Brakes make Scanmaster go Crazy

[oneslobuick]

I know this sounds crazy but I have had this problem for a while but I just found the source. Ok I noticed that when the car is idleing in park and press the brake the scanmaster numers go crazy (all parameters are off, the numbers just bounce around and the ATS goes to dashes)when the peddle is released every thing goes to normal.


any suggestions

 

[gbsean]

where is the gound for the scanmaster..

 

[hrhoward]

Brake pedal messing with some wires?

 

[oneslobuick]

grounded to the grounding screw under the dash on the left side near the floor lamp.

 

[hrhoward]

Does it do it when the key is in the "On" position without the engine running and brake pedal depressed? (And I don't mean sad)

 

[BlackMetal]

Sounds like you might be pressing on the wire or somehow moving it when you move the brake pedal. When my ScanMaster wire starts to get loose I get hilarious readings like 89 degrees of knock retard. Still need to get the proper terminal soldered on the end of it so it stops moving.

 

[TurboBob]

unplug the center relay on the drivers side inner fender and see if the problem goes away

(seriously.......)

Bob

 

[hrhoward]

unplug the center relay on the drivers side inner fender and see if the problem goes away

(seriously.......)

Bob

Got to ask..........???

 

[TurboBob]

Got to ask..........???

Ok, the long answer is:

the low speed fan ralay is controlled by the same chip in the ECM as the serial data output.

the low speed fan output is not a typical on/off, but a PWM, that is set to 99% to turn the relay on.

Every cycle of the PWM, the relay coil kicks back a voltage spike. The spike bleeds over from the relay control section of the chip to the section that handles the datastream. Some ECMs are worse than others on this.

When the brake pedal is pressed, the spike is worse (don't really know why on this one)

The scanmaster has noise filtering circuitry, but if the noise is bad enough it will corrupt the datastream.

Bob

 

[hrhoward]

Bob,
Would an external suppressor help with this problem?

 

[TurboBob]

yes, but you want to supress the relay, not the datastream. A reverse biased diode across the relay coil works. Or replace the relay with one with an internal diode. (verify polarity)


Bob

 

[hrhoward]

Like this?

 

[Shane]

I think he means to use the diode as more of a short to ground for any interference.

In other words connect the diode across the contacts for the coil side of the diode with the anode on the ground contacts and the cathode on the signal contacts.

 

[TurboBob]

this is what I mean


Note: DO NOT GET THE DIODE BACKWARDS OR YOU WILL SMOKE YOUR ECM

 

[L917]

unplug the center relay on the drivers side inner fender and see if the problem goes away

(seriously.......)

Bob

Thanks for the tip! I was about to lose my mind. I unplugged the center relay, like you said, and found the plug was melted- but my scanmaster works fine when it's unplugged. I'm going to replace the plug and relay box to see if the problem goes away.

 

[NY Twin Turbo]

Ok, the long answer is:

the low speed fan ralay is controlled by the same chip in the ECM as the serial data output.

the low speed fan output is not a typical on/off, but a PWM, that is set to 99% to turn the relay on.

Every cycle of the PWM, the relay coil kicks back a voltage spike. The spike bleeds over from the relay control section of the chip to the section that handles the datastream. Some ECMs are worse than others on this.

When the brake pedal is pressed, the spike is worse (don't really know why on this one)

The scanmaster has noise filtering circuitry, but if the noise is bad enough it will corrupt the datastream.

Bob

Bob,
Would an external suppressor help with this problem?

yes, but you want to supress the relay, not the datastream. A reverse biased diode across the relay coil works. Or replace the relay with one with an internal diode. (verify polarity)


Bob

What?

Are you guys from Mars?

 

[L917]

Thanks for the tip! I was about to lose my mind. I unplugged the center relay, like you said, and found the plug was melted- but my scanmaster works fine when it's unplugged. I'm going to replace the plug and relay box to see if the problem goes away.

I cleaned out the gunk from the existing plug and replaced only the hi/low relays with a cheapo generic from the auto parts and that did it. Problem gone. Scanmaster reading real numbers now.

 

[NY Twin Turbo]

I cleaned out the gunk from the existing plug and replaced only the hi/low relays with a cheapo generic from the auto parts and that did it. Problem gone. Scanmaster reading real numbers now.

These cars can sometimes do the weirdest things.
Glad you got it fixed.