Hello All, I am still chasing a engine cut out problem on my 1985 Jaguar XJ6 S3. I decided to have a look inside the red diode pack from the firewall as its the cover was held on with a cable tie which also held it in place in it’s socket. On opening I found what looked like really poor soldering and what looks like a jumper bypassing the resistor in the pack. Check out the pictures
I have disconnected the jumper and measured the resistor at 1127 ohms. I am not sure what this is supposed to be or why the OP would have bridged the resistor. Does anyone know if this is/was a common modification and what the resistor value should be? On reading the colour code on the resistor I get red, violet, brown, red which I think translates to 27 and the the red is x10 so 270 ohms? The final red band is 2% tolerance. Never could get that right, can any one correct me?
Con
Hi Doug, This has been a long affair. I think I am suffering from a lack of fuel but am currently waiting for a fuel pressure gauge to walk from China to Australia . It is seriously taking a long time. The car will start and run for a short time then just die. It originally started doing this on a drive and quit along with back firing. Had to get brought home on a flatbed. I have changed plugs, ignition module, related wiring and connectors, fuel filter, ignition coil, tested the CTS, checked and cleaned the ignition switch, removed an aftermarket security system (just in case) tried running the motor with the fuel caps opened in case there is a pressure problem in the tanks. The pump is pumping but I am wondering if if it isn’t pumping enough. I have tried priming the system by cranking with the trans in drive and it does start easier when I do that but still conks out. I feel like this is a fuel problem but need the pressure gauge to start testing.
Con
Thanks Andrew, looks like the one in there has been cooked a bit. will replace and see if that achieves anything. I’m thinking the resistor is to limit the current but not completely sure.
Mine is RED VIOLET GOLD GOLD = 2.7R 5% on the 86XJ6. It measures as such. It appears to be just a current limiter to protect the ECU. Here is a schematic I did a while back showing both the Pektron pin numbers and the standard Bosch pin numbers in red.
It would appear by the shit stains on the board and the side of the resister; The resister let it’s smoke out. It would appear the PO figured that out, priced the resister pack ($$ Yikes), monkey’d with the wiring configuration, to discover the car runs… WooHoo!! Made the it permanent by soldering the jumper and called it good.
Hi Ron, I was just trying to figure out how to contact you. I found that old post and was going to ask you to confirm. Thanks.
Right then, thanks for your help fellas, A visit to the electronics suppliers for a 2.7 ohm resistor and some diodes is in order tomorrow. Hope it hasn’t cooked something in the ECU as well.
Con
**
The sole purpose of the diode pack is to operate the fuel relay, Con from two sources - with the diodes preventing unwanted feedback between the sources…
Like Doug I don’t know the purpose or value of the resistors - but if the pump is running; the diode pack is passing current to the fuel relay. One function test of the diode pack is that the pump runs in ‘crank’ and stops when the engine stops…
For further testing, the diode pack is easily bypassed - just remove the pack and jump the pump to run with ign ‘on’…
Frank is correct that the fuel pump relay SHOULD just operate or not. However, somebody badly butchered your diode pack and who knows what else. It’s worthwhile sorting out, given that it’s straightforward and cheap to rebuild. If worse comes to worse, for a brief test you can always just jumper the fuel pump to constantly run to verify/eliminate that issue.
Well that’s interesting, Replaced the resistor and double checked the diodes which tested ok. Plugged it all in and tried to fire the beasty. No go. Hmm, couldn’t hear the pump running checked for power at the pump and I’ve got a nice healthy 12V there so I’m thinking that the pump has finally quit on me. What is the best way to check this? I’ve got 12V when cranking and the earth is good, seems to me the pump has everything it needs to do the job but knowing the Jaguar electrics there is always a little surprise circuit or relay that needs to be checked. Actually I’ll be kind of glad if it is the pump dead it’ll mean that most of my running and starting problems may be solved finally. Still waiting for the fuel pressure gauge.
Con
I take it the nice healthy 12V is measured at the pump directly across the fuel pump terminals?
If so and the pump isn’t running then yes it does sound like it has failed.
Regards,
Hi Andy, I didn’t measure at both terminals. I connected to the +ve terminal and then connected the -ve to earth on the body so I could crank the motor. I guess I could hold the flap on the AFM open and measure that way or get my wif email to crank it.
Con
**
To make sure, Con - check power at the pump with a test lamp…
A multimeter may show 12V without actual power, current, going through. Also; verify pump ground - there might be power, but no ground…
It’s most unusual for a pump to seize up - it’s a tough customer. But the fuel contamination previously mentioned may have done it. You may try reversing pump polarity; if that spins the pump - I suggest a pump replacement without waiting for the fuel pressure test…
Hi Frank,
We had a warm day here in Adelaide today when I was testing earlier and then the temp dropped lower this evening. I went out to check the ground points and to try switching polarity and also tried cranking again when the pump started working again, then the car fired up and ran for 5 or so minutes before dying again. I really feel like I’ve got a dying pump that is temperature sensitive. Perhaps the bridged resistor allowed too much current through that may have caused some internal damage? Have ordered a replacement so we’ll see,
Con
**
Any modifications to the diode pack would only affect the fuel relay, Con - not the pump itself…
The pump is cooled by fuel flow through it - so is hardly likely to be heat sensitive. However, erratic pump behaviour with power to it confirmed does warrant a pump replacement…
There is one exotic thing related to the diode pack. A failed diode may feed power back into the ECU; which may cause the ECU to go into constant ‘crank’ fuelling enrichment. This enrichment may cause overfuelling as the engine warms up - maybe causing engine stalling, drowning the spark…
All diode anomalies can be bypassed for testing by removing pack and, removing also the fuel relay, connect fuel relay socket white to socket white/green…
. It is seriously taking a long time. The car will start and run for a short time then just die. It originally started doing this on a drive and quit along with back firing. Had to get brought home on a flatbed. I have changed plugs, ignition module, related wiring and connectors, fuel filter, ignition coil, tested the CTS, checked and cleaned the ignition switch, removed an aftermarket security system (just in case) tried running the motor with the fuel caps opened in case there is a pressure problem in the tanks. The pump is pumping but I am wondering if if it isn’t pumping enough. I have tried priming the system by cranking with the trans in drive and it does start easier when I do that but still conks out. I feel like this is a fuel problem but need the pressure gauge to start testing.
