matt_ledbetter

According to the repair manual, at 3.9" of vacuum, you should see 2.0-2.5 volts at pin D of the connector.

The '87 Turbo II I just got feels sluggish, although it might be because I am accustomed to my '93.

When I check mine, I get almost 5 volts at 4" of vacuum. It doesn't change much at all when I change the vacuum level. Inside the car, the stock gauge doesn't budge when I turn the key to on as it should according to the manual. Also, while driving the gauge only moves a bit. At full boost and high RPMs, it goes just a tick or 2 in to the positive. My autometer gauge reads about 7psi at full boost though.

I'm assuming the sensor has failed and it is running in fail-safe mode of 5psi, which is why it feels like a slug.

Has anyone else had the sensor fail?

-Matt
'93 Touring
'87 Turbo II

Ranzo

I personally have never seen a boost sensor fail. If your stock guage is working then the sensor is probably good. Maybe the sluggishness you feel is due to the older heavy rotor low compression engine.

Randy...

matt_ledbetter

As I said, the stock gauge does almost nothing while I'm driving... It normally doesn't move, but goes up a couple of clicks on full boost. The autometer gauge reads almost 7psi.

-Matt

Scott 89t2

stock the car only runs 5psi. if the gauge isn't moving then something is wrong.

akageals

Matt,

I JUST fixed this problem on my TII. I chased the low pressure problem for months only to find out that it was my gauge....it did exactly what you describe. I fixed it by removing the gauge cluster and cleaning the contact screws for the boost gauge. There are screws that go through the printed circuit and carry the cignal to the gauge. The contact points were obviously dirty AND the screws were loose. I cleaned the contacts, tightened the screws and now the gauge goes all the way to the top! Here are the voltage specs.
0 PSI = 2.25 volts
1 PSI = 2.5 volts
2 PSI = 2.75 volts

Basically it goes up .25 volts per PSI after that....my graph stops at 10 PSI at 4.0 volts.

Todd

PS. I'm sure that '87 does feel sluggish compared to your '93.

matt_ledbetter

It's got a downpipe and FCD... it runs about 6-7psi according to my Autometer and Blitz gauge...

-Matt

matt_ledbetter

Hi Todd and thanks for the good info...

When I test my boost sensor at the sensor, it puts out about 5 volts regardless of vacuum or boost. I'm guessing the sensor is bad, although that is not to say that after replacing it I won't be cleaning the contacts as you did.

-Matt

akageals

Are you checking voltage at terminal "D" according to the FSM? It's the lower left terminal if you are looking at the back of the plug with the connector pin at the top (Per the FSM). That is....IF...I remember correctly. I have an extra BPS sensor that is known to be good (I bought it because I thought that was my problem). I checked the output against my graph and it was perfect as is the one on the car. If you need the sensor let me know.

Todd

akageals

Sorry I just re-read your post.....I guess you are using terminal D.

matt_ledbetter

Yep, using terminal D....

This morning I got different readings. I get 1.9 volts at 3.9" of vacuum. This is *just* below spec. At 20" of vacuum, it only drops to 1.7 volts. With any pressure applied, it doesn't go above 2.0 volts.

How much for your used sensor?

-Matt

akageals

Well....I paid $25.00 + shipping. I'll sell it to you for the same but be sure you need it first! I t'd in a pressure gauge and a large syringe on the BPS line and then measured voltage across the entire positive range...1 PSI, 2 PSI, 3 PSI etc. . I found both sensors working perfectly. I'd clean those gauge contacts first and THEN see if you need to buy the part. Just FYI I got bad readings on my sensors at first but later found out I needed a new batter in my DMM....then they both checked out perfectly (don't know if that fits your situation though). You can get that cluster out by only removing the surround, electrical connector, and speedo cable. You don't have to remove the steering wheel or steering column cover. Took like 1 hr. total.

Todd

matt_ledbetter

Hi Todd...

I'm using a Mity-Vac or whatever its called... It can do 30" of vacuum to 30lbs or pressure.

When I checked it this morning, I got right around 2 volts regardless of pressure... I even cranked it up to 20psi and it didn't budge more than .1 or .2 of a volt.

According to the manual, it should be between 2-2.5 at 3.9" of vacuum... According to your specs, 1psi is 2.5 volts.

Regardless, mine doesn't move from 2 volts enough to convince me its working.

I will clean all the contacts tomorrow and re-evaluate.

Thanks,
Matt

matt_ledbetter

More info....

With the boost sensor disconnected:

Turn the key to On, stock boost gauge shoots up very close to the top. 2 lines down from +40.

Start the car, stock gauge reads about 1/2 way between 0 and 40 and stays there regardless of vacuum or boost.

idle is identical to when the sensor is connected...

driving the car feels MUCH stronger than with the sensor connected....

According to the manual, it should be in a fail-safe mode with the sensor disconnected. Going to 5psi in the maps regardless of boost or vacuum. The car SHOULD feel sluggish off idle, but it feels much stronger than when the sensor is connected.

-Matt

akageals

Hmmmm...weird...seems like that gauge should not move off the pin with the BPS unplugged. I'm sure of those specs. . Got them from fc3s.org in an article dealing with the DIY FCD. Confirmed the specs. on the two BPSs I have. If I remember correctly disconnecting the BPS disables the secondary injectors, be careful! Good luck....let me know what you find out.

Todd

PS. Doesn't the manual say not to exceed 14 PSI on that sensor...could have damaged it.

matt_ledbetter

Any chance you can disconnect yours and see if it shoots up to 40 when you turn the key to "on"?

The sensor can't read over 14psi, but it shouldn't damage it when applied.

Thanks,
Matt

matt_ledbetter

Obviously nobody is very interested, but I like to keep my own thread alive.

Looking at the wiring diagram (50-40), I'm pretty sure that the pressure sensor sends the signal to the ecu and the ecu sends a signal to the stock boost gauge.

This would explain why the stock boost gauge didn't move much and the car was sluggish. My worry is that it appears the ecu would only supply enough fuel for the 2 volts it was putting out, but I suppose the boost sensor is 2ndary to the air flow meter.

More evidence is that the stock gauge shoots up to near the top when the pressure sensor is disconnected. This being a result of the ECU going in to a failsafe mode of 5psi and sending that signal to the ecu.

Tonight I'm testing the pressure sensor on another Turbo II, so that should help a bit.

-Matt
'93 Touring
'87 Turbo II

akageals

Matt,

I'll check mine and see what happens with the sensor unplugged. Ted just answered another post and I think he said that the gauge was spliced into the signal coming directly from the sensor. Some guy wanted to replace the stock gauge with something else and I THINK he said it would be fine. Anyway...i'll let you know what happens when I unplug mine.

Todd

HAILERS

Lots of possibilities here. First of all the output wire is brown/red. With the plug off the max you should read on that wire is .52 volts. Note the decimal. If the plug is off, the boost gauge should read zero, zip, nada. If it reads anything at all, then you have a wiring problem. You say you read 5volts on the plug in an earlier post. I think maybe you are on the brown/white wire perhaps, and that would be normal. I guarantee that the gauge should not read with the plug disconnected from the boost/pressure sensor. The brown/red wire , output signal goes from the sensor to the ECU. On the way there, there is a splice in the brown/red wire that goes to the boost gauge. The only time I ever saw the boost gauge go to the position you talk about i.e. almost to the top with the key on, is when I put a n/a boost sensor in the turbo car. Different internals, thank you. First thing to do is take the plug off, and put your meter on the brown/red wire with the engine running. Write back and tell us what the voltage reading was and we'll tell you how to fix.

HAILERS

Here's the schematic. See the pressure sensor. See the brown/red wire. See the splice headed for the boost gauge. See how if the input is from the pressure sensor and the plug is off the pressure sensor that there will not be a reading on the boost gauge. Do you have a good less than one ohm ground on the brown/black wire? Do you have 5v on the brown/white wire? What is the N number on your ECU/??????? Should be N332. Its written in very large letters on the top of the ECU. If you take the middle plug off the ECU and turn the key on, do you get the boost gauge readings still???? Honked up ECU I'm betting.

HAILERS

Dummy forgot to add the jpg:

HAILERS

GRAPH:

matt_ledbetter

Hailers,

I can't check the sensor until later, but I will run all the tests again this afternoon.

I agree with you on the ECU wiring diagram, but if you look at the gauge cluster diagram (50-40), it makes it appear that the stock boost gauge is controlled by the ECU.

It's quite possible the initial 5volts reading was on the wrong pin, but the follow-up tests were done correctly. I got right around 2volts regardless of vacuum or pressure.

-Matt

HAILERS

Signal for the boost gauge comes from the splice off the brown/red wire, goes to the gauge and the ground is from the gauge to the brown/black wiring. Ever notice how the brown/black wiring is spliced at all those precious sensors like the boost sensor, variable resistor, afm, thermo sensor, atmospheric sensor blah, blah. Those are all solder joints made by some guy who didn't get any last nite and he could care less if you have any problems with your car as long as he gets paid so he can go find some comfort girl on pay day to get some relief. Those brown/black end up at pin 2C on the ECU. 2C ain't an output, unless you work at it. Thats why on some cars its advisable to add an additional ground to the 2C wire. Not all cars, just some.

matt_ledbetter

The number on top of the ECU or the pressure sensor?

-Matt

matt_ledbetter

Update:

You are correct, with the pressure sensor connector plugged in to my FCD, but not to the actual sensor, my boost gauge shoots towards the top.

With it plugged in to neither the FCD or the sensor, it doesn't budge.

The number on top of the sensor is N328.

Where is the Ecu in this car? Behind the kick panel like my '93?

-Matt