Hi folk,

I’ve been browsing. these forums for a while as a guest and decided it was time to post my own question as I can’t seem to find a resolution on here anywhere. I could really do with some expert help in tracking down this intermittent gremlin as I can’t find any posts on this forum where this error has been resolved

my car is throwing up a P1236 code (no other codes) intermittently which is listed as communication problem between the ECM and the Fuel Pump Driver Module. The MIL is on as a result but the car drives perfectly including under hard acceleration to 70 mph and constant motorway speeds etc. I have the Jaguar SDD software, service manual, electrical handbook and lists of DTC codes etc. I’m also pretty handy with mechanical stuff and especially electronics but this problem has got me stumped. There doesn’t seem to be any pattern as to when the MIL comes on, usually I’m just driving down the road at a constant speed.

I have followed the recommendations in the service manual for recommended resolution ‘pinpoint tests D and E’, which basically checks all of the wiring and also ended up replacing the FPDM. The only last thing to check is replacing the ECM which sounds a bit drastic when the car is running perfectly. I have also monitored the fuel pressure which sits around 50 PSI comfortably so I am assuming the ECM/FPDM is controlling the pump fine but something is failing in the feedback circuit and the ECM isn’t receiving all of the signals back from the FPDM. I have also cleaned the contacts on all connectors in the chain (except one labelled CA10 which I can’t seem to locate but it should be under the glove box according to the manual (is that for RHD or LHD vehicles?)

One thing I did notice when I was graphing the fuel pressure was a sudden drop for a few seconds during deceleration, this does not correlate with vacuum, throttle position or vehicle speed but it directly correlates with PWM cycles which increase accordingly, suggesting that the correct commands are being sent to the pump. I don’t know if this drop in pressure is normal or not as I’m not experienced in these types of fuel systems.

another thing I noticed is that the shield (and signal ground) on the signal wire from the ECM to the FPDM measures < 0.5 ohms to ground and has good continuity over it’s length. However, after cranking it measures about 50 ohms to ground. The manual says this should only be a problem if we were having Electromagnetic interference problems which we are not, I’m not sure what could cause this but it is worth noting. Possibly a bad earth at the ECM (or even the FPDM)

Anyway, I’m out of ideas so any thoughts or help would be appreciated...

Thanks in advance

 

I forgot to mention, the only other thing that may (or may not) be related is I experience flakey connectivity to the ECM with my diagnostic tools. The other modules in the car read fine but the ECM is a bit hit and miss....

 

One other possible cause of a p1236 =

1. The crankshaft sensor signals the fuel injection computer or the ignition control when the cylinders are firing. This causes the ignition coil to provide a spark and the injector to inject fuel into each cylinder at the right time.If either sensor isn't working correctly, the car will run rough and the engine will be less efficient. In later car models, the car's computer can usually keep the vehicle running, but the engine warning light on the dashboard goes on to warn the driver.

2. Faulty (or failing) Powertrain Control Module (PCM).

P1236 Meaning

The P1236 code indicates the Fuel Pump Drive Module (FPDM) detected an invalid or missing Fuel Pump (FP) circuit signal from the Powertrain Control Module (PCM). The FPDM sends a message to the PCM through the FPM circuit, indicating this failure was detected. The PCM sets the DTC when the message is received.

 

Thanks Dell for your reply.

The fault description you posted is really good and slightly different from any other descriptions I’ve read. Where did you get that from? Was that in JTIS? It refers to the PCM but I’m pretty sure all x-types have an ECM (although there is not a lot of difference in reality)

If you are right about about the crankshaft sensor, it would follow that a lot of other engine sensors could affect that code also, in particular the fuel pressure sensor would have a direct affect on the PWM commands to the FPDM. However, I’m not convinced this is an issue for the following three reasons:

1. The car is running perfectly, other than a little reluctant to start when cold but I will change the plugs next week and see if that changes.
2. There are absolutely no other codes thrown up which suggests no other sensors are operating outside of their range etc. Occasionally I’ve seen a P0430 code which is the bank 2 cat efficiency but it is very rare to see that.
3. nowhere in the documentation I have does it mention other sensors or engine performance issues as a possible cause for this code. It does however mention the fuel pump as a possible cause, I’m not sure how this affects communication between the ECM and FPDM but that’s what they say, the pump itself seems to be fine and maintaining consistent pressure.

Your second point regarding the PCM is where I’m starting to go with this problem now, I would like to know it was faulty for sure before splashing out for a replacement

 

Today the car has thrown up an intermittent code for the ABS sensor circuit on the right rear wheel. I’ve cleaned the sensor and the connector contacts but it still throws the code up randomly. I initially suspected this was completely unrelated to the FPDM issue but upon further inspection, both this sensor and the FPDM signal circuit feed back through the cabin harness and connect to the Central Junction loom through a common connector (listed as CA10 in the electrical manual). It is possible that if there was a problem with this connector (partially unplugged, dirty or corroded etc) it would explain both faults... As far as I can ascertain, there are no other ‘critical’ circuits likely to cause codes running through this connector so it could very well explain my issues.

During the week, in the process of trying to sort the FPDM issue, I attempted to locate this connector to clean the contacts. I simply can’t find the connector, does anybody know where it is located? Maybe a photo? The Electrical manual lists it as ‘under the glove box’ (which is LH side on my car) but I can’t seem to find a 22-way grey connector which matches the description anywhere near the glove box or central junction fuse box.

 

You are right that our Jags have an ECM paired with a TCM versus other cars who have an all-in-one PCM which controls ALL functions. The explanation is still valid.




Connector (assuming yours is RHD) same location for petrol or diesel =

 

Thanks Dell,

That is really helpful, my Electrical Guide (2003 - 2004) doesn’t have that drawing which labels the connectors in each location.

That is the general region I’ve been looking, I removed the glove box and side panel of the footwell and although there are a lot of connectors in that area, none of them match the description of CA10. The cabin loom disappears through a steel panel in the footwell and it is difficult to follow where it goes from there. I guess I’ll need to start a process of elimination to find the right one.

My wife is away with the car for a week so I’ll have another look when she is back next week and we can go from there.

 

SOLUTION:

This problem now appears to be resolved, After quite a few hours of diagnosis, we have tracked the issue down to a faulty solenoid in the aftermarket fuel return that was fitted for the LPG conversion, I have never run the car on LPG and didn't realise the fuel system had been modified.

When running on LPG, the fuel is returned directly to the tank to avoid excessive fuel pressure build-up and fuel pumps burning out. The solenoid that controls this had failed so that the fuel was always returning to the tank, this meant that although there was enough resistance to build up almost 50PSI of pressure at the fuel rail and the car ran perfectly, the pressure was not measuring as the ECM expected judged by the commands it had sent to the pump. Therefore the car threw up an error code (P1236) and the engine light came even though I wasn't running on LPG because the solenoid valve that controls this process had failed and was permanently open.

A simple ten quid solenoid of Ebay and we're back in business. when running on petrol I have no error codes.

The interesting thing is that if I run on LPG the problem comes back immediately, so the next project is to fit a fuel pressure emulator for when the car is running on LPG

Thank all for your help