88 GMC 1500 no start

[Double Snowman GMC]

I have a 1988 GMC Sierra 1500 truck with a stock 1227747 ECM that now won’t start.


Here’s the problem, I was trying to solve a progressively worse idle/ stall situation:

• Tried to install “tuner pro” software from a purchased disc, anti-virus would not allow the full install
• New ALDL cable plugged in from truck to laptop
• Started the truck, ran for 15 seconds, then died.
• Downloaded “winALDL” from the internet, “freescan 207”
• Now I can see 4 rows only of data while cranking......no start!
• Tried the paperclip trick.....no codes
• Tried resetting the ECM by disconnecting the battery.
• Confirmed no spark via timing light, now fuel via visual

After a couple of days off from the above, now it fires for 3 seconds and dies.


Hopefully someone here can help get my truck back up and running. Did I fry my ECM?? What should I be looking at to troubleshoot?


Thanks in advance!
88 GMC

[Double Snowman GMC]

Thanks in advance for any help.....did I fry my ECM?

88 GMC

[dave w]

Maybe the ignition module is faulty?

The ignition module "Tells" the computer the engine is spinning with a "Distributor Reference Pulse" (DRP).

No DRP = No Start
No DRP = No Spark
N0 DRP = No Fuel from Injectors

dave w

[Double Snowman GMC]

Thanks for the prompt reply Dave,

I just swapped out the ignition module (I had installed a new one in May 2020).......sadly still no start.

Bob W

[dave w]

If you are 100% positive there are DRP's from the Ignition Module, then the next likely part is a bad computer. Most mechanics don't have the equipment to verify DRP signals, but an Oscilloscope will.

Maybe it's a mechanical problem, like a bad timing chain sprocket or stripped distributor gear that's keeping the distributor rotor from turning?

dave w

[Double Snowman GMC]

The only thing I changed in an attempt to tune my rig was connect a cable & computer and the result was quick and obvious (to me).....no start.

Could an EBay cable & software take out my computer? This is a second owner Cal truck, just passed smog, 120k miles, well maintained.

I can easily check the distributor gears....but it seems like such coincidence.

Bob W

PS I did buy a refurbished ECM from the local parts house last year, and swap over the chips.

[tayto]

do you have fuel pressure? do you have spark? many are quick to blame EFI when they can't even troubleshoot an engine with a carburetor

[Double Snowman GMC]

No spark
No fuel
Good Ignition module

[tayto]

check pickup coil in distributor. also, when you turn your key ON engine OFF does your fuel pump run for 2 seconds? at the same time does your check engine light blink on and then eventually off? Or does it stay lit all the time? Have you checked all fuses? i believe your era truck still has an ECU related fuse underhood, near heater box in a weatherpak-style connector.

[dave w]

Was the ignition module installed with dielectric grease or heat sink grease?

https://www.fullsizebronco.com/threa...c%20components.

dave w

[Double Snowman GMC]

Dielectric grease in place, I have not checked the pick up coil (yet).
I do not hear the fuel pump with the key on.
Check gauges & check engine lights stay on.
Fuses are checked good
Ran the paper clip test again.....no codes.

So I took the liberty of checking continuity on my new ALDL connector cable.......I find a dead short between pins M and E!
Does this seem correct? I would think these terminals would be separate. Could it be a defective cable?

Thanks again,
Bob W

[dave w]

Maybe ALDL information in this thread will help: http://www.gearhead-efi.com/Fuel-Inj...05-ALDL-wiring

dave w

[dave w]

OBD1 Serial Communication is either 160 Baud Pin "E" or 8192 Baud Pin "M" It makes sense the ALDL Cable is manufactured to work with both 160 baud and 8192 baud.

The ALDL Cable is supposed to have CONTINUTY between Pins "E" and "M". The ALDL cable IS NOT defective because there is a short between Pins "E" and "M". It very unlikely the ALDL cable damage your computer. The ALDL cable IS NOT the reason for No Start.

dave w

[tayto]

i believe the CEL should come ON and then turn off after a few seconds but my memory is fuzzy and I don't have a TBI vehicle at this time to confirm. you can try reseating the PROM pins E & M are for data and different vehicles use different pin for data. i would test pickup coil see attached imagine. i have seen pickup coils test ok but fail when hot.

[tayto]

distributor can be tested in situ. im not a fan of dielectric grease as a heat sink compound, I use the stuff they use for CPUs on personal computers.