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Cat 3126 no start problem
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Posted 6/12/2019 22:18 (#7555987 - in reply to #7555738)
Subject: RE: Cat 3126 no start problem

SW Iowa

John.jr - 6/12/2019 20:44 I'm working on a cat 3126 7as prefix, heui injectors. Truck died going down the road like the key was shut off. The engine is in a 99' Chevy topkick/c6500 with an obd style connector, no 9 pin Deutsch connector anywhere I can find, no dealership has the right connector, I ordered one but is 3 days out.

 Today I checked the 70 pin ecm plug for power, ground and ignition, checked good. Builds 1500 psi at icp sensor with analog gauge.

 I was told to throw parts until it was running, I replaced timing sensors, icp sensor, ipr valve.

Fuel primer pump is hard, was hard immediately after dying. Not one bit of smoke out of the exhaust.

5v at all the engine sensors. Any ideas?


I can't believe a True CAT shop truck dealer doesn't have those adapters, in their shop. (we had at least 2 for GM products)

You mentioned about the fuel hand pump being "Hard" = Does that mean you can't "pump it" reasonably?  (some fuel hand pumps and their fuel bases, could allow one of the "press-fit" one-way check valves to "flip over" and that could block the fuel or really, really restrict the fuel flow; BUT I personally haven't seen this issue on a 3126 model. { I'll add a picture of a typical fuel hand pump and you will notice one check valve is one direction and the other check valve is pressed in the opposite direction (this would be correct)}


Have you tried just a squirt of ether, into inlet manifold, while cranking; and see if it acts like it wants to run?

Have you pulled the top valve cover and looked at valve train? (while cranking)

Have you looked for rubbed wires, that go from the "RPM/Timing sensors"; back to the CAT engine 70 pin connector? (I've seen this once or twice) CAT even sells this little harness, individually, too.



Not being a butthead: but during your 12 volt DC testing and ground testing at OEM plug at CAT ECM, did you have some resistance (test light or old headlamp) tied into circuit while you have your Voltmeter/Multimeter monitoring the voltage? [I use an old Hella Work lamp/Spotlight that I know pulls a little less than 5 amps DC, and a circuit might show 12 volts DC with ONLY a multimeter attached (at OEM plug pinouts), BUT then you add in an old halogen headlamp or my 12 Volt DC Spotlight and the voltage drops to almost ZERO, on a corroded or damaged circuit. A good circuit will light my Spotlight, using either or Both the OEM's plug for 12 volt power and Ground wires, and ignition wire, too.]

I'll add pictures of this lamps, so you get an idea of what I am speaking of.

(yes, I know you are showing 1500 psi of HEUI pressure(mechanical gauge), thus you must be getting something into the ECM, But I've had low voltage cause an unusual stumble and lack of RPMs above 1400 before= OEM wiring corrosion problem, but that 3126 engine did start, in that scenario)

(fuel hand pump (full).jpg)

(Hella_Offroad_Ultra_Beam_H3 (full).jpg)

(Old halogen headlamp (full).jpg)

Attachments fuel hand pump (full).jpg (11KB - 4 downloads)
Attachments Hella_Offroad_Ultra_Beam_H3 (full).jpg (17KB - 4 downloads)
Attachments Old halogen headlamp (full).jpg (58KB - 4 downloads)
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