NVRAM getting scrambled when driving

[someguy360]

Hi Guys,

Got a weird one that I'd been discussing on and off with Gareth for a while can't seem to track down the fault.

Hoping to put it out to the brains trust to see if anyones got any ideas.

I've got a genuine pcmhacking NVRAM in my VR (Delco 424 PCM). I took the car off the road for paint about 4.5 months ago, the car had the battery disconnected for about 2 months and when I reconnected the battery the NVRAM had seemingly dropped it's tune (Car wouldn't start, thermo fan firing etc same symptoms as if you tried to start the car with no memcal inserted at all).

I had assumed the battery had gone flat in the NVRAM so I ordered a replacement (genuine DS1245) and was able to reflash my 11p bin onto it.

However I got around 1km down the road and in the middle of a highway the car just died and wouldn't restart, same symptoms as with the old NVRAM. I put my EEPROM back in and managed to get it back home but when I took a dump of the bin on the NVRAM it was completely scrambled (tunerpro was showing jibberish values).

I can flash the bin back onto the NVRAM again, and go for a drive and bam within about 10 minutes the car just dies and the data is scrambled again.

I then assumed it must be a PCM issue for that kind of behaviour, grabbed a spare 424 from the wreckers, same issue took it for a drive and the car died.

I've tried:
* flashing the bin then leaving the R/W jumper off, still scrambles.
* Tried with both the factory memcal piggy backed with the M jumper and with it off, same result however at least with the memcal piggybacked the car goes into limp mode to at least pull over safely.
* Tried with both modes of bin switching via the jumper.


I'm pulling my hair out trying to work out whats going on. I really need the NVRAM working to be able to road tune the car and also we're adding Nitrous for the track later in the year and need to be able to live tune that when we get it up on the dyno.

It's a factory 304 VR wiring loom, standard PCM etc nothing special no strange hackery to the looms or conversions etc.

Any ideas at all would be greatly appreciated.

Cheers

[charlay86]

Have you tried burning the tune onto a regular memcal?

[someguy360]

Yes I've got the exact same tune on a traditional EEPROM which works fine, it's what gets me home again when the NVRAM dies.

That tunes rock solid and done about 3000km on an EEPROM, it's only when flashed to the NVRAM it makes it about 1km or maybe 10 mins if I'm lucky and the car just dies and the data on the NVRAM is scrambled when I try to do a dump via my burner.

[vlad01]

I had the same issue once with VE learn function. Check to see if that is not been set as enabled.

[someguy360]

I had the same issue once with VE learn function. Check to see if that is not been set as enabled.

Thanks mate I'll check that tomorrow, Did 11P have VE learn? I thought that was a 12p thing.

[immortality]

When it happens do you have a laptop hooked up for tuning?

[someguy360]

When it happens do you have a laptop hooked up for tuning?

Nah, I had thought of that but I've made sure to leave the ALDL cable disconnected when driving, same thing unfortunately.

[antus]

the RW jumper is a direct connection to the RW pin on the nvram. The chip shouldnt/cant be writing with that off without a short to the case. I remember speaking to Gareth about this one and I know you have checked this and confirm it still faults with no case on the pcb and ive seen the pic you sent of the soldering which looked good. So we can be sure there is no physical short. The hardware has been changed, and is all genuine, so that rules that out too. I think that is all the normal things. So I'm digging the bottom of the barrel for ideas now, as this is the only time Ive heard of this in the 23 years or so of 12P/11P. But I figure there has to be some kind of external influence that is related to your car. Logically without the RW connected and no physical shorts it has to be bad electrical noise/problems getting in there somehow, either bad power or radio frequency interfearance (RFI). Even so this isnt normal, but I cant see that it could be anything else. For power the most likely sources of extreme electrical noise are alternator/regulator, and also possibly maybe the battery.

To test this, can you cause it to glitch just at idle or do you need to be driving around? If at idle maybe you can disconnect the alternator and put a car battery charger across the battery and let it idle. The battery charger will introduce some noise itself, but it wouldnt normally cause a problem so I think its still a valid test. This will only be helpful if know you can cause it to glitch at idle.

For the battery, Im thinking maybe its a broken cell and as the car moves around it shorts out and pulls the whole voltage levels of the car down momentarily. Maybe thats enough to cause enough of an extreme low voltage power spike that could glitch something. Try another battery on the road to test (the physical movement of the battery would be needed to move a broken cell around).

For RFI you can let it it idle in the dark with the bonnet open and look for blue spark from anything high voltage arcing in the engine bay. Leads to engine around the outside of the spark plugs, coil pack around the edge of the coilpack to the base. If you see spark where its not supposed to be that could be causing RFI (though having said that I have seen this on a sprint car around the coil packs, and still the nvram was fine and the car still ran, just was missing from the bad spark).

[someguy360]

Thanks for the advice Antus,

The battery was replaced about 3-4 weeks ago so I can rule that one out however you may be onto something with the alternator.

It's a reasonably cheap Aeroflow alternator with a high amp rating for my thermo fans. Maybe I'll chuck the OEM alternator back in and go for a drive and see what happens there. The issue only occurs while driving, so far I haven't experienced it idling.

[Gareth]

Do you have resister plugs fitted?