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post #1 of 36 Old 10-08-2019, 07:38 PM Thread Starter
Andy-WhiteTJ
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Obd2 - no link

1998 Wrangler with 4.0 Check Engine Light came on. A little sputtering so I put my Scanner on and I get NO LINK. There is power to the Scanner but I can't read any codes. I did go through water earlier that day and the idle seemed a bit off.

A bit more to the story is that it went in for inspection about 2 months ago and they had problems linking up to get it inspected (NY). After several tries they were able to get it. They mentioned they had a tough time last year as well.

Where to look? I guess I can trace and see if I get continuity on D20 & D21 from the Data Link Connector to the PCM. Any other ideas?


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post #2 of 36 Old 10-08-2019, 08:07 PM Thread Starter
Andy-WhiteTJ
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I have continuity on wires D20 & D21 (SCI Receive & SCI Send) from the port to the PCM. I have 12 Volts at the port. I have continuity on both ground wires on the port to the ground on the engine. The CCD BUS(+) and CCD BUS(-) wires are not used on the scanner I have.

BTW, the Jeep runs and starts no issues there. I just can't get any readings with a NO LINK to figure out why the CHECK ENGINE light is on.

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post #3 of 36 Old 10-09-2019, 12:05 PM Thread Starter
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I'm going to try to remove each sensor, one at a time, and re-check to see if I have a link. I was told that a shorted sensor could prevent the scanner from linking to the PCM.

Anyone have any other ideas?

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post #4 of 36 Old 10-09-2019, 12:31 PM
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Are any of the terminals in the data port dirty, damaged, bent, recessed, or otherwise would provide a poor connection? How about the SCI pins at the PCM and its connector?
For your continuity check of the SCI xmit and rcv, are you getting less than five ohms resistance from the port to the PCM connector?
With the PCM connector C1 disconnected, are you showing a short to ground on either SCI wire?

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post #5 of 36 Old 10-09-2019, 12:54 PM Thread Starter
Andy-WhiteTJ
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Originally Posted by mukluk View Post
Are any of the terminals in the data port dirty, damaged, bent, recessed, or otherwise would provide a poor connection? How about the SCI pins at the PCM and its connector?
For your continuity check of the SCI xmit and rcv, are you getting less than five ohms resistance from the port to the PCM connector?
With the PCM connector C1 disconnected, are you showing a short to ground on either SCI wire?
I was hoping you'd chime in.

I took the OBD2 port apart and put each wire individually on the Scanner pins so I know I was getting good contact. The pins at the PCM are all clean and so is the connector.

I'll have to check the resistance tonight. I didn't get to that last night. I'll also check the short to ground. I assume that would tell if there was a short on one of the sensors?

Thanks

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post #6 of 36 Old 10-09-2019, 02:25 PM
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Like the CCD and later PCI communication busses, the SCI bus relies on specific voltage ranges and low electronic "noise" in the circuit to work. Shorts to ground/voltage or excessive resistance on either the transmit or receive wires will alter the voltage and possibly the timing of the data bits, leading to the bus not working. I don't believe one of the sensors would pull down the SCI bus on your '98 as it's only used between the PCM, ABS module, and the OBD data port, but it definitely couldn't hurt to check -- sensors pulling down the data bus usually pertains to the CCD or PCI bus.

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post #7 of 36 Old 10-09-2019, 03:58 PM Thread Starter
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Like the CCD and later PCI communication busses, the SCI bus relies on specific voltage ranges and low electronic "noise" in the circuit to work. Shorts to ground/voltage or excessive resistance on either the transmit or receive wires will alter the voltage and possibly the timing of the data bits, leading to the bus not working. I don't believe one of the sensors would pull down the SCI bus on your '98 as it's only used between the PCM, ABS module, and the OBD data port, but it definitely couldn't hurt to check -- sensors pulling down the data bus usually pertains to the CCD or PCI bus.
Forgive me as my electrical knowledge isn't great. Mechanically, I can do almost anything including setting up gears. That said, if there isn't a short to ground on the SCI Receive & SCI Send and there isn't excessive resistance, you're saying it could be in the PCM since I don't have ABS and the data port seems fine.

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post #8 of 36 Old 10-09-2019, 04:11 PM
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Correct, the SCI circuitry in the PCM may be bad.

I'd press on with checking the wires and trying the sensor disconnect test on the off chance it may be the cause, as well as try the scanner on a different car to make sure it's working normally. If the PCM SCI circuit is bad, you may also consider trying a different scanner that uses the CCD bus wires. With your TJ being a '98, you may alternatively have the option of using the key trick to display your codes on the odometer.

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post #9 of 36 Old 10-09-2019, 05:29 PM Thread Starter
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Correct, the SCI circuitry in the PCM may be bad.

I'd press on with checking the wires and trying the sensor disconnect test on the off chance it may be the cause, as well as try the scanner on a different car to make sure it's working normally. If the PCM SCI circuit is bad, you may also consider trying a different scanner that uses the CCD bus wires. With your TJ being a '98, you may alternatively have the option of using the key trick to display your codes on the odometer.
Well, I would guess the SCI circuit is bad. I checked both send and receive wires and they're not grounded. I set my Meter to 200ohms (smallest setting) and it read 00.6 and the wire alone to check the circuit read 00.5 so I assume I subtract that from the other and my reading is quite low.

I tried checking a reading after removing almost every sensor on the engine with no change. I also tried the Scanner on my son's 2003 Jeep and my wife's 2018 Hyundai and it worked fine.

1998 can not get readings from the odometer with the key trick. I've tried over the 18 years I've had this. I need to get a scanner to read for inspection here in NY so I guess I'm off to buy a PCM.

Anyone have a recommendation from all the places out there? I've read bad reviews for just about all of them.

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post #10 of 36 Old 10-09-2019, 05:42 PM
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be sure no pins in DLC pushed out, bent, continuity from wire to pin, pin is clean no corrosion.
Will CEL turn on @ 3seconds then go out at every start?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #11 of 36 Old 10-10-2019, 11:08 AM Thread Starter
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be sure no pins in DLC pushed out, bent, continuity from wire to pin, pin is clean no corrosion.
Will CEL turn on @ 3seconds then go out at every start?
Jeep starts and runs fine. I just have a Check Engine light that came on.

The main issue is connectivity to the PCM from the OBD2 port. All pins, sockets and wires have conductivity. The PCM seems to be the culprit.

I've got a new one on its way. We'll see what happens. I'll post here when I get a correction

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post #12 of 36 Old 10-11-2019, 11:15 AM
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Keep the old one as long as you can - lets make sure it the solution.

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post #13 of 36 Old 10-11-2019, 09:51 PM Thread Starter
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Keep the old one as long as you can - lets make sure it the solution.
There's no core charge and I don't need to send it back. If it does take care of it, I'm going to see if I can get it repaired as a back up or keep the new one as a backup.

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post #14 of 36 Old 10-18-2019, 06:53 PM Thread Starter
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New PCM and still no link. I'm baffled.

When I did check the SCI send for continuity and ground, I decided to check also to the CCD bus (-) and (+). I do get a slight clicking on my meter for sound continuity but it's not a constant sound like I get with a good connection. Could there be issues with the CCD bus and the SCI interfering?

Anyone have anything else to check? All connections are fine and I get good continuity to the PCM from the Data Port.

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post #15 of 36 Old 10-18-2019, 10:37 PM
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Ignition on, what voltage readings do you get at pins 3, 4, 5, 6, and 7 of the data link connector?

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