functioning pressure sensor?

Anyway, I'll have another look through the FSM section you suggested.

HAILERS

Paul Stoakes in the attached article explains it better than I could: http://www.teamfc3s.org/info/articles/fcd/diy_fcd.htm

Another thing a boost/pressure sensor signal to the ECU does, is let the ECU know when the car is under load, so when 3800 rpm come along, the secondary injectors will come on. If the thing does not detect LOAD at 3800 they won't come on.

There's a catch to that though. There is a default value the ECU will look at if there is no boost/pressure signal. Duh. I forget what it is. It's given somewhere in the series five FSM. I'm not sure it's shown in the series four FSM though.

If you have a digital meter, you can back probe the input wire to the ECU either at the boost sensors plug or at the ECU's middle plug at pin 2B. Just turn the key to ON and engine off and look at the voltage. The value is given in the FUEL SECTION of the FSM, subsection called CONTROL UNIT. It's somewhere b/t 2-3 vdc. Can't look at the FSM right now. It's there though.

Instead of pullig the rug up and looking at the ECU, just unbolt the boost sensor and flip it around where you can backprobe the BROWN/RED wire. That is the output wire on the plug. Look there.

zetecr

Curiouser and curiouser...

I had a good look this afternoon and found there is a small plastic box that seems to intercept the pressure sensor signal. When I remove the box and plug the sensor directly to the loom, I get 2.3V at the pin. Still no reading on the boost gauge, however. When I unplug the sensor, though, the gauge jumps up and reads "0" - this might be the default value at work?

Looking at the manual, page F2-82 says 1.9-2.1V @ 100mmHg, but in the ECU terminal description table on page F2-78, it says terminal 2H should have 2.8-3.2V @ 100mmHg. Isn't terminal 2H wired directly to the sensor?

Anyway, a quick test drive with the mystery black box disconnected gave noticeably reduced performance, and I believe I could feel/hear the boost being limited. I guess this means the pressure sensor is working properly and it may well be the gauge itself that is faulty..

Obviously, the box must be a device that modify the pressure sensor signal to fool the controller into delivering more boost. In your wisdom, how harmful to the engine/turbo is this likely to be?

Edit: Doh! I just read Paul's article. I have an FCD, then...

SureShot

It sounds like the type of FCD that lowers the entire sweep of values.
One option is to tap the gauge signal to tbe pressure sensor before the FCD.

HAILERS

*****ys terminal 2H should have 2.8-3.2V @ 100mmHg. Isn't terminal 2H wired directly to the sensor?*********

I said 2B in an earlier post because I thought it was a series four car. Series five ECU pinouts are different as you noticed.

Sounds like the ECU is seeing an output from the boost sensor. No foul there. That same wire from the boost sensor to the ECU , splits off and goes directly to the boost gauge. THEN the negative wire for the boost gauge comes back from the boost gauge and ties into the gnd circuit for all the sensors in the engine bay. Sensors like TPS, boost sensor, afm etc.

Engine off, key to ON should result in the boost gauge going from dead bottom ,, up to the zero. Mine never does. It gets a couple of needle widths of the zero. I believe that's the way the manual states it. Go into boot the thing should rise up on the gauge. Only way to find out is to put a meter on the input wire and watch it rise or fall.

Using a FCD on a car that does not boost over 10-11 psi should do no harm. Has'nt hurt my car at least. Might check the fuel pressure when your at full boost to make sure it rises up to approx 47psi at 10psi of boost, and maintains that pressure in the fuel rail. Seeing as how the previous owner has been driving it like this it's probably good to go. Who knows though?

zetecr

How can you tell? The way I understand it with help from Paul's article, an FCD that lowers the values proportionally across the range would cause some degree of lean burning throughout the rev range. Not something I would expect to be able to tell?