Author Topic: TBI electrical troubleshooting  (Read 6197 times)

Offline Leevon

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TBI electrical troubleshooting
« on: March 18, 2009, 09:50:25 pm »
I didn't want to start another "it won't start thread" but Vile talked me into it by making the point that others could learn too. 

So I pulled my engine to seal up leaks.  It is a 305 TBI in an '87 silverado, factory stock, no modifications whatsoever.  I put everything back like it was and now it cranks but won't start. 

So far I have:

1) verified that the injectors are pulsing fuel (assuming this means the ECM has power)
2) verified there is no spark at the plugs
3) checked ECM A & B, IGNITION, CRANK, IDLE/GUAGE fuses visually and for continuity
4) checked resistance of the coil wire
5) verfied connection of all plugs, sensors, and grounds

Next I plan to:

6) check resistance of the coil itself (does anybody know what it should be?)
7) check continuity of the ECM to distributor module
8.) check voltage at the relative ECM pins
9) have the distributor module tested

Am I on the right track, any tips or does anybody have the correct troubleshooting procedure handy? 

Thanks!
   
1950 3100
1971 Cheyenne
1987 Silverado
2018 Silverado

Offline Captkaos

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Re: TBI electrical troubleshooting
« Reply #1 on: March 18, 2009, 10:53:22 pm »
#2 - did you verify that there WAS spark or found there was NO spark?

Offline Leevon

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Re: TBI electrical troubleshooting
« Reply #2 on: March 19, 2009, 10:03:19 am »
no spark.
1950 3100
1971 Cheyenne
1987 Silverado
2018 Silverado

Offline VileZambonie

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Re: TBI electrical troubleshooting
« Reply #3 on: March 19, 2009, 10:27:26 am »
OK, you verified the pick up coil is plugged in and the ignition module is connected and the coil is connected to the module. If you have no spark at the coil check to see if the coil is being triggered.

Actually here, I'll post the actual step by step

CHART C-4B - IGNITION SYS. CHECK (SEALED MODULE CONN. DIST.)



 Test Description : Numbers below refer to circled numbers on the diagnostic chart.

Two wires are checked, to ensure that an open is not present in a spark plug wire.
1A. If spark occurs with EST connector disconnected, pick-up coil output is too low for EST operation.

A spark indicates the problem must be the distributor cap or rotor.
Normally, there should be battery voltage at the "C" and "+" terminals. Low voltage would indicate an open or a high resistance circuit from the distributor to the coil or ignition switch. If "C" term. voltage was low, but"+ "term. voltage is 10 volts or more, circuit from "C" term. to ignition coil or ignition coil primary winding is open.
Checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned "OFF", so normal voltage should be about 12 volts. If the module is turned "ON", the voltage would be low, but above 1 volt. This could cause the ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small amount of voltage will leak through the module from the "Bat," to the tach terminal.
Checks for an open module, or circuit to it. 12 volts applied to the module "P" terminal should turn the module "ON" and the voltage should drop to about 7-9 volts.
This should turn "OFF" the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester could determine which, is at fault.
 Diagnostic Aids :

The "Scan" tool does not have any ability to help diagnose an ignition system check.

See Diagnosis By Symptoms - No Trouble Codes Stored.




,                           ___ 
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              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline VileZambonie

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Re: TBI electrical troubleshooting
« Reply #4 on: March 19, 2009, 10:28:20 am »
Capt, can you override the image restriction?

Here's the direct link Click Here and enlarge
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline Captkaos

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Re: TBI electrical troubleshooting
« Reply #5 on: March 19, 2009, 04:31:48 pm »
I moved it up some but I don't want really HUGE pics taking up all the space so you have to scroll left/right.  92% users based on forum stats browse the forum with a max of 800x600 screen size.  You could link it and it will open as you did in the second pic...
If you are using Mozilla you can right-click and view it fullsize...

Offline Leevon

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Re: TBI electrical troubleshooting
« Reply #6 on: March 19, 2009, 04:59:35 pm »
WOW!  Vile, did anybody tell you that you're the man today?  I was able to print out the chart on 11x17 and the ignition diagram as well.  I also have a diagram for the whole harness on 11x17.  I am armed and dangerous, should get some time in tonight.  I'll let you know how it goes. 

Thanks a million!

Update:  I fixed it.  There was voltage drop during the ignition module diagnostic (check voltage at tach lead, key on - 12v, then crank).  It dropped to around 6v.  This prompted me to remove the module, doing so I found the connector from the pickup unit was a corroded blob.  Took the module to the parts store and it tested fine...so I put it back in and doing so realized I had put a broken connector on backwards.  This was the problem the whole time  ::)

Bad news is the oil leak is still there  >:(

« Last Edit: March 21, 2009, 08:41:34 pm by Leevon »
1950 3100
1971 Cheyenne
1987 Silverado
2018 Silverado