Boost Gauge and Sender Testing

sigchuck · 09-08-2022, 08:56 AM

Hi folks, I'm attempting to add a 930 tach with boost gauge (and all of its associated shortcomings) to my 911 chassis.

The tach is generally good and functioned fine for many thousands of naturally aspirated miles. During this time with no wire on the boost terminal, the boost gauge would peg with the ignition on/car running. I think this would generally indicate the tach is functioning as intended.

I have a Koklen intercooler on the engine with a factory 930 sending unit. Since my car is a 911SC, I ran a single wire from the engine compartment to the front trunk and connected the ends to the appropriate terminals (single spade on sensor and boost location on gauge).

I'm unfortunately still getting the same behavior from the gauge - it pegs with the key on/car running.

I've read that it can be difficult to get sufficient ground for the sensor via an aftermarket IC. I think the sending unit has a separate (and currently unused) ground post that I could run an external ground to.

My understanding of automotive electronics is somewhat limited, so before I take the IC back off, I wanted to see if you guys might be able to confirm the following:

1. Un-grounded, the factory boost gauge should be fully pegged with the car on or running.

2. A single wire from the boost terminal on the tach to the single spade on the sender should be all that's needed to carry the signal, assuming the sender is otherwise correctly grounded.

Thanks for your help! Happy to post any additional information that might be needed!

spuggy · 09-10-2022, 01:29 PM

Quote:

Originally Posted by sigchuck

I've read that it can be difficult to get sufficient ground for the sensor via an aftermarket IC. I think the sending unit has a separate (and currently unused) ground post that I could run an external ground to.

Sounds like a plan. Good grounds never hurt. I don't recall mine having a ground post or two terminals. You sure it isn't just a random sensor to plug the hole in the intercooler?

Quote:

Originally Posted by sigchuck

1. Un-grounded, the factory boost gauge should be fully pegged with the car on or running.

2. A single wire from the boost terminal on the tach to the single spade on the sender should be all that's needed to carry the signal, assuming the sender is otherwise correctly grounded.

Correct AFAIK.

The gauge will/should peg with no signal to the boost terminal on the tach. No sensor ground would have exactly the same effect - eg no ground means no circuit, and no resistance for the gauge to display.

FSM says to ground the boost terminal wire from the tach. eg, pull it off the sensor end and find a ground in the engine bay (which also lets you test if the Kokelyn is grounded).

The boost gauge should drop to zero (as does resistance between the sensor input and chassis ground, which is what the gauge is displaying).

If the boost gauge doesn't drop to zero when the wire is grounded, one of the following is true

the wire is broken or otherwise not connected to the tach terminal,
gauge is defective, or
you didn't find ground.

FSM says that at 20C and atmospheric pressure, resistance of the boost sensor should be between 5-13 ohms (measured between the connector and the housing).

Alan L · Alan L's Garage

Take care to look how your after market IC is mounted. I had a similar issue with fuel pumps and o/boost sw after changing to a 965 IC. The mounting lugs were rubber mounted. There was no grounding thru the IC. So the boost sender had the same issue. A single grounding wire from one of the bolts holding the IC fixed all that.
Alan

sigchuck · 01:44 PM

Thanks, guys! The sensor is correct per the part number. I unfortunately was mistaken and there’s not a separate ground terminal on my sensor.

I unplugged the sensor wire and grounded it to both the car and IC, which zeros out the gauge with the key on. I think this is indicative of the IC having a good ground.

I’ve read a few threads on sensor failures and I haven’t seen any symptoms of a bad sensor that mimic bad ground. Any other thoughts? I’m not using thread tape, etc., so there should be good contact between the IC and sensor.

EDIT - missed the FSM test procedure - I’ll give that a try!

sigchuck · 09-11-2022, 07:08 PM

I tested and was having some difficulty getting a reading initially, but ultimately ended up with around 380-400 kilo ohm - so apparently not in spec, to say the least.

Since I’ve checked the gauge and confirmed the IC is a viable ground source, it must be the sender. I did find other threads that mentioned resistance rises with pressure (only into the low 100s of ohms), so presumably the increased resistance that I have in the bad sender is also maxing out the gauge and mimicking the same symptoms as an ungrounded sensor? Sound plausible?

spuggy · 09-12-2022, 07:52 AM

Quote:

Originally Posted by sigchuck

I tested and was having some difficulty getting a reading initially, but ultimately ended up with around 380-400 kilo ohm - so apparently not in spec, to say the least.

Since I’ve checked the gauge and confirmed the IC is a viable ground source, it must be the sender. I did find other threads that mentioned resistance rises with pressure (only into the low 100s of ohms), so presumably the increased resistance that I have in the bad sender is also maxing out the gauge and mimicking the same symptoms as an ungrounded sensor? Sound plausible?

LOL. As the resistance at 20C should be ~10 Ohms and it's reading ~400 KOhms, as you say, a long way out of spec. The gauge appears to be doing the right thing; it's trying to display "lots" of bars....

Certainly looks like the sensor should be replaced to me.

Unfortunately, the Porsche-branded part isn't no-brainer cheap; seems to be about $200 a pop.

However, Porsche don't make sensors, in the same way that they don't make tires, brakes, seats or gauges...

They source them from vendors. Sensors tend to be either Bosch or VDO (latter now owned by Continental, and parts can be ordered from O'Reilly - perhaps the very cheapest way to get them, based on my ordering a boost gauge and a combined oil pressure warning switch/10 bar sender recently).

I'd be very inclined to look at the threads that screw into the intercooler and match up the 930 spec boost pressure sensor with the generic VDO part and order that. Tend to be a lot cheaper, so worth a little time.

For example, VDO #360-081-032-058C is a 2 bar pressure sensor with a resistance of 10-180 Ohm for 0-2 bar, and has an M18x1.5 thread. Which seems somewhere in the ballpark of what I remember (big old hole in the intercooler, right?).

It's not identical to the factory part, as it has M4 screw terminals, and a dedicated ground post; just a thought.

$49 on Flea Bay. Little more from the vendors kind enough to publish the VDO catalog details:

https://veratron-marineshop.nl/en/pressure-senders/359-vdo-pressure-sender-0-2-bar-m18-4103590691217.html
https://gaugesvdo.com/products/453067962-360-081-032-058c-vdo-pressure-sender-2-bar/