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Discussion Starter #1
My '94 k3500 has developed some problems recently. The truck still runs but it isn't reading the crank sensor or cam pulse counter. I started looking for bad grounds and found a fusable link from the junction block ( cicuit c10 from ECM ) grounding itself out before the fuse. This circuit powers the fuel pump relay and this short may have done some damage to the ECM, as it still doesn't work after the short was fixed. The fuel pump stopped working during the glow cycle five months ago but the other sensors worked till just recently.


I'm interested in how others might solve this problem.


I would like to know if a new ECM would solve this or will there be other damage in the system.
 

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Lift Pump Relay pin A is tied to the start circuit thru a pur\wht wire to the 5amp Crank Fuse. It is enabled only when the key is turned to START.


The orange wire feeds raw battery power to the relay on pin A, and to the PCM on E1, supplied for PCM volatile memory.


ALDL pin G is Fuel Pump Test pin - should have voltage in START if relay is good, and when engine is running if OPS is good.
 

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Discussion Starter #3
The orange wire (power to pin A on fuel relay) is good at the fuel relay, but the purple/white wire that is fused by the crank fuse shows no power at the fuel relay, pin G ALDL, or crank fuse. My manual shows this wire originating at the fuel pump prime connection. I'm not sure what the prime connection is.


The OPS is good and fuel is pumped as long as the engine is running. Edited by: quantum mechanic
 

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Fuel Pump Prime is the ALDL connector in the cab, pin G.


Crank Fuse and circuitry has power only when ign switch is in START postion
 

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Discussion Starter #5
gmctd said:
Fuel Pump Prime is the ALDL connector in the cab, pin G.


Crank Fuse and circuitry has power only when ign switch is in START postion

Pin G and crank fuse/circuitry arn't powered when ign. is in start. I replaced the IGN switch with no change to this circuit.
 

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If the starter solenoid pulls in when the switch is in START, the big purple wire has +12v on it - simple soldered and taped splice between there and CRANK Fuse.
 

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What do you mean by this?


The truck still runs but it isn't reading the crank sensor or cam pulse counter.

If neither of them are working then the truck won't start.


They should read 0 on a scan tool if the truck is running correct.(unless something is different on your scanner)
 

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Discussion Starter #8
These are my current readings with the engine running. I could have sworn that last time I checked this I had a reading on cam pulse and crank reference. Last load I hauled it gave code 35/36 the whole trip and even sounded different (weaker) when climbing hills. I havn't hauled since and it drives fine empty. no codes.





 

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QM I don't know what those old codes are mine are OBD2, but I had similar to what you describe the truck ran fine empty staggered while towing, loss of fuel to IP from partial plugged fuel filt, it was semi new only had 5K miles on it.


Have you gauged the lift pump output? I temp gauged mine by running a tube to the filter bowl drain line out back of hood into cab to a 0-15 gage,(make sure you don't pinch tube will give false reading BTDT) opened the "tee handle" and read press normal 4-6 psi, my semi-plugged fuel filt down to 1psi under load. Maybe you have a filt prob or weak lift pump, delivery min in my 98 manual 3psi min, > 1/2pt in 15 sec.(15gph) just another idea to try.
 

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Discussion Starter #10
code 35/36 are OBD1, injection pulse width error, too short/too long. I've seen thses codes when the optical/IP (crank reference?) isn't working right.


I know all is not well in denmark. I was hauling everyday no problems and then I put the turbo/exhaust manifold I had internally resurfaced on and it started acting up. It took me a moment to realize that the two probably didn't have much to do with each other. I've looked at most of the EFI connectors on the engine harness/ grounds and the like. I'll clean my fuel system manually to see if I can't find some blockage. I had to replace most of my soft lines recently dryrot or biodiesel degradation, it all looks the same.
 

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I would try getting someone else to check too but I think that if you get a cam count or crank reference, it will give you a code.


What about your voltage? In the picture above, it shown only 12.6 volts.



When I got water in fuel and killed my IP, the cam reference would slowly count to 255 while cranking. Once it hit 255 then it would start and run bad. If I would unplug the crank sensor, it would kill the engine.


I was told that it uses both to make sure that the cam and crank are lined up. I has to have at least one for the engine to run since that is how it runs the IP timing.


With my new (used) pump, the numbers stay at zero.
 

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Discussion Starter #12
Whatnot,


What you're saying is logical. I had the suspision that perhaps I was misinterpriting the data. I would like to scan another OBD1 to see what it gives. The only other truck I did had a bad optical and I might have been seeing numbers when I scanned it and then gotten confused when I scanned my truck for its code 35/36 and didn't see any numbers.
 
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