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Discussion Starter #1
Got an issue with my Jeep TJ. It will crank but not start. I replaced crank and cam sensors. I checked the asd and fuel pump relays. I also checked the socket terminals for power where the relays plug in. Constant power terminal checks out good on both sockets....but when I turn the switch on to check power in the terminal sockets for the relays... one relay socket shows no power. Could this be a bad pcm? Any help would be appreciated.
 

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Switched power to those relays comes from the same fuse behind the glove box which should be labelled PDC Coils.

If one relay is getting power first check both coil terminals on the other socket, as the sockets may be wired differently. If still no power there may be a bad splice or bad crimp in the PDC.

Without knowing what model year you have I can't check wiring diagrams. Adding some basic vehicle information like model year, engine and transmission to your signature would help this and future posts.
 

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Discussion Starter #3
Thank you. It is a 97 TJ wrangler sport. 4.0 Ltr with manual transmission.
 

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Crank briskly no start
1 make sure fuel in tank
2 check for spark
3 check fuel rail pressure
4 check with noid lights if injectors are pulsing

Never start replacing parts randomly it just clouds the picture as not all new parts are functional out of the box

With a 97 not uncommon for pcm 5 volt output to fail which results in no signal from sensors dependent on that 5 volt supply including crank position sensor

If that happens an external USB port sold at amazon or Walmart for a few bucks can be used as an alternate 5 volt supply


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Discussion Starter #5
I checked and the wiring is different on one of the relay terminals, it’s the terminal the shows power when the switch is on.
 

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Check wiring to coil it gets a constant plus 12 with cranking and a pulsed ground

If only missing pulsed ground suggests crank sensor which suggests possible loss of 5 volt supply to sensor


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Also got fuel pressure is not checking for 49 psi on rail with gauge


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This image shows where you should have live (red) and switched (yellow) power for ASD, fuel and horn relays. The horn relay can be swapped with the ASD and fuel pump relays to make sure the relays themselves are ok.

4433456


As you can see, the switched power is on opposite sides of the ASD and fuel pump relay sockets.

The fuel injectors, coil and O2 sensors get 12V from the ASD relay. You can short the terminals numbered 1 and 2 in the ASD relay socket and see if it starts. If it doesn't start with the ASD socket shorted to supply power you may have a bad cam or crank sensor. If it does start the PCM is not switching the ASD relay.

The crank sensor can be checked by unplugging it and measuring the resistance across the B and C terminals (look for lettering on the connector) which should be open circuit (or over 10K ohm at least)
 

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If no spark, start simple and check the rotor arm under the distributor cap. I've had a rotor arms that looked good short out in the past. With the rotor fitted measuring from the center to a good ground should be open circuit. If that's ok the coil output can be checked. The coil could also be checked for continuity and cracks. Might be easier to get to the pins to test with it removed, and it would also be easier to see if it has cracked which seems to happen more on the engine side anyway from what I've seen.
Resistance between the two pins at the connector should be 0.95-1.2 ohms, resistance between either of those pins and the HT terminal should be 11,300-15,300 ohms.
 

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Simpler to just check for spark from coil before it reaches dist(cap rotor and wires)

Pull dist end of spark wire that goes from coil to dist and check spark there on dist end of that wire


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When you turn the key on do the gauges power up? Easy to see if the fuel & volt gauges are working. If not that is a good sign that you are losing the 5v supply to the various sensors as Digger noted earlier (see post #4). I would check that before going too far into the diagnostics.
 

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Discussion Starter #13
I checked the pcm. Everything’s good except for voltage coming out. Instead of 5 volts, I’m getting.85 on pins 17 and 31. Does this mean the pcm is shot?
 

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Generally if the voltage is low like that there is a short in one of the sensors or in the wiring to one of them.

Start unplugging each of the various 3 wire sensors. They are the TPS, MAP, speed, cam & crank sensors. Hopefully unplugging one will bring the voltage back up to 5v & you will know where the problem is.
 

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I see you have already replaced the cam & crank sensors so start with the others. While you are doing that check the wiring at & near them for any wire damage.
 

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Simpler to just check for spark from coil before it reaches dist(cap rotor and wires)

Pull dist end of spark wire that goes from coil to dist and check spark there on dist end of that wire


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It may be simpler but spark from the coil doesn't mean spark at the plugs. Cap and rotor would still need to be checked.
 

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It may be simpler but spark from the coil doesn't mean spark at the plugs. Cap and rotor would still need to be checked.
Bad logic only need to check cap and rotor if actually have spark from coil

The goal of a test is to rule in or out a problem not endless data points

Visual inspection of a component is inferior less conclusive to actual functional test

By checking for spark at coil you know problem is coil or before or past coil

Looking at cap and rotor and seeing no problem does not do that as problem could still be anywhere


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