WTF Is Going On Here?

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WTF Is Going On Here?

Postby Mark Dalton » Wed May 11, 2016 9:42 am

Short Story Long.....but bear with me.
We had a car come in for not starting (no crank) at times and noisy (sounds like the starter motor gear clashing with the ring gear) when cranking at other times.
It had been to another Workshop who had replaced the Starter Motor twice.
The Car had logged a P0615 : Starter Motor Relay Control Circuit Malfunction.
We swapped the Starter Motor Relay but the fault persisted. The basic tests were carried out, checking for intermittent opens, shorts and high resistances, all of which tested OK.
We set up a LED test light (the test light lights up green or red depending on the direction of polarity) into the back of the Starter Motor signal wire to see if the Starter Motor was actually getting a signal voltage even when the engine would not crank. So we couldn't duplicate the no crank condition but did witness the noise during crank.
We noticed something odd. When the car cranked normally the test light lit up green, when the car cranked but was noisy the LED had some red flickering in it, as though the polarity was changing.
I didn't know what to make of it, so put it to the side for the time being.
So we decided to backprobe the Starter Motor Relay Control Circuit at the Engine ECU and removed the casing from the Starter Motor Relay to test at the terminal inside the relay, so we could test for any voltage drop across that circuit.
We managed to replicate the no crank condition and there was no signal voltage coming out of the Engine ECU, fair enough. Then the Engine started to crank again but was noisy during crank.
When i looked at my scope capture there was large voltage spikes, but the opposite polarity??
Can any explain what might be going on here? I it just the Starter Solenoid inducing it back into the circuit because the Starter Relay is being switched off and on rapidly?
Have attached a GIF and and a screen shot to aid my novel of a story above.
Attachments
2016-05-11.png
YUL697.gif
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Re: WTF Is Going On Here?

Postby STC » Wed May 11, 2016 10:54 am

Hi Mark

What Car and Engine Code is this ?

P0615 will only ever be pointing at the "Control" Side of the relay. The ECU can't "Directly" see what the Load side is doing.

Is the Relay(s) fitted the correct part ? Is the Internal Diode faulty or missing ?
Did the Garage fit Genuine / Quality Starter Motors or that Junk in a Red Box from ECP ??

Would be good to see all 4 channels back probed into all 4 pins of the relay. Then swap the Term 30 (Live Feed) channel with an amp clamp on Term 87 (Load). Then a High Amp Clamp on Starter Current.
Where is that opposing voltage / polarity coming from if it is not the Starter motor Internals ?
Next would be Volt Drop the Starter Supply and Earth
(More Channels Please Pico :D )


Add to that the Noise that you can hear !

What happens if you pull the wire off then use a power probe to excite the starter motor ?
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Re: WTF Is Going On Here?

Postby Mark Dalton » Wed May 11, 2016 10:26 pm

Hi STC,
Your absolutely correct of course, about the control side of things, wasn't thinking about it enough.
There's multiple relays in that fuse block that are all the same number and we've swapped it more than once, so we're reasonably happy it's not the relay.
Starter Motor is a Genuine Part, the Starter Motor / Battery Harness has been replaced as well.
My Scope is only a two channel, so I can't scope all 4 relay terminals.
I have operated the starter by just bridging 30 & 87 of the starter and cannot duplicate either the no crank condition or the noise during crank.
Voltage drop for the starter supply and earth was around 100mV give or take a little bit.
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Re: WTF Is Going On Here?

Postby STC » Wed May 11, 2016 10:45 pm

Hello again Mark.

If you cannot replicate the fault by bridging the relay then we are back to a "Control" Side Issue. Have your scope connected to Terminal 50 at the Starter just to confirm it isn't kicking out odd / negative Voltages.

What can you see with scope probed into 85 & 86. ?

I think, for now, I would be fixated on the Relay & the Diode in it !!! the alternative being the ECU itself !!! Perhaps Scope ECU Powers & Grounds Whilst Cranking.
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Re: WTF Is Going On Here?

Postby STC » Tue May 24, 2016 8:19 pm

H Mark

Just wondered how you got on with this.

I looked at a Ford SMax today with a Glow Plug Fault P0380 set at ignition on, each time without fail.
Simple set up with a Permanent Live from a fuse to one side of the Control Side, the other a Switched Earth from PCM. I have load tested all relevant wiring and drag tested each female terminal - All good, swapped Relay with another in the fuse box, same part number. Load Side Supply and Consumers all fine and dandy too.

I don't have a capture to share as I used a Handheld scope as a quick look, the trace was similar to yours with a massive negative kick.
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Re: WTF Is Going On Here?

Postby Mark Dalton » Thu May 26, 2016 10:38 pm

I replaced the Engine Control Module and hasn't missed a beat since. I suspected the ECU before I loaded the post, as there was no signal from the ECU at times. But I'd never seen anything like the big negative voltage spike before and didn't know how that would come about.
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Re: WTF Is Going On Here?

Postby STC » Fri May 27, 2016 7:05 am

I too thought ECU with my Glow Plug fault, as with yours, if its not the Wiring or Relay, what's left ?

But I'd never seen anything like the big negative voltage spike before and didn't know how that would come about.


Indeed that goes over my head too, like a giraffes gonads. A negative voltage in a DC circuit is best achieved by having your meter connected the wrong way round. In your case we see -20v in a 12v Circuit. My guess would be along the lines of a Collapsing Magnetic Field, only the wrong way round !! ?? Which doesn't really add up or make much sense to me either.

Your Scope trace shows the Negative Spike at the "Control" wire and the LED you set up at the Starter Solenoid - The "Load" side of the Relay, Both playing up ?

Yet the Whole point of a Relay is that the 2 sides, "Control" and "Load" are kept separate ??

One theory is that with the relay energised pins 30, 86, & 87 will be hardwired to the Battery + terminal so not so separate after all, in this application anyway,
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Re: WTF Is Going On Here?

Postby Mark Dalton » Sat May 28, 2016 4:54 am

Drives me a bit nuts not knowing what's going on, but I guess it's going to be one of those ones where you just have to be satisfied knowing you fixed the issue.
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