4586

Everything was going great with my sons' LT1 (95 Z28) drive-line swap in a street rod.

He drove it into a shop this morning to have shock brackets welded to the frame.

Battery wasn't disconnected... but they indicated that the mig was properly grounded.

Now the car turns over and fires erratically but shuts down when key starter position is let go. If I keep the starter cranking, it starts very hard and I can keep the car running - but when the key forward tension is released it shuts down. (like no ignition power sympton)

Fuel pressure is good and has spark. I even re-flashed the PCM in the car without a hitch, but no condition change.

At one point noids would flash intermittently, one time they would flash, next time they wouldn't. Before I left they all flashed during cranking.

No DTC codes at all.

As I mentioned..if I turn the key forward to the crank position car will eventually run with starter engaged, back off to the "on position" and it shuts down.

All fuses are good, have 12v(plus) to ignition module with key on and about 10v while cranking.

The car ran perfectly before the welding....TPS at closed position is .58, changed MAP and MAF same condition, checked PCM connectors and wiggle tests.

One item I noticed on the 95 Z28 wiring diagram is that the Park Neutral Position ORN/BLK wire has a diode between the PCM Pin C15 and the Neutral Safety switch (C230). I never installed one...but since the car ran fine before..don't feel it is the culprit.

Have any suggestions...bad PCM possible?. Working on locating another PCM tomorrow.

Thanks

Fatdog2

Sounds like an O2 sensor wire got bumped.I would disconnect the negative terminal on the battery for a few hours and see if this helps.Your guys hit or burned up something I think.I've been wrong before,but I would do a visual check of all the wiring.Hope this helps since nobody else chimed in.My02.

4586

Thanks FatDog..obviously I went through the wiring..but you are the 2nd person to mention O2 sensors...they are new but who knows.

I have a pile of used O2 sensors that I can R&R after checking the wiring again.

Shame - was running like a bear

Injuneer

You don't have to disconnect anything for "hours" to reset the PCM. 30 seconds is more than adequate.

Is possible the welding current fried the O2 sensors. They can be destroyed by using a multi-meter with a very low impedance..... very delicate, electronically.

4586

Thanks Fred....I'll get back down there maybe tomorrow.

BTW: What procedure do you guys recommend to safe guard the electronics when welding on a vehicle. Sounds like disconnecting the battery might not be good enough?

Injuneer

I've had welding done on the exhaust system, and they disconnected the battery. The O2 sensors were in place (even though I don't use them for the ECU - but still can monitor them with my ScanMaster), and didn't seem to suffer. But then they also seem to have survived huge amounts of highly leaded fuel. Neither the stock PCM or the MoTeC ECU suffered any damage. The Opti optical module was not harmed.

GaryDoug

Sounds like a defective ignition switch or one with the linkage out of adjustment. I didn't think the O2 sensors had any effect until after closed loop was obtained, several minutes later. It has spark but only verified while cranking, correct? Have you checked power to the needed pins on the pcm while the ignition switch is on?

4586

The ignition (pink wire) is getting 12V with key on and approx 10V while cranking at the module so the key related power checks out.

I'll have to pull the connectors off the PCM and verify power with key on and while cranking at the PCM point. Whenever I make a wiring harness I always solder connections or use OEM style pins with a factory correct crimp....but even factory wiring is bad occasionally.

The park neutral switch (org/blk) wire is grounding to PCM when placed in park or neutral.

Understand that the O2 works in closed loop but is it possible that a fried, shorted or open circuit O2 sensor could prevent the run condition?

I have to wait a few days until I pick up a good PCM and then hope it flashes properly. Wondering if a bad PCM will accept the flash like this one did.

Fatdog2

Well just for kicks check out this utube thing(soldering or crimping with butt connectors)cars,stereos,electrical).Seems the soldering voltage drops off a bit.There is also a utube thing on how to check an 02 sensor with a blow torch.This is all Greek to me but is some interesting looking I think.Hope this helps.(Also previous post have stated some o2's are not really any good for these cars.Don't know why.My02.)

4586

In a nutshell the car was fine until it was up on the lift and shock brackets welded to the frame. It was lowered and now won't run.

Been doing TPI conversions and like the prom burning procedure for the 730 ecm...burn the eprom and insert.

These 16188051 PCM's have me nervous, first one fried, 2nd one has flashed 3 times now without a hitch, the 3rd one fried again.

Sending one out today to Tuner Cat for repair.

Tried repairing one myself, soldered in sockets, flashed the proms but still can't get WinFlash to recognize the PCM. Could be the file I'm using to flash the two eproms for initial recognition.

If anyone here has repaired a 16188051 sure could use the bin(s) you used to get the pcm started.

Not doing this to make money..just like doing as many repairs myself as possible.

Injuneer

Gary's right about the O2 sensors.... the car should start fine without them, unless they have driven the long term fuel trims to an extreme. The LTFT's are used at startup unless the PCM has been reset. But the LTFT's would only have changed if the car was driven an appreciable amount AFTER the welding was done.

TGGodfrey

See if you can find a spare ECU to try. Whenever you weld on a vehicle it is important to disconnect the battery. The ground system becomes part of the current path due to the battery and you can do a lot of damage to harnesses and electronic equipment. I personally have seen harnesses that are no where near the welding light on fire. It doesnt matter if the ground (MIG) is right in front of the wire fed torch either. Based on the brackets your son was having welded on, I can safely bet they were using 150amp+ settings and to a harness/battery/ECU, that is a ton of current.

Even if the O2 were damaged you should still get the car to run and move under its own power while cold.

Since your dealing with a rat rod, check all the chassis grounds your son used for the ECU. See if there are signs of heat or damaged conductors.

Whenever I do any type of chassis welding the battery MUST disconnected at a minimum. If its easy I disconnect the ECU. I have never had an issue. Welding exhaust is kind of a hit or miss thing because while welding an exhaust the impedance towards any type of electronic equipment is very high and although it is possible, its less likely to do much damage. Frame welding has very little impedance to your ground circuit.

As for your diode in the neutral switch....put the diode back in. Its not the cause of this but it does serve a purpose that lies in a discussion on DC theory.

Terry

4586

TGGodfrey

Thanks for taking the time and responding. One area that I really am fixed on is establishing good grounds. On the engine alone I have three separate ground straps, then frame to body, battery to frame and engine. Learned the importance of grounds years ago when I changed a thermostat on a GM TBI engine with the ground wires not being tightened on the thermostat housing.

I have a replacement 16188051 en route..USPS indicates a Thursday delivery. I just hope it flashes without a problem. Also sent one flash fail PCM to Tuner Cats yesterday for repair..but the repair person is out this week. (Have zero problems flashing the removable MEMCAL type ECMs)

Attempted this repair on a failed flash PCM:

How to fix the PCM Flash Memory

Didn't work and only created procedure questions - the author can't be contacted.

From reviewing the article I assumed only one eprom needed to be R&R, tried that, then the guys on an efi forum said I needed to R&R both 32 pins eproms..that didn't work either.

The modification is pretty straight forward, the external chip flashing isn't a problem..tried different BINs...but can't get that specific PCM to fire up.

One of those things on my bucket list I wanted to try.

Thanks..