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Boost Gauge Pressure Sending Unit Problem

Hello Pelican -

I am having an interesting problem. Since I've purchased my 930 in January, I've gone through two pressure sending units for the boost gauge. First one I put in lasted <100 miles, and second probably even less. Both work properly, then suddenly the boost needle gets stuck at 1.5 bar (77 Turbo Carrera). Luckily our host is awesome enough to warranty the piece... but is my car killing these pressure sending units? Of course, no fuses are blown because as soon as I plug a new one in, everything is dandy! Piece is Made in Germany by VDO. Host Part # 993-606-103-01-M100. First sending unit had reminisce of dap oil on it (blow by?), but nothing else major.

Any guidance would be much appreciated. While I get the warranty work, it's annoying to keep sending things back and forth. Perhaps it's just a coincidence?

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Old 04-30-2014, 05:26 PM
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Why not use a 2 bar gauge, then you won't be slamming the other one so hard it sticks?
Old 05-01-2014, 04:03 AM
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By the rear fuse panel there is a 14 pin plug,

the Sender wire to the boost sender inside that plug is brown with white spots

looking at the rear of the engine it will be the wire that is far bottom right

This wire is fairly thin and only soldered into a round cap , over time the cap turns in its socket wearing the strands of the wire away and either shorting out on the adjacent wire inside the 14 pin plug or sending a weak signal o the guage,, or other problems,

Some folk say to wiggle it and it comes good, ,,
not good idea because every time its wiggled that wire loses another strand or two,
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Old 05-01-2014, 07:09 AM
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Quote:
Originally Posted by Porsche 935 View Post
Why not use a 2 bar gauge, then you won't be slamming the other one so hard it sticks?
I'd like to stick with the factory 1.5 in the speedometer, just to keep things more original. I did think about adding a separate gauge somewhere, possibly in the clock opening, but the clock works... so decided against it. Still may be an avenue I pursue though!
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Old 05-01-2014, 07:13 AM
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Quote:
Originally Posted by Robbbyg View Post
By the rear fuse panel there is a 14 pin plug,

the Sender wire to the boost sender inside that plug is brown with white spots

looking at the rear of the engine it will be the wire that is far bottom right

This wire is fairly thin and only soldered into a round cap , over time the cap turns in its socket wearing the strands of the wire away and either shorting out on the adjacent wire inside the 14 pin plug or sending a weak signal o the guage,, or other problems,

Some folk say to wiggle it and it comes good, ,,
not good idea because every time its wiggled that wire loses another strand or two,
Thanks Robbbyg,

I'll take a look at this wire when I get home. Hopefully this is the issue, however, I still can't explain why I have to replace a sending unit every time it shorts, or sticks at 1.5 bar.

Thanks all for suggestions!
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Old 05-01-2014, 07:15 AM
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If I recall, should a person pull the wire off the sender, the gauge will peg. The sender provides a ground path for that wire, so if the sender itself is poorly grounded you could get erratic gauge action (as in pegging to max whenver the ground is lost). This is especially true when the boost sender is mounted to the Intercooler, since the IC is pretty much rubber mounted.

If you suspect poor ground, then run a separate ground wire attached directly to the body of the sender. You guys will have to help me 'cause I don't remember precisely and don't have a sender to look at, but memory says there are two spade connectors on the sender. Perhaps one of them is intended as a separate ground point?

You can also test your sender by removing it and applying air pressure, and with a VOM reading the resistance. The resistance should increase as pressure increases. If you need it, somwhere I've got the resistance values across differing boost ranges.

You may very well have returned two perfectly good senders!
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Old 05-01-2014, 11:45 AM
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Good point, Mark. But the IC must have a good ground for the overboost switch, otherwise the fuel pumps would never run, assuming the fuel pump circuit hasn't been modified to bypass the overboost switch. Hopefully the OP isn't using thread sealant on the boost gauge sensor.

SR
86 930

Last edited by S1000RR; 05-01-2014 at 12:53 PM..
Old 05-01-2014, 12:49 PM
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Quote:
Originally Posted by S1000RR View Post
Good point, Mark. But the IC must have a good ground for the overboost switch, otherwise the fuel pumps would never run, assuming the fuel pump circuit hasn't been modified to bypass the overboost switch. Hopefully the OP isn't using thread sealant on the boost gauge sensor.

SR
86 930
Stock location for the OB switch is on the boost recirculation manifold (and actually, the same holds true for the boost sending unit now that I think about it). But when people remove that manifold in favor of running with a long neck IC and a 965 BOV, then the intercooler becomes the mounting point for those problematic gizzmos.

Good point on the thread sealant, by the way!
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Old 05-01-2014, 01:10 PM
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Thanks again for the guidance. The car is a 77, so there is no IC. It sits on the manifold, and what you mentioned about the grounding problems do make sense. The only weird thing is that if it was a grounding problem, a new unit wouldn't make everything go back to normal, right?

It almost seems like the setup damages and breaks the pressure unit?
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Old 05-01-2014, 02:32 PM
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Quote:
Originally Posted by spitz View Post
Thanks again for the guidance. The car is a 77, so there is no IC. It sits on the manifold, and what you mentioned about the grounding problems do make sense. The only weird thing is that if it was a grounding problem, a new unit wouldn't make everything go back to normal, right?

It almost seems like the setup damages and breaks the pressure unit?
A new unit will/should only show the same problem,
so that part of your description has me stumped

But if you want to check the cintinuity anyway
Ive just resoldered mine but see the pic below where im pointing, if you take the black cap off the cover of the pin connector and test continuity from the brown wire straight to the lug on the sending unit youll find that youll have 100 percent -0 continuity and that is your problem, ive just done mine ive also got 1.5 bar on my Euro 77 and mine was always stuck on bout 1/4 bar then it went to 1.5 and sat there for a while.. now it sits squarely on zero after fixing the brown wire

While your in there take a pic of BEFORE you start taking wires out if you decide to do the lot. Ive been awake all night because of another unforced error on my part

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Last edited by Robbbyg; 05-01-2014 at 05:18 PM..
Old 05-01-2014, 05:13 PM
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Quote:
Originally Posted by S1000RR View Post
Good point, Mark. But the IC must have a good ground for the overboost switch, otherwise the fuel pumps would never run, assuming the fuel pump circuit hasn't been modified to bypass the overboost switch. Hopefully the OP isn't using thread sealant on the boost gauge sensor.

SR
86 930
Good point re thread sealer,

To overcome that i use Loctite medium the light Red one, if used sparingly a good Ground is still maintained,

That ob switch is a pain twice ive lost boost from it coming loose in the past
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Old 05-01-2014, 05:16 PM
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Your sender should not fail. It's just a simple potentiometer that moves and changes resistance in response to the pressure it sees. The more resistance it sees, the higher the resistance and the higher the gauge reading. It is not under any + voltage, just the negative ground lead from the gauge is attached to it. So if it continues to give problems, most likely it has something to do with the wire from the sender to the gauge (cracked or broken wire, bad connection somewhere).

Here is the full range of the resistance readings of the senders, for bench testing, that I picked from a previous post of mine a few years ago.

0.0 Bar 5 - 13 Ohms
0.9 Bar 117 - 129 Ohms

Of course you would have to take it off the car and put air pressure to it to see if the values are in line. Somewhere I have a complete mapping of resistance values for each psi
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Old 05-01-2014, 05:53 PM
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Quote:
Originally Posted by Porsche 935 View Post
Why not use a 2 bar gauge, then you won't be slamming the other one so hard it sticks?
That might sound like a good idea, but like putting in a 20AMP fuse into a circuit designed for a maximum of 15AMPS, the results may be less than desirable.

The root cause should be found first as to what is the problem.
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Old 05-04-2014, 07:19 AM
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Quote:
Originally Posted by Robbbyg View Post
A new unit will/should only show the same problem,
so that part of your description has me stumped
New unit always operates correctly, then dies after a few 100 miles.

Quote:
Originally Posted by mark houghton View Post
Your sender should not fail. It's just a simple potentiometer that moves and changes resistance in response to the pressure it sees. The more resistance it sees, the higher the resistance and the higher the gauge reading. It is not under any + voltage, just the negative ground lead from the gauge is attached to it. So if it continues to give problems, most likely it has something to do with the wire from the sender to the gauge (cracked or broken wire, bad connection somewhere). [/COLOR]
Looks like I am still running into issues. I'll look at all the suggestions highlighted in this thread again and do more testing.

Our sponsor sent out a new piece to me (second unit under warranty) and the unit died AGAIN. I have now gone through three separate VDO units, all failed. I don't understand why the units would die, since mark houghton mentioned that the unit was receiving no + voltage.

Again, the boost pressure unit is dying, not the gauge itself. Every time I put a new unit in, it works for a few hundred miles then dies. Is it possible that something is getting into the unit itself?

I'm stumped. Here is a video of the result, although it probably won't tell much:

Boost gauge problem, Porsche 930. - YouTube
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Last edited by spitz; 05-20-2014 at 02:23 PM..
Old 05-20-2014, 01:58 PM
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Weirdness.....Put a voltmeter between the sender wire and ground; see if you have some voltage that shouldn't be there. I still feel that's nothing more than a ground connection but hey, I've been known to make mistakes and don't have an electrical schematic in front of me.

Here's another pregnant thought: It may be a fault with the gauge itself, drawing too many amps through that sender/variable resistor ground contact, and frying it as a result. Put an amp meter between it and ground, see what she's drawing. Just a long shot....
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Old 05-20-2014, 02:52 PM
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Quote:
Originally Posted by mark houghton View Post
Weirdness.....Put a voltmeter between the sender wire and ground; see if you have some voltage that shouldn't be there. I still feel that's nothing more than a ground connection but hey, I've been known to make mistakes and don't have an electrical schematic in front of me.

Here's another pregnant thought: It may be a fault with the gauge itself, drawing too many amps through that sender/variable resistor ground contact, and frying it as a result. Put an amp meter between it and ground, see what she's drawing. Just a long shot....
Good call on the gauge being iffy - I did notice some erratic behavior. It would have random jumps to 1.5 bar on full throttle, although most of the time it holds to stock .8 bar at full boost. It looks more like something funky is going on with the electronics, as I don't feel an over boost. These are electric gauges, and not mechanical... so I'll check that out.
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Old 05-20-2014, 03:06 PM
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Quote:
Originally Posted by mark houghton View Post
Weirdness.....Put a voltmeter between the sender wire and ground; see if you have some voltage that shouldn't be there. I still feel that's nothing more than a ground connection but hey, I've been known to make mistakes and don't have an electrical schematic in front of me.

Here's another pregnant thought: It may be a fault with the gauge itself, drawing too many amps through that sender/variable resistor ground contact, and frying it as a result. Put an amp meter between it and ground, see what she's drawing. Just a long shot....
Alright all - here are my continual attempts at preventing my boost sensors from dying (three killed boost pressure sensors!)...

Without the boost pressure sensor, I measured voltage of the brown wire (that is supposed to attach to the sensor) to ground. I got a reading of 9V. In addition, I read the current to be 10Amps. Lastly, the boost gauge went to 0 when the brown wire was grounded.

Measuring the source of the boost gauge to be 14.5V and using ohm's law, I deduced the boost gauge's resistance to be 0.55 ohms and the resistance of the "noise" that is causing a 9V drop at the brown wire to be 0.9 ohms.

Knowing from the previous posts that a 0 Bar reading from the boost pressure sensor is 13 ohms. If this is the case, the 0.9 ohms that is causing the 9V reading isn't enough to cause the boost gauge to read max at all times.

With the current (broken?) boost pressure sensor connected to the brown wire, there is a 14.5V reading where the brown wire meets the sensor and 0 Amps across it. This is the characteristics of the sensor being at high resistivity with low current, which in turns causes the boost gauge to peg at high. So it seems the boost pressure sensor is operating abnormally. There should have been 13.9V and 1A measured where the brown wire meets the sensor at 0 Bar and at with the resistivity given.

Questions:
1. Obviously a 9V reading at the brown wire should have been 0V. The common ground seem to have some minute resistivity. Does anyone think this is why the boost pressure sensors are killed?

2. Is the 10Amps of current too high for the sensor (and causes the sensor to malfunction)? The original fuse in this circuit is 15Amps. That would be 145 Watts if the sensor was at 0 ohms at worse case.
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Last edited by spitz; 06-09-2014 at 09:20 AM..
Old 06-09-2014, 09:17 AM
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This issue has me pulling my hair, anyone?

Someone surely has to have a Boost Pressure Sensor killing 930!
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Old 06-13-2014, 09:48 AM
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Hi where is the location of the Pressure Sending Unit for Turbo Boost in 1986 930?
the boost gage is stock in 1bar and it was working fin few weeks ago not is stock there

some help please
Old 08-08-2014, 01:13 PM
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Max boost reading of OEM sensor

Anyone know range is factory boost sensor ?
1979 T

I have a 1.4 bar spring and the factory 1 bar gauge in tach face -
Thinking about external gauge or maybe have the tach changed by Palo Alto gauges if they do such a thing

I am also wondering if I can attach two gauges to the sending unit - so the factory gauge will be operational and a 2 bar gauge will tell me whole story. I am really clueless with electrical - so my guess would be no. If I split the wire then each gauge would get half the reading ??


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Old 05-04-2016, 01:31 PM
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