Doing pinpoint tests on the CAN, SPC and Serial. Everyone tests both short to ground and B+. I have disconnected many modules and still get the bus shorting to B+.
Has anyone come across this. As you would expect all christmas tree lights are on and car won't start! Many things still work, such as radio, lights, climate control etc. I am sure I have a short in the DSCM however, evem with it disconnected, the B+ is still there.
One interesting thing is the effect on battery voltage drop vs which modules drop out. As the battery goes down below 12.6 or so more and more modules drop out. That is almost a fully charged battery. I would expect the modules to function quite a bit below this voltage. I guess the shorts are causing the sensitivity.
I have much of the car torn apart to get at the module connectors and am running out of modules to disconnect.
Any help much appreciated.

 

I'm not sure that I understand what your problem is, but if your B+ is shorting to ground, I would try removing each fuse individually until the faulty circuit is pinpointed.

 

Unfortunately B+ is not shorting to ground. Only to each bus. Same with ground. The problem is isolating the modules without all of their controlling/controlled circuitry. The bus circuits actually check out with the proper resistance in each circuit as well.

 

The pinpoint tests have you check resistance from each bus wire to ground. It should be more than 10k Ohms - mine are not, Also it has checks from each bus wire to B+, most are supposed to be less than 3v. Mine are in the 6 to over 12v range. This degrades the signals. When I look at the signals, with the scope, they are malformed and often weak. It is a wonder any modules work properly with the poor signals however some are not complaining.

 

Can you disconnect the CAN bus and narrow down where the short to B+ exits? The IP cluster is the network gateway, so that might be a handy midpoint. Or if you suspect the ABS module, connector IP55 would break that end of the chain off.

 

When I found that both Can and SCP bus had similar shorts I figured it would be the Instrument Cluster, since that is a common point for them. I have disconnected it without any change. Most of the interior is now removed as I try to isolate the problem by disconnecting modules and connectors. The Jag training manuals say it is easy to find with a DVM and the wiring diagrams!
It will probably be something obvious when I finally find it.

 

Check the electrical diagram manual, there is a good network diagram and I believe there are modules (other than the IP) that use both CAN with SCP.

 

I am well and truly deep in the electrical diagrams. As far as I can tell the IP is the only one with both SCP and CAN. There are others that have CAN and Serial or SPC and D2B. I think, in the end, that them both having similar faults may be a red herring.
Each module, in the series, seems to reduce the resistance until it drops below the minimum 10k ohms. Adding the DSCM (ABS), to climate control/J gate run, drops it to 11K. Adding TCM it goes to 8K and with ECM it drops to 5k. before adding any of the last three the resistance is in the meg ohm range. It is as though each module has a short to some other circuit internally. I do not know how they all could 'go bad' at once.

 

By B+ do you mean 12v battery?

have you disconnected every module that uses that bus yet?

 

Yes you're right--sorry I was thinking the serial bus instead of the SCP bus. So you are finding low resistance readings to B+ on the CAN and SCP networks... I assume you have the battery disconnected for all this?

I've gone through that diagnostic tree for my CAN bus and I can confirm that the measurements from CAN to gnd, CAN to ign, and CAN to B+ were 10s of megaohms.

I think I would not go crazy trying to ID why the SCP and CAN have issues, but do them separately. If you can put everything back on the network except the IP and have no shorts, that sounds like your answer.

 

Virtually every module uses B+ or/and B+Switched. Both are direct to battery 12V. The switched one is with Ingition Switch i position 2. I have most modules disconnected many are hard to get at and I am leaving them til last!

 

Thanks for the info that you see high resistance when doing those tests. I was wondering if I was not interpreting the tests correctly. Yes the battery is disconnected for the Ohm tests. And connected for the B+ testing which gives me high voltages for the busses.
Oddly enough the IP does not seem to cause the low resistance in either bus. I also wondered if the modules were programmed to do that when a theft was suspected.
I am now doing them seperately since the IP did not cause issues. I am leaning to the modules going bad due to a voltage spike. However for a multitude to go bad it would seem unlikely.
Thanks for your responses.

 

Based on what you said so far I’m not sure you’re actually Reading resistance during your testing because if every module is in Series, There is no way that the resistance would go down as you add more modules to the chain. In a series resistor measurement Anything you add to that chain adds resistance and cannot reduce the resistance. Only a parallel connection will reduce the overall

 

Disconnect the positive lead of the battery for several minutes then physically touch the positive lead to ground and hold to the drain off any charge on any modules that are still connected and try it again

 

So I just looked at the schematics and both the serial dater link and the can bus R parallel connections not series.
So that’s why every time you add a module the overall resistance goes down.

 

Where are you making your measurements from? Both leads? According to my manual with the battery disconnected, the can + and can - signals should be more than 5 ohms to either B+ or ground. What do you read?

also with the battery disconnected but the abs module connected the resistance between can+ and can- at the dtc should be 50 and 75 ohms. Whatcha got?

 

The modules are actually in parallel. Some are used as a pass through. Others are spliced in parallel with another module. It looks very much like a series circuit but is not.

 

Where are you making your measurements from? Both leads? According to my manual with the battery disconnected, the can + and can - signals should be more than 5 ohms to either B+ or ground. What do you read?

also with the battery disconnected but the abs module connected the resistance between can+ and can- at the dtc should be 50 and 75 ohms. Whatcha got?

 

Each bus checks out properly as far as resistance from Can+ to Can-, which is 60 Ohms. The problem is when checking B+ to Can+ where the pinpoint test calls for it to be more than 10k Ohms. Mine are around 5K. In actual fact they should be much more than 10k but will function down that low.
The other test,with battery connected, is between B+ and CAN+ where the voltage should be less than 3V. Mine are close to B+.

 

Also Check for the correct termination resistance inside your engine control module it should be between 101 140 ohms