Boost Gauge Off Scale.

[lotusflasherman]

Just driven home with my Boost Gauge showing around 0.4 bar boost on light throttle & low revs and then went off the scale when I stopped on my drive. 'Engine off & ignition on' gave me this

offscale.JPG

I was a bit worried about it damaging the gauge so thought I'd pull off the supply from the Boost Pressure Transducer but it obviously works in the other direction as it stayed the same. Then I noticed the bottom screw was loose allowing a 'floating earth' and a few turns to tighten it up and every hunky-dory again. Offending screw shown in this one..

bpt.JPG

Just thought I'd share...

[Saltire]

Thanks, Phil, off to check the offending screw now

[pmb]

Tick another box in the "success" column for tightening the ground screw! Erratic gauge operation completely tamed by a simple turn of the screwdriver.
Many thanks to the knowledgeable contributors on this forum.
Peter

[sinannasirli]

Any ideas if the following happens and if you have a good ground?

[John_W]

Any time I've seen people reporting a "boost off scale" problem, it has always been to do with a faulty earth connection at the transducer, as lotusflasherman's photograph shows.
John

[sinannasirli]

I did make sure that there is good ground.. Measured with multimeter... from the bracket to iback of the gauge..

It has ground.. I am going to get crazy....

Next thing i will do is to take the transducer apart.. or try with a new transducer..

MIne is an S2 car but the transducer is orange instead of grey, so somebody have messed up with it already, as they told S2 transducer is grey..

Regards

Sinan

[dapinky]

Sinan,

As others have stated, the issue is usually with the earthing of the transducer.

I know you have checked continuity to the guage earth, but when this happened on one of my cars and I couldn't see what was happening, I ran a seperate earth wire between the transducer and the earthing point at the bonet hinge. This solved the problem, but I still don't understand how it made a difference as (like you) I had already checked the full continuity of the circuit.

[lotusflasherman]

I did make sure that there is good ground.. Measured with multimeter... from the bracket to iback of the gauge..

It has ground.. I am going to get crazy....

Next thing i will do is to take the transducer apart.. or try with a new transducer..

MIne is an S2 car but the transducer is orange instead of grey, so somebody have messed up with it already, as they told S2 transducer is grey..

Regards

Sinan

My photo at top of this thread is a grey transducer on a '93 SE , so it may be an early or late issue ... earlier SE has orange and think I have a spare orange, but don't think colour is important. Working properly is.

[sinannasirli]

İs it possible for you or for anyone else to measure the resistance between G terminal and earth of the transducer?

What is important, it shouldnt be hooked up to the gauge when measuring, at least not the G terminal..
Maybe measuring any spar s you might have?

do you have connectivity with some resistance or no connection?
Thank you

Sinan

[lotusflasherman]

İs it possible for you or for anyone else to measure the resistance between G terminal and earth of the transducer?

What is important, it shouldnt be hooked up to the gauge when measuring, at least not the G terminal..
Maybe measuring any spar s you might have?

do you have connectivity with some resistance or no connection?
Thank you

Sinan

Sorry I can't for a while. I'm in south of France, Elan is in UK. Mr Pink is good at these things..... Dave?

[sinannasirli]

I did make the test with a friends car.. I was right and transducer is at fault.. So I am the unlucky one out of the all remaining people with a grounding problem..

If anyone bumps into a similar problem, here is how to troubleshoot with a multimeter..:

viewtopic.php?f=35&t=25442

Regards

Sinan

[shakadad]

Wow I just had my boost gauge stick up at max when turning the car on. This thread seems to have it solved, I'll go out to check that screw now.
Video of Gauge jumping to max...

[lotusflasherman]

Wow I just had my boost gauge stick up at max when turning the car on. This thread seems to have it solved, I'll go out to check that screw now.
Video of Gauge jumping to max...

Hope tightening that screw sorts it out for you ...

[Vaughnc]

Hello all on the LEC.
This is my first post to the LEC community since acquiring my M100 in spring this year (from muley if I am not mistaken).
I have had the same boost gauge problem mentioned in this and other posts. It was working as far as I recall when I bought the car, but at some point its stopped registering boost at all. I have solved it and proven that it is the earth connection. In my case it was not as simple as cleaning and tightening the earth connection on the boost gauge transducer (orange in my case). It needed a new Earth connection. I wired mine into the earth strap on the nearby hood/bonnet hinge. The original earth connection screws into the fibreglass bodywork. Presumably the nut/assembly behind the fibreglass bodywork has an earth connection of its own otherwise this could never have been earthed. But clearly this has failed or dropped off. It seems impossible to look at or get behind this assembly to investigate, hence my alternative approach.
Its great to see so many posts and helpful information on LEC!

[Brit-Car-Nut]

Since the earth circuit was damaged, you really should have repaired it rather than create a new source for that earth/ground.
In the future, someone else might never find your modification.
Other electrical systems that also get their earth/ground from that connection might not work correctly either.
It is best to put it back to the original design.

The earth / ground connection for that harness is at the RH Longeron common earth connector.

Here are the two circuit diagrams you should have referred to:

Instrument Circuit.jpg

Earthing Points.jpg

Manuals link is in my signature below

[Vaughnc]

Thank BritCarNut for your comments. You are probably correct but I could see no way to get behind the bodywork to check/fix the original earth connection. I've not removed the original earth connection from the transducer, just added an extra earth connection so it should not affect anything adversely I believe. Whenever I get chance I'll certainly look to see if I can get behind the bodywork to correct whatever has (presumably) gone wrong with the original earth.
All the best, V.

[Vaughnc]

An update.... apologies if I've been misleading. I was half asleep when I first investigated the issues I have. I had mistakenly assumed the earthing was via the supporting nuts into the bodywork rather than the obvious black earth wire (doh!). Tested said black wire with the multimeter - it's OK. But it's not tightly connected due to the old grommet-based fixing into the bodywork. Took all apart, sanded all connections, refitted and it works. But there is another issue as gauge stops works after 5-10 minutes. Perhaps the boost is failing when hot? More investigations needed.

[Vaughnc]

Finally resolved. I've found several issues with the turbo gauge and its sensor that may be helpful to know:-

1) if the wires to the Boost Pressure Transducer (aka sensor) are not connected, or are so furred up that there is no connection at all - then the boost gauge can register off the scale.
2) the black ground/earth wire may not make a sound contact - either it is oxidized with age, or too-loosely connected. The latter happens because the mounting screws for the transducer fit into nut within a rubber grommet. In my case the grommet perished and everything was wobbly and would not tighten properly. SOLUTION:- replace with a new grommet/nut fixing for a more secure connection if you can, OR drill a hole in the mounting bracket and fit the earth wire directly to this with a small nut/bolt. Don't forget to sand the connections to remove oxidation.
3) In noticed a small split in the rubber sheath that encases the transducer. This is a sort of bellows that flexes as the boost pressure changes. Luckily this was a small split but if large it will affect the performance of the sensor. Newer sensors by the same manufacturer (VDO) use a different technique for venting the boost which does away with the rubber sheath.
4) my problems were the earth connection at the back of the gauge itself! It would loosen after a few minutes of driving, so a working gauge stopped working then started working again next trip. SOLUTION:- recrimp the earth wire and refit.

I hope this post is helpful to someone! Its been a learning experience working on this issue!

[Brit-Car-Nut]

So as to not confuse anyone, the earth/ground to the Boost Transducer is NOT effected by attaching the ground wire to the mounting bolt for the transducer. The body structure where the rubber insulated rawl/well nuts are used does NOT offer a negative/ground/earth. The ground for the transducer is part of the harness and get's it's ground from the chassis near the right front suspension area (underside of the chassis).

The two rawl/well nuts are there only to hold the transducer to the car body.

[John_W]

Vaughnc,
Good catch about the earth at the rear of the guage itself - I’ve not seen that one before.

I’ve never thought of the rubber cover to the boost transducer as being critical to the performance of the component. I always thought it was basically a dust cover - although a bit over-engineered for that! If the split you found was affecting the performance of the transducer, I wonder what effect the hole that mice chewed in mine had. I didn’t notice any difference in performance or in gauge response. I repaired the hole with silicone just to seal it, and installed mouse traps in the garage.

John