Digital Boost Gauge Fluctuating Signal

[ChrisP]

Hello Everyone

One of the problems needed to be attended to in my newly aquired Eclipse would be the recently fitted Autogauge Digital Boost Gauge.

Everything is wired up and I'm getting a good but fluctuating reading between 13 - 16 psi at full throttle. I managed to trace the cause and it's not mechanical.

Judging from previous electronic faults and numerous posts and threads on this forum, I know that the Eclipse doesn't have very good Earth. Simply using electronic components such as the electric windows or the indicator proves this as you can see that dash lights seem to dim when it engages. Not alot though.

If the car had to be idling/ driving and the boost gauge steadily reads -5 psi, being electronic, if I operate the windows or any other interior auxilar, the boost gauge drops to -3 psi and respectively returns to -5 once I stop using the windows or other. Now my argument is, the engine ecu and ignition constantly uses power to run the engine and if the windows or other components have a 3 psi affect on the reading, then surely the engine electronics have a constant effect on it, adding to the incorrect reading.

I wired it as follows:

Gauge had 4 wires
1. Positive 12v ignition (that I took to the rear wiper switch as one of the wires had a positive battery voltage when I switch on the ignition)
2. Dimmer (did not connect)
3. Earth (Connect it to the closest earthing point, passenger airbag bracket)
4. Signal wire to sensor in the engine bay

Three wires are on the sensor:
1. Power to the sensor 12v+ (I did a devac on my car so I used one of the plugs from the vacuum solenoid as positive ignition power)
2. Earth (Connected to intake manifold) bracket
3. Signal wire to gauge.

Any advisce on how to ensure the gauge won't fluctuate? Should I rather be connecting the sensor and gauge to a different earth and positive?

Thanks for reading.

 

[dsmgst1996]

If you think its a bad ground you could always run an extra ground strap off the battery to the engine and then you would make sure you have good ground . Also sounds like you need a a few extra grounds to your body from what you are describing.

 

[Breezio69]

One thing to remember when your at idle and messing with power window buttons etc, the draw in current puts a load on the engine and the vaccum goes down. Same thing as lightly tapping the right pedal.

Also, if you get a correct reading it should show a vaccum of around -14 or more if there are no leaks or anything, I run a +/- 30psi gauge and my vac at idle is between 15-16psi. Maybe the fluctuation of the gauge is a leak and nothing is actually wrong with the gauge at all?


Edit, also... lots of eclipses have the "problem" you are describing with dimming lights and even rpm fluctuation when you do electronic functions. Mine dims when I push buttons, turn on headlights, turn the heater on. But it reads 14+ volts leaving the alternator on mine.... thats just how it is

 

[ChrisP]

dsmgst1996: I will run some earths from the battery to the body. Which gauge size wire should I run and where do you recommend I run it to from the battery negative? There is a 4 gauge currently from the battery earth to the shock pillar, mounted to some thread which seems like the factory mounting spot.

breezio69: Thank you for the info. The mentioned vacuum makes sense. I thought that would have been the case until I saw it doing it whilst under constant boost. Like driving in 5th and flooring the pedal to max boost and operating the window or rear dimister causes a 3psi fluctuate. I will build the boost leak tester and carefully check for leaks at the same time.

I have DeVac'd the eclipse and I'm currently using the large outlet towards the fuel rail to supply vacuum/boost to the gauge. Is this fine or should I eather use a different outlet? There are 3 others available that routed to some solenoids on the firewall. The reason I'm asking is because I dont know whether those outlets are fitted with restrictors and may give an incorrect reading.

Kind Regards

 

[Breezio69]

Typically people tap the vac line into the fuel pressure regulator line with a tee.

Also for reference... my boost gauge is only vacuum/boost operated (mechanical), but my oil pressure gauge is fully electronic. When I operate windows or put an electrical load on the car it fluctuates this gauge by 4-6psi... I can sit and toggle the window switch up and down and the gauge will just bounce from its "real" reading to the reading with the load on it.

I guess its better that its the oil pressure on mine.... can you hook up a mechanical gauge in tandem with the digital to make sure the digital gauge is even accurate?

 

[ChrisP]

breezio69: Ill tap it in with a tee then and see if there are any changes.

Im going to try and find out exactly how the gauge changes pressure to voltage. maybe there's a way we can ensure 12v constant to the gauge for dead accurate readings on boost/oil pressure. Like installing a electronic component/voltage reducer on the + wire that always keeps the voltage at 12v.

Great idea! I should have another mechanical gauge lying around somewhere. Ill install to compare the readings. I deviated from mechanical because I wanted to ensure accuracy and at times, with mechanical, you have difficulty pinpointing the exact reading quickly under acceleration(after making adjustments) especially if you are boosting like a specific pressure for instance 0.8 or 1.3.

 

[jeff1717]

Some gauges come with a restrictor that fits in the vacuum line. To smooth out the signal and keep the needle from bouncing.

 

[Breezio69]

Some gauges come with a restrictor that fits in the vacuum line. To smooth out the signal and keep the needle from bouncing.

I actually did this in my own way with my gauge. The old one had a hard platisc line (autometer) and the new one that matches my others had rubber, I took a 1" piece of the old one and stuffed it in the rubber line then hooked it all up. Has a really smooth read and I had both gauges hooked up at one point to check accuracy... no problems since

 

[ChrisP]

Please excuse the delay but I managed to pop the turbo on my eclipse. The turbo decided to supply boost to the oil line and the oil decided to lubricate the exaust ... Absolutely wonderful!

Okay, we would need a Zener 12v Diode. One would expect the gauge to have one (I dont know if it is fitted with one) but at the price of 40c a diode, it cant hurt fitting one. It's a simple solder on between the gauge and voltage supply line. It will only allow through 12v, regardless of the input voltage (up to a certain point ofcourse!) Will try this the weekend and see if this solves it.

Regarding the restrictor. My gauge came with a unit that looks a lot like a mini filter. I have added this to the system prior to this post. So I wouldnt know the effects of not using it.

 

[luv2rallye]

If your gauge and sensor each have a signal ground wire (separate from chassis ground wire), connect them to each other only. Otherwise if all else fails you might try connecting your sensor's ground to ECU signal ground through one of the other sensors signal grounds that go there. No guarantee it will fix your problem since your sensor doesn't go to the ECU but at least this signal ground wire has very little noise and small current in it.

On a 2g (but not 1g) ECU pin 92 (note: this pin is incorrectly labeled Ignition switch on some 2g ECU pinout diagrams) goes to the signal grounds of the following engine sensors: manifold diff pressure, engine coolant temp, front O2, rear O2, TPS, volume air flow (MAF), and on the 98 & 99 fuel tank diff pressure. Pin 92 is grounded inside the ECU but all these sensors signal grounds must NOT be grounded anywhere but through pin 92 to avoid electrical noise and ground loops from affecting the sensors differential signals and to guarantee that the sensor's ground is at the exact same voltage potential as the ECU ground (which it might not be with large current flowing through any other ground).