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Old 04-20-2007, 11:10 AM
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350 TBI spark problem driving med crazy!!

I have a 350 tbi installed in a -72 Jeep CJ-5. The engine is from a 1991 Chevy van G20. It all runs fine but one day the problems started...
The truck gets harder and harder to start but it runs perfect when it at last fires.. One day it wont fire at all.
I had no spark so I changed : plugs, cap, rotor, ignition coil, p/u, ignition module and still no spark!!!!! After some faultfinding I get spark if I disconnect the 4-wire plug that connects the ignition module (in the distributor) to the ECM (1227747). I also tried to have the 4 wire plug connected but with bypass disconnected - no spark . I don�t think there is a fault in the wiring harness, if I disconnect the ECM completely I get spark -even if the wiring harness is connected to the ignition module. Could the ECM tell the ignition module to make no spark? Even if the bypass wire is disconnected?
I have also tried to change the ECM but it doesn�t helped, could it be some other sensor problem??
Any suggestions?

Kind regards
Erik from Sweden

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Old 04-20-2007, 12:08 PM
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Ohm the pickup coil and compare to specs. also you can have the igniter module tested at some parts stores (if they know how to hook up the machine). I used to work at autozone and we had an igniter module tester, and I have tested many. Sometimes it isnt right but I used to compare new ones to the old ones on the tester. If it tested good I would test a new one to see if I got the same results, and the same if it were bad. all the By-pass wire does is send ignition timing, so you can only test it if the engine is running to see if it is advancing. Is there any trouble codes? also see if you are getting power to the distributor. if all else fails pull the distributor and turn the key on and hook a spark tester to the coil wire. with all the wires connected to the distributor except the plug wires and the coil wires see if the coil is producing spark and see what color the spark is. also Ohm the coil.
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Old 04-20-2007, 12:36 PM
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Thanks for answer!
There are no problems with either the pickup, the coil or the plugs ! I got good spark when i disconnect the wires between ECM and the module. I have also tested with a new ignition module twice! The module is a Standard LX-340 and it looks exactly as the orginal GM-module. I donīt get any codes because I dont get the engine running.


Last edited by Erst; 04-20-2007 at 12:57 PM.
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Old 04-20-2007, 02:18 PM
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whats wierd is you shouldnt get any spark from the distributor without the igniter module connected. The igniter module acts like a set of points. if you take the points out you cant get spark. all the igniter module does is ground the coil. In your case the igniter module has been built to where the computer can send a signal before the the circut should close. ohm the wires on the coil and the igniter module to see if its grounded somewhere. in this case when you unplugged it the wire might rub a ground somewhere and send a false spark that wouldnt be in time with anything. other than that pull the distributor like i mentioned earlier to check to see if the coil is sparking like it should which should be 8 times to one revolution. if not than the computer is either not getting the signal or its not sending the signal. the only way I know to tell is by using a dwell meter on the coil and on the wire coming from the igniter or the pickup coil.
you have to check each component individually.
1st is the coil. you never told me what color the spark was. and you didnt tell me if you had power to the coil and how much.
second is the igniter module. you said it was new and you had it tested. did you put di-electric grease on it before you put it on?
and last is the pickup coil and pole piece. you never told me if they were in specs when you ohm'd it
next is wiring. is it fine? are there any shorts?
last is the ecm. is it getting or sending signal? but more than likely it isnt the ecm.
let me know what you find.
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Old 04-20-2007, 03:14 PM
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There are 4 wires between the ecm and the module:
(1) is ground
(2) is "HEI ref" , it sends a tach signal from the ignition module to the ecm, the ecm then modifies the signal by changing its duty cycle and returns it to the module in the (3) "EST" wire. In the ignition module this signal is used to control timing.
The (4) "BYPASS" signals the module to ignore the EST signal when the engine is being cranked. (0V=ignore the signal, 5V = use the EST-signal)

With the bypass = 0V the timing is set to initial value.
The Bypass should be disconnected when setting the the initial ignition timing.

The sparks are clear blue and looks fine when the 4 wires are not connected to the module. This make the ignition system working without the ecm, the module works just like good old points. I have power to the coil, even when the engine is cranked (12V). The pickup is new and the ohm is correct. The coil is new and have correct values. The fact that I get good spark with the ecm disconnected from the module tells me that there are no problems with coil, spark wires, pick up or 12v to coil.
I put on the dielectric grease delievered with the module but I donīt realise why it should be there? Is the underside of the module not allowed to be electrical grounded to the distributor?

I have checked the wires between the coil and the module and also checked the 4 wires between the ecm and the module and havent found any damages or dents. I have also ohmeīd the wires and they are ok.

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Old 04-20-2007, 04:19 PM
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Doc here,

Pull and disconnect the Tach, The calibration board may be shot and canceling spark at the HEI, and ref pulse to the computer..

Be sure the white wire going to the tach is pulled AFTER the computer..or you will lose the ref pulse also..for testing.

If it runs, get a new tach,AND maybe a tach adapter is required.

Check the Injectors..The Red wire should be 12 volts on a DVOM set to DCV, V X 20 or higher, with the key on..The Black wire will "Flash" when tested with a "Noid" lamp while cranking.

If not Pull ONE Injector and try to start..

If not, connect it and pull the other..If it starts (and runs poorly) you have a shorted injector..

If not..check the fuses.

Check your Fuse Buss, Inspect (WITH A METER!) all fuses, especially those marked ECM, ECU, INJ 1, INJ 2, IGN, COIL, ECM A,B or C, CRK SENDER. DO not just eyeball these, Ohm them out..any fuse can open under the blade foot, and you'll never see it and blow it off as good..you'll be troubleshooting for the next year for a blown fuse you should have fixed on day one!

Have the module tested..and..YES, I know it's new..

AND Yes, I know several guys that never had it tested after "Shotgunning parts" ... only to find out some $1500 later and 100 hours worth of work, the module was thermal (heating up under load and shutting down) AND, that the failure rate on these is like 1 in 3 or 5 right out of the "NEW" box from the store...

IF you need a new one , HAVE that tested before you leave the store, Once you leave, it's your problem..

ALSO! Don't forget to CLEAN off the old Heat sink compound and apply new on either replacing the new or old module, OR you'll be doing this again real soon!

Do you have a crank sensor? Check the Ohmic Value on this, It is the MOST common culprit on degraded spark ending up in losing of spark..BE sure it plug is clean, burnished, and tight fitting. Be sure the sensor is tight, not full of grease, and / or bent from hitting road fodder.

Check the coil..measure (if remote type coil) with all the wires removed, DVOM Set to R X 1, OHMS scale, Calibrated to "000" , the primary side of the coil, It should read LESS than an ohm, but more than "000" Out of that range, replace the coil..it is bad or headed that way.

Next measure the secondary, set the scale to R X 20K or higher, It should read 10,000 to 11,000 Ohms..out of that range replace the coil.

If it checks good, Ohm out the Magnetic pickup..you'll need to look up the spec for your year truck engine (two values are given with a cutoff year in between those values) at any rate, It should not be open or short..but somewhere around 5 K ohms IIRC..if not replace it..it requires removal and disassembly of the dizzy.

Check to be sure the IGN is Not dropping OUT during cranking mode..not knowing how a JEEP ignition system is wired, you MAY need a secondary Ignition to support the COIL and COMPUTER during crank mode..

Does this engine have a MAF? If so inspect and clean it..if it's bad will kill starting..

You should not just throw parts at the problem..99% of the time, it won't fix the problem, and 50% of the time CAUSE more problems..(bad parts, Wrong Parts, breakage of things that didn't need to be messed with in the first place, and electronic issues)...

AND 100% of the time drain your wallet and produce no noticeable improvements..PLUS..you still end up going to a Shop to repair the original Problem, Plus all the ones you caused in shotgunning..

The only way to approach this type of problem is "Logical" troubleshooting..ONE step at a time, eliminate or include each step as you go by testing, then move on..Like:"Is it Fuel or fire"..Fire..."Is it power to the coil?" Good.."Is It spark in/out of the coil?"..you get the idea...

Write a flow chart if need be so you DON"T double your work.

Replace ONLY those parts that Don't meet Spec.

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Old 04-20-2007, 04:51 PM
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Thanks for a long answer Doc!
First of all this is not a issue of fuel delivery problems. I have fuel but no spark. I have measured all fuses, they are ok. I have ohmed the pickup, its fine (and new). I have MAP not MAF.
Could someone answer me if the ECM should be able to shut down the ignition even with the bypass disconnected? I donīt think so and this make this problem very strange. Why do I get spark with the ecm disconnected but no spark with it connected?? Even if there is a problem with the crank sensor (Iīm not sure if I have one) the ecm shouldnīt turn down the ignition (with the bypass disconnected), right???
The IGN is not dropping out when cranking.

I have tested two new modules and I canīt beleive both of them could be bad from the shop. And if a module is thermal I suppose it wouldnīt give spark even without the ECM disconnected. If itīs bad it is really bad, or?

Could there be a ground problem? Is the module grounded through the ecm or through the distributor? I have 0 ohm between the backside of the module and the ground connection at the module.
I think I will make a testharness and disconnect each of the 4 wires one by one to see wich of them that cause the problem...
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Old 04-20-2007, 04:53 PM
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Didn`t the original equipment on these vehicles shut down spark if no oil pressure was detected? I don`t remember exactly, but there was a friend of mine, his chevy pick up did this same thing when the oil pressure dropped too low, but it`s been so long ago, I really can`t recall how the system worked, I don`t know if it shut down fuel or spark, but whatever it did, it wouldn`t start.
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Old 04-20-2007, 05:19 PM
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Doc here,

IIRC, if the oil pressure switch goes bad, It shuts down the Fuel , not fire..BUT I don't have the Schematic here in front of me to verify that..I will locate that and verify and post back.

IF you loose the Ref Pulse..(anywhere) IT will shut the Confuser Down (Well, sorta..) in as it won't have timing pulses on which to base data on the I/O ports (from Sensors To Computer Data) This Instructs the Computer How to "Adjust" to certain engine demands, Like Time/Vs amount of fuel to the injectors..More Fuel, more demand..more go~Fast..

By Disconnecting the EST, you may be forcing the "Limp Home Mode" allowing it to run..but very badly..good enough to get it somewhere off the road..

If not already, Clear the computer, pull the power, and touch the cables together for about 1 minute then reconnect..On SOME computers this will work on others you need the reader..

Don't rule out the possibility that the PROM and / or CALPAK has taken a dump..Without the instruction set input the MPU will just say, "You want me to do What?"...

I will try and locate my GM schematic for more specifics on this and post back what I find, maybe get you some pins to try on the computer..

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Old 04-20-2007, 07:02 PM
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I have a pretty good wiring diagram I will post it in my picture gallery.

Yes its very possible that you can have spark with computer 4 wire plug disconnected from distributor. That exact distributor setup is used in marine applications with a different module so the advance curve isnt so abrupt, its used along with a carburetor.
Its called Delco Voyager Ignition.

Your TBI setup cant actually run that way because computer would never get a RPM signal from distributor. But you will get spark.

Have you done continuity test on those 4 wires I think thats what I would try and be sure to peek into the connector too there might be a pin bent over or starting to back out.
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Old 04-20-2007, 07:37 PM
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Doc here,

OK, here are some check points..DVOM set to DCV, VX20 or higher:
  • 12 volts at pin A6 of the computer (key on)

  • 12 volts at pin B1 of the computer (key on, off or start..)

  • 12 volts at pin C16 of the computer (key on/off or start)

  • 12 volts at pin C19 of the computer (Key in Start, pull the "S" wire from the solenoid to test w/o cranking)

Next, Configure your Meter for Ohms scale, R X 1, Calibrated to "000", and measure to ground the following:
  • Ground at pin D6 of the computer

  • Ground at pin A12 of the computer.

  • Ground at D1 of the computer.

IF you have those , the computer is (Should be) Up and thinking...

The Dizzy four prong plug Should be :
  • White to D4 of the computer (this , I think is Ref pulse)

  • Tan/Blk to D5 of the computer.

  • Blk/Red to B3 of the computer.

  • Ppl/Wht to pin B5 of the computer.

Check those for continuity between the Dizzy and the Computer backplane connector.

You show a CTS (coolant temp sender ) on A11 of the computer and C10, also Blk into the TPS..IIRC, if that is thinking he engine is hot, it won't start..

Your Coil & Dizzy module gets power From the pink wire..no magic going on there, measure it for 12 volts..

White #1 goes to the Dizzy module from the coil -, White #2 goes to the Tach calibration card..make sure this wire is not shorted or grounded it WILL kill spark as well as a bad calibration card..unplug it..

The Oil sender only controls the Fuel pump relay, Fuel pump prime, and fuel cycler module. Since you say fuel is NOT the problem, it should be just fine..But the signals can be located at B2 and A1 of the computer should you need to check for any reason.

Hope that gives you some direction..

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Old 04-21-2007, 05:11 AM
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I Iīm at my work all sat and sun so I canīt measure anything until monday but I know the following:
*I have no bent pins at the computer connections or in any other wire connections.
* I have cleared the computer
* I have done a continuity test on those 4 wires between the distributor and the computer.
* the pink wire gives me 12 V, I have also tested with a direct wire from the positive battery terminal.
* I have measured the CTS, its ok!

Things I donīt understand:
*Doc, what is a tach calibration card?
*How could the computer turns down the ignition even if I have a bad calpak, prom, temp sending unit, oil pressure sender or whatever. Could anyone explain how the computer turns down the ignition with one of the three cables: ground, EST and HEI ref ? I would say itīs impossible to turn down the ignition with the bypass disconnected!

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Old 04-26-2007, 10:18 AM
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OK, now I have done some measurements!
First of all I tried to unplug each by each of the 4 wires between the ecm and the dizzy. With the EST-wire unplugged I got spark and the engine started!

Next thing was measuring the i/o- signals from the ecm, both with engine off (ign on) and engine running (idling, working temp).
Everything was as it should except:

* D5 on the ECM (est bypass) was 0V when engine was running (should be 5V) but that canīt be the cause of my problem. That cable SHOULD be 0V when starting and it is when starting I donīt get spark!

*B3 on the ECM (common ground between dizzy and ecm) was 0,7 V. When ohm-measuring it was 0,1 ohm. With the ground disconnected from the dizzy B3 measured app 0V.

This is what I get when measuring the signals:
ign on/engine running

A1 (fuel pump relay) 0/13 OK
A6 (ign-ecm fuse) 12/13 OK
A11 (sensor ground) 0/0 OK
A12 (system ground) 0/0 OK

B1 (batt voltage) 12/13 OK
B3 (Est ref low, GROUND) 0,7/0,7 maybe not OK? see note above
B5 (HEI ref) 0/2 OK(?)
B7 (ESC knock retard signal) 9/9 OK

C10 (Coolant temp signal) 4V cold 3,5V hot OK (?) (itīs about 75 deg F here)
C11 (MAP signal) 4,9/fluctuating suppose to be OK?
C13 (throttle positioning sensor) 0,6/0,6 OK

D1 (System ground) 0/0 OK
D2 (sensor ground) 0/0 OK
D4 (EST) 0/1,8 OK(?)
D5 (bypass) 0/0 not OK se note above

I have measured the 4 wires between ecm and dizzy, they are ok.
Of course the pink wire is 12 V

Any ideas?


Last edited by Erst; 04-26-2007 at 10:30 AM.
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Old 09-27-2016, 04:39 PM
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I see this thread came to an end without a solution! I too had the same problem. Here is what solved my problem .the magnets in the distributer relucter had cracked so the magnetic field was not strong enough to create spark enough to fire. It did have enough energy to send the ECM a pulse to turn on the fuel pump. So simple but we had to take the distributer to see the cracks!
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