91 K1500 5.0 No Spark

RageZro · May 19, 2010

New to the forum here and desperate for some advice!

A little history on the truck... Just bought a 1991 K1500 short bed with the 5.0 in it. It was hard starting, previous owner had to pump the accelerator like it had a carb on it to get it to start and even try to run. After it was warmed up a bit it ran just fine. It would though be tricky to restart sometimes requiring partial throttle.

I bought it as it was a Texas truck that was the most rust free body I have seen for a 10 year old truck let alone a 20yr one! Needless the to say the interior was dry rotted though and I replaced it, including the entire dash except the wiring, ECM, etc. I did however remove an after market remote door and alarm system from the truck.

Now for the new problem, after putting the interior back in it, the truck will crank and pulse fuel but no spark. Everything else works on the dash and all external wiring seems OK except the left front turn signal and side marker lights are on solid when the ignition is on. If I turn the running or head lights on the side marker goes out but the turn stays on solid. It will blink when I turn on the left turn signals. I'm guessing that is a separate issue due to the hack job the PO did putting that remote entry/alarm system on the truck.

What I have replaced:

New ACDelco Stock Plugs

New Cap & Rotor

New Pickup Coil

New Ignition Control Module

New MSD High Output Coil

New Coolant Sensor for PCM circuit

New Ignition Switch

New Knock Sensor connector (found old one dangling under the engine broken)

In addition to the above I repaired all of the strip-backs the PO did to the wiring to add the aftermarket junk plus I found that several of the wires on the engine harness had been also stripped back for no apparent reason unless he had been trying to check for a why it was hard starting.

I have 12VDC at the coil and out of it to the ICM. When I code the ECM it only gives me the "12". While the jumper is in to code the ECM, I can hear the Idle Air Controls Valve making noise constantly.

Any ideas?

grampadirt · May 19, 2010

A long shot but you never know....check the wires going to the oil pressure sending unit.If one is broken or grounded it will show no oil pressure.No oil pressure = no start,ECM safety thing.Ask me how I know

RageZro · May 19, 2010

Thanks, I'll recheck that as it is hard to see but I can see the oil pressure gauge bounce with each rotation during cranking. It seems to bounce a lot, is that normal? My gauges are the disk type instead of the needle. Don't care for them much and when the truck is off they don't "zero".

If photos will help just ask I can take some really good close ups with the wife's digital.

Also I would like to know where the white wire that's in the same connector with the pink 12VDC power supply to the coil goes. I know it is supposed to be the tach wire but I don't have a tach and just like the pink wire is hot all the time when the key is in the run position I found this to be at 12VDC and about 3VDC. The Haynes books test is a bit confusing to me as it doesn't exactly explain the on/off description as to whether that is a pulse or when exactly it is to be high or low. Seems odd that before I started replacing parts I had the 3VDC now each time I check it it is 12VDC but still no spark.

Can the ECM give me a fuel pulse but no spark pulse? I read somewhere that the ICM controls the spark independent of the ECM up to 400 RPM. If that is true you would think it would at least pop once or twice but it has no spark whatsoever. I've checked it at the coil wire too, nothing.

RageZro · May 21, 2010

Well I replaced the ECM/PCM and now I have no Check Engine Light at all and no spark at all either though the injectors are pulsing. Since there isn't a check engine light I can't check for codes and I'm guessing there is something wrong with this unit as the probing the OBD1 to get the codes doesn't even cause the Idle Air Control Valve start pulsing. I'm taking this unit back and retrieving my old one as it at least seemed to be working better than this one.

I looked at the oil pressure sending unit wire as best I could and it looks good, I still get a bounce on the gauge while cranking too.

I pulled the ECM/PCM and checked the continuity of the wires from there to the Ignition Control Module.

Purple/white oxygen sensor = good

Tan/black ignition bypass = good

Black/red distributor lo = good

White IC = good

I'm reading 12VDC on the pink to the coil and on the tach wire at the coil. The wires from the coil to the ICM are good too. My meter isn't fast so if there is supposed to be an really fast interrupt on the tach lead to fire the coil it isn't going to show up.

I'll pull the dash back apart on the driver side and trace that wire to look for issues. It has to run back to the ECM/PCM so I'll check that first for continuity.

If all that checks out then what???

Selling it is starting to come to mind....

Mike G · May 21, 2010

Nothing worse than electrical houndoggin'. I would look back into what you did to disconnect the remote door/alarm system. Seems to me that they build in a "kill" for the system. Perhaps something didn't get hooked back into its proper place.

Just brainstorming... --Mike

RageZro · May 22, 2010

Well I got the old ECM back in and I have a Check Engine light when cranking like I should and I can code it. Still getting just the "12" though and no spark.

I started pulling the dash back out and will comb the harness for issues. I'll hook it back up without the dash so as to eliminate any outside of the start/run circuit issues that might be playing havoc. I'm also going to hook a tach onto the tach wire to see if there is a proper signal on that wire since my meter says it is a constant 12VDC.

RageZro · May 24, 2010

The good and bad...

I've got spark now just not much of it. The PO had switched the battery to a top post type and though I had 12VDC when I started to crank the engine the more times I tried the more it kept dropping so I checked the connection by pulling the wire out of the battery terminal clamp. Oxidation had corroded the wire and clamp pretty bad so I replaced them both and cleaned the wire ends or stripped them back where needed.

All of that helped but not much so I pulled the MSD high output coil and put the old original one back in and that's when the truck fired up! It ran like crap at idle for maybe 30 seconds or so then died. I figured it was just the ECM relearning and/or the timing was out since I had the distributor out to replace the pickup coil. Tried to restart it and it just cranks. Checked for spark and it looks pretty yellow by putting my xenon tester between the coil and the distributor. It will backfire and speed up the crank a little sometimes but nothing like any real attempt to fire enough to actually call it trying to run. No codes showing either.

My question is why did it start and run if the old coil won't do it now? I've never heard of a coil heating that fast and self destructing. I'm not even sure what to think about the MSD coil as it passes the 3 tests given in the Haynes book. It too would have to be intermittent. I don't want to install another coil if something is toasting them so if anyone has an idea on this please post it!

The other weird electrical problem where only the left front turn signal stayed on if the ignition was turned on, turned out to be due to the instrument cluster that was in the replacement dash I installed. Identical dash and cluster but there must be something wrong with the cluster in that one as it went away once I put the old cluster back in.

I'm almost there but there is still probably one thing not right and I can't even guess what it might be.

RageZro · May 29, 2010

The saga continues...

Found the 2nd coil to be bad like I had indicated earlier. I changed it out with a standard replacement coil and now it tries to start and run.

New problem = no fuel except pre-charge when you turn the key to run. I could hear the pump but when I changed the fuel filter to make sure it wasn't blocked, so I let it pump into a can with the key in the run position. Seemed rather weak so I replaced the pump & screen. No, I didn't check the pressure as I think someone has borrowed and failed to return my gauge.

Still won't supply fuel. You can turn the key to the run position and hear the pump, turn it to start and it fires long enough to burn the pre-charge of fuel but then no further fuel flow until you turn it off and repeat the sequence.

Looking at the wiring diagram it would seem the oil pressure switch and the pump pressure cutoff are connected so that if there isn't enough oil pressure the pump kicks out. I think I'm having an issue similar to what grampadirt mentioned earlier. It might be that the switch is faulty and kicking the pump relay out. (I replaced the relay with a known good one just to be sure it was OK).

I'll get the switch swapped out this weekend when I get a chance and post the results.

I did find myself looking at the different for sale signs at Autozone tonight...

Badass69 · May 29, 2010

The saga continues...

Found the 2nd coil to be bad like I had indicated earlier. I changed it out with a standard replacement coil and now it tries to start and run.

New problem = no fuel except pre-charge when you turn the key to run. I could hear the pump but when I changed the fuel filter to make sure it wasn't blocked, so I let it pump into a can with the key in the run position. Seemed rather weak so I replaced the pump & screen. No, I didn't check the pressure as I think someone has borrowed and failed to return my gauge.

Still won't supply fuel. You can turn the key to the run position and hear the pump, turn it to start and it fires long enough to burn the pre-charge of fuel but then no further fuel flow until you turn it off and repeat the sequence.

Looking at the wiring diagram it would seem the oil pressure switch and the pump pressure cutoff are connected so that if there isn't enough oil pressure the pump kicks out. I think I'm having an issue similar to what grampadirt mentioned earlier. It might be that the switch is faulty and kicking the pump relay out. (I replaced the relay with a known good one just to be sure it was OK).

I'll get the switch swapped out this weekend when I get a chance and post the results.

I did find myself looking at the different for sale signs at Autozone tonight...

Every TBI truck I have ever owned will run just fine without the oil pressure sending unit connected. If it's in question, just disconnect it and open circuit it. If the truck is going to start it will start.

RageZro · June 2, 2010

Well it seems your right about the oil pressure switch, the fuel control / oil pressure relay is not present on my truck.

It has came down to this...

I have spark and it will try to start from injector leakage.

I have a good fuel supply to the TBI.

I have test hot wired the injectors and they will spray just fine.

I have 12VDC on both the red and white leads to the injectors when the switch is on.

That leaves the blue and green leads to the injectors as either open or the ECM is not pulsing the injectors.

I will check those leads for continuity and if they are good then I'm back looking at a defective ECM as I can't see any other reason why there wouldn't be an injector pulse from the ECM. If the ICM controls the firing up to 400rpm then maybe it isn't even giving a spark signal once the engine reaches that speed.

I'm not looking forward to trying another ECM after that last one being a dud.