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Discussion Starter #1
I have read a lot of the threads relating to starting problems. Now I have the following information from owner:

He has changed lift pump and PMD. Truck will start after prolonged cranking, and once started, runs good. Computer gives a code for faulty crankshaft sensor. One mechanic says change the wiring harness.

How does the crankshaft sensor affect starting ? Can this function be bypassed or corrected without changing all the wiring ?

Owner is giving me the truck for use on my farm, but it has nearly 500,000 km and I don't want to waste my money on a "dead horse". I may try to drive the truck from Vancouver to Vancouver Island and if I get stuck and can't start it, call BCAA for a tow. Once here I can go through all the ckecklists found in your forum.
 

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Whistle Pigs Are Cool
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OPS tells the LP to run, and the OPS has a nasty habit of failing.

Do you have any air leaks? My '94 had super brittle fuel hoses at the FFM & IP.
 

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Discussion Starter #3
Is OPS the crankshaft sensor ? What if I wire the lift pump directly to an ignition controlled circuit, therefore bypassing some of the the problems I have read about lift pump ?

I don't know about air leaks. Do you mean between the fuel tank and lift pump ?
Is FFM the fuel filter ? Is IP the injection pump ?
 

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Whistle Pigs Are Cool
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Oil Pressure Sender. Its behind and under the Fuel Filter Manager which houses the fuel filter, water in fuel sensor and fuel heater. I did relay wire my LP to Key ON & Crank which takes the load off the OPS and I know when the LP is on as it clacks as it is pumping.
Yes, the hard fuel lines from the chassis use rubber fuel hose to & from the FFM as well as the hose from the FFM to the Injection Pump.
 

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1995 GMC Suburban
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Crankshaft Sensor is CPS (that is a "C" and "P").
Oil Pressure Sensor is OPS (that is an "O" and "P").

The abbreviation needs to reflect the name of the part.

Just in case, C and O are 2 different letters.

LP is Lift Pump
FFM is Fuel Filter Manager aka Fuel filter bowl (a common leak point due to aging hoses and o-rings)
PMD is Pump Mounted Driver sometimes called FSD (Fuel Solenoid Driver) if it is not Pump mounted anymore.
CTS is Coolant Temp Sensor.
 

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Try taking off the fuel cap and see if that helps.
 
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Discussion Starter #7
I still don't know about the CPS and how that affects starting and how to clear that error code. Thanks for all of your help.
 

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I still don't know about the CPS and how that affects starting and how to clear that error code. Thanks for all of your help.

Crank Position Sensor is one of the sensors the PCM (computer) uses to manage engine functions, primarily the Injection Pump. If you are going to replace it, use either GM: 12557046, or AC Delco: 213337
 

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Discussion Starter #9
Thanks. Where do I find the CPS ? Is there a test to tell whether fault is sensor or wiring ? Would the CPS cause starting problem ? Once started, motor seems to operate normally.
 

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Thanks. Where do I find the CPS ? Is there a test to tell whether fault is sensor or wiring ? Would the CPS cause starting problem ? Once started, motor seems to operate normally.
The CPS is low down on the drivers side timing gear/chain cover. You can follow the wiring harness down. Look for it under the back side of the power steering pump.

Always check wiring first and the connectors.

Hard starts can also be caused by:

 

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#8 is the CPS.
Look where the wires enter the sensor for missing insulation/short.
If the CPS is bad there should be a code 19 and you will be in limp mode.
It will be harder to start than normal.
Unplug the CPS and see if anything changes.
If not, plug the CPS back in, then unplug the OS and if it will not start then the CPS may be bad.
Search GM 12557046 to see what the CPS looks like.

CPS01.jpg
 

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Discussion Starter #12
Thanks for all the help. The timing cover is off. Would this cause a CPS fault code ? Would a faulty CPS cause starting problems but then allow engine, once started, to run normally ? This doesn't make sense to me.
 

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Discussion Starter #13
Thanks for the diagram and advice. I didn't see that there was a page 2 of my thread. If I follow your suggestion, a bad CPS may cause starting problems, so if I unplug there should be no change. I don't know what OS is, but bad CPS and OS unplugged results in no start. Does this mean that good CPS and unplugged OS may still start ?
 

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OS is the optical sensor on top of the injection pump (6 pin connector). The OS has 2 engine speed sensors for IP control and combined with the CPS the computer keeps things running.
Either the CPS or the OS can quit and the other will keep the engine running but the computer will go into a limp mode, set codes, and be harder to start, long cranking times.
 

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I can't stress enough how cheap GMTD scan software and a DIY cable are compared to time spent guessing. You can figure out your issues with the free version.

If it shows any missed cam or crank references, then you know what to fix. Pending codes and live data make troubleshooting a breeze.

Did you say your timing cover is removed but you are still running it??
 

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Discussion Starter #16
No, the truck is not mine yet and timing cover was removed as part of the troubleshooting. Two shops told the owner about the CPS fault code and one recommended a new wiring harness. The owner is gifting the truck, but I don't want to own a lemon. I have learned a lot from this thread in case I do own a GM diesel. Thank all of you very much. Now I am working on a Zetor tractor and will not bother you again about this truck unless I bring it home and start working on it myself.
 

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I can't stress enough how cheap GMTD scan software and a DIY cable are compared to time spent guessing. You can figure out your issues with the free version.

If it shows any missed cam or crank references, then you know what to fix. Pending codes and live data make troubleshooting a breeze.

Did you say your timing cover is removed but you are still running it??
Are you saying a free programs to read the pcm. Also, what about this dyi cable?
 

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Are you saying a free programs to read the pcm. Also, what about this dyi cable?
Yes I would love to hear about this myself
 
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