Mac valve and relay question.

s300 and SManager software questions & answers
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originaldroidster
Posts: 12
Joined: Fri Jul 27, 2018 11:35 pm

Mac valve and relay question.

Post by originaldroidster »

Recently I noticed in the install faq that you should not should not have a relay to power mac valve. I have been running one like this for a long while with no issues, however I do not want to damage the ecu.

I also use relays that are ground triggered by ecu which can be a bad thing if you're not using a relay with a flyback diode which I am using. If I am using arelay with flyback diode for the mac valve, is it safe to continue using relay to power the mac valve, or is the relay not be used for other reason beyond voltage spikes when the relay coil shuts off?
crluver123
Posts: 155
Joined: Tue Oct 10, 2006 10:13 pm

Re: Mac valve and relay question.

Post by crluver123 »

mac valves work off duty cycle at specific frequencies. Relays can not react fast enough to provide accurate duty cycles or frequencies. The Mac valve needs to see raw the square wave directly from the ecu signal.

The only reason to use relays on anything is when the ecu trigger is a low amp circuit, and the device being operated uses a higher amperage than what the ecu can provide.
Mac valves use much lower current than the boost control circuit in the ecu, so it's also totally unnecessary to use a relay for the boost solenoid.
originaldroidster
Posts: 12
Joined: Fri Jul 27, 2018 11:35 pm

Re: Mac valve and relay question.

Post by originaldroidster »

Right I understand the concept of a relay, and I understand sometimes many devices have a low amp draw and relay would not be needed. I have ecu ground triggering fan relay and also fuel pump. Many cheaper relays do not have a diode and the the coil disconnects on relay it can get a large voltage spike on the trigger side which can damage ecu's, which Is why i use flyback diodes. So I was curious if that was why the faw specifically stated not to use a relay, which now i know it's not

I have no problem connecting to a switched 12v, One of my relays in my fuse/relays box powers up some gauges and other things. I do realize for something with a really small amp draw a relay would not be needed. When I made my switch panel and fuse relay box I just had the switches trigger a few relays, and put more then one devices on a few of the relays.

I also realize the ecu use pwm to control the boost controller,

My frame of thinking was the relay is already activated when car is turned on because its powering a few gauges, so power is already running through the coil of the relay, and the other wire of the boost controller is going directly to ecu.

Once the relay has been activated with power and the connection is made then the response is just the same for the power going through it right? When you say relays are much slower, you're referring to the activation, correct?


I understand if the ecu was having to cut the relay on and off then it would be far , far to slow for pwm. I guess I was thinking about it the wrong way.

Thanks for the input, really appreciate it. My post may sound like im arguing against using a regular switched 12v feed, but i'm not, i was just trying to understand.
originaldroidster
Posts: 12
Joined: Fri Jul 27, 2018 11:35 pm

Re: Mac valve and relay question.

Post by originaldroidster »

I re read the post and just wanted to make it clear that i was not referring to using the pwm output wire to trigger a relay, one wire of the mac valves wire is connected to an already triggered relay , and the other wire from mav valve to ecu. I'm already going to move it over to switched 12v feed, at this point i'm just curious on some of my questions.
crluver123
Posts: 155
Joined: Tue Oct 10, 2006 10:13 pm

Re: Mac valve and relay question.

Post by crluver123 »

I misunderstood your original explanation. I did think that you were using the ecu to trigger a relay on the pwm circuit, and the relay trigger the solenoid.

After reading everything here now, your setup is totally fine with the solenoid connected to a relay for its power source. Afterall, that's how all the stock wiring is controlled anyway; via relays that turn on with ignition on.

What you were reading about not using a relay for the solenoid was referring to the pwm circuit controlling a relay, and the relay controlling the solenoid. Which it appears you definitely seem to understand would be horribly wrong haha
originaldroidster
Posts: 12
Joined: Fri Jul 27, 2018 11:35 pm

Re: Mac valve and relay question.

Post by originaldroidster »

Yeah man reading your response for a second time earlier made me get the idea you may of thought I was talking about using pwm output to hit the trigger side of a relay, so that is why I put the 2nd response, thanks a ton for the input man I appreciate it. Yeah I do under that that would be totally wrong, hell even if using a solid state relay on the trigger side I'm sure it would still be totally wrong.

I built my stand alone fuse relay box to control, ecu power, sensor power, coil power, etc. I am still learning, and enjoying it very much. I knew it was probably ok to have the mac wired like it is, but seeing that no relay comment in the faq section got me thinking. I mean I at least knew it would work the way i had it because, well it worked :) After reading that I wanted to make sure I would not eventually damage ecu.
crluver123
Posts: 155
Joined: Tue Oct 10, 2006 10:13 pm

Re: Mac valve and relay question.

Post by crluver123 »

no you won't damage the ecu running it the way you have it.
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