When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
X350 Super V8 - My gearbox goes into limp mode, if i read gearbox date i come up with the following code; P1794 battery supply malfunction. If i watch data and watch the volts go up to 15.8 the gearbox light comes on. I have changed the alternator as I think its overcharging, but it still reading high. The fault is random and intermitant
Anybody got any suggestions, as I am completely out of ideas!
Hi Pkoko i have a new battery from what i have reserched these cars have a high charging rate of 15 volts If i have my diagnostic gear connected reading gearbox data i can watch power supply go up higher than 15 volts soon as it hits 15.8 volts gearbox warning light comes on and gives me a fault code of p1794 battery supply malfunction i have cleaned all ternimals on battery leads ect thanks
15.8 is too high of a voltage... Can measure the voltage at the terminals? Measure with car off and on without load. Also measure it with electrical load with car on. It should be less than 15V; IIRC it should be 14.8V
hi yes way to high but cant understand why it will creep up wirh no load and so intermittantbe think it might be an earthing problem or bad connection at ecu
You have a real mystery on your hands, but I think with good methodical diagnosis you can figure it out.
First of all, the Electrical Guide and Diagnostic Trouble Codes Summaries will be helpful, and you can download them at the links below. You don't mention the year of your car, but the 2004 Electrical Guide is generally representative of all X350s:
Here's what the DTC Summaries have to say about P1794:
According to the Electrical Guide, the ignition switched power supply for the Transmission Control Module (TCM) arrives on a Green wire with Orange tracer stripe (color code GO), which connects to Fuse 33 (5 Amps) in the Passenger Junction Fuse Box (PJFB), which is located at the base of the right-hand (U.S. passenger side) A-pillar. Always-on battery voltage arrives on a Brown/Red (NR) wire from Fuse 17 (10 Amps) in the Front Power Distribution Fuse Box (FPDFB), located in the front right corner of the engine compartment.
You could start by measuring the voltage at the terminals in the socket for each of those fuses. Test Fuse 17 in the FPDFB with the key off. Test Fuse 33 in the PJFB with the key in position II (Ignition ON) but do not start the engine. Are the fuse spade terminals and socket terminals clean and free from any signs of whitish corrosion/oxidation? Do the fuses measure zero or near-zero ohms of resistance across their terminals? Is the voltage measured across the fuse socket terminals close to the voltage across the battery terminals with the engine off?
Another test would be to start the engine and measure the voltage across the battery terminals. Does the voltage rise to 15.8 volts, or is that possibly an erroneous value seen by the TCM?
The Workshop Manual Electrical section can be downloaded here:
Here's what the Workshop Manual has to say about Battery Charging:
You could also check the voltage at the TCM electrical connector. The ignition switched B+ voltage arrives on the Green/Orange wire, and the always-on battery voltage arrives on the Brown/Red wire from. Measure the voltage from those wires to ground with the key in position II. Turn the key off and measure from the Green/Orange wire to ground - if the resistance is low, there may be a short somewhere.
If you determine the charging voltage really is reaching 15.8 volts, I would suspect resistance somewhere in the circuit causing the ECM to read the voltage lower than it actually is. The ECM receives the alternator status signal over the CAN bus, so it can't be measured with standard tools. However, faults in the CAN may trigger DTCs with a U- prefix (Undefined, mostly Network-related). These are proprietary Jaguar codes that cannot be read by generic OBDII scanners, and require either a dealer-level diagnostic system such as Symptom Driven Diagnostics (SDD), or high-end third-party systems such as Autoenginuity with the Jaguar enhancement, or possibly some of the more powerful scanners from companies like Snap On, Mac, etc. If you can find a good independent mechanic with such a system , it would be helpful to know if any U codes are flagged since they may help narrow down your diagnosis.
HI, Were on the same wave length as that. Have tested the voltage across the battery, and it corrosponds with the live data voltage when pluged into a scan tool. Im in the middle of checking all the earths at the moment, am using a Snap-on Solous Pro and have also read with a Lanch machine. Its so intermittent, some days its fine and other days it does it straight from start up. Sometimes it even seems temperature dependet (the car is kept in a garage) with colder weather making it worse. If i load the system up i.e with the heated rear screen on and the day time running lights on the voltage stays normal and i have no driving faults although the codes remain. Thanks for getting back to me, have only just seen this.. I have also recently read another intermittent code of B1676 in the drivers door module battery voltage out of range. When I have the ignition on the batttery light was pulsing and the interior lights, which makes me think that there is a bad earth or bad contact. I have only seen this once i have had this problem for over a year on and off. Thanks for the infomation.
gearbox fault - gearbox goes into limp mode
