Newcomer XJS V12 - 1988 and some problems ; start, but no crank

Hello everyone,

I have been reading the forum for a long time and learned a lot about this model. The forum is very interesting. So I registred today !

I have in the garage a XJS V12 from 1988. The car has been stopped in a garage for 15 years
the car has always undergone proper maintenance and has stayed dry.

Before to put her back on the road, there was a lot of work to do that didn’t give me any problems : brakes, rear suspension unit, interior, and others things.

Then I moved to the mechanical part.
What I done until today :

  • Clean fuel tank and little fuel tank
  • New fuel pump
  • New fuel hoses in th etrunk
  • New main fuel injection relay (in the trunk)
  • New water hoses
  • New water radiator
  • New belts
  • New distributor pinion gear
  • New distributor shaft (in the middle of the V)
  • New plugs
  • New HT wires
  • New distributor rotor
  • New electrical socket for injectors
  • I used a distributor head twice, but very thin, they split. I put the old thicker green Lucas back
  • I cleaned all the electrical connections that I could find
  • I check (with continuity) all the wires I could find
  • Second hand inertia switch
  • New water sensor
  • New air sensor
  • Some new vacum hoses
  • all electrical connections are properly secured, the grounds are good

Now, the car start, crank and after 2 second died.

What I observ :

  • when start : fuel is coming in the rail and injectors open
  • when crank, the fuel in the combustion chamber is burnt and the engine starts,
    But at the same time, fuel pump is cut off, and the electricity to the injectors is interrupted, and the engine dies. Why ?

I understand the mechanics, but I don’t understand on this car what could be the cause.
Help is welcome !

Jean

Welcome Jean.

Remove the fuel pump relay and with a piece of wire connect the pump terminals so that the pump will run all the time. If the engine starts and runs beyond 2 seconds it’s most probable that you have a problem with the fuel pump control circuit inside the ECU.

It’s nice that you did all those things before you attempted to start it, I usually put in another battery listen for the fuel pumps and try to start it up after disconnecting the coil HT lead and cranking till I see oil pressure then reconnect the coil and start if no start trouble shoot the obvious, like if there’s an ignition override that puts power to the injection or coil, if the car has an imobilizer or an oil pressure cut off for the inj and pump as to the rest there are so many disturbed systems that its a toss up, You intimate that it starts if you hold the key in the start position there may be a circuit out that controls running and that’s out though the system energizes when cranking sleepy eyed here good health to all

Did you clean the filter screen in the little fuel tank… or sump tank as it is sometimes called?

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@ aristides : I made, always fuel pump, same problem. I think something cut the electricity in the injectors. But what ?

@ rlysmthn : OK, but which one ?

@ pknellie : already change with a new one

Does the ignition system involve a resistor in the run circuit?

I don’t know right now. But I guess I have a problem in the front wiring loom.

I take out all the injector wiring loom. There are two circuits, one that goes directly to the ECU and another that goes to the resistor, located on the right front engine bay.

I would like to figure out, on the injector connector, where is the direct circuit with black/pink wire ?

posirtion 1 or 2 ?

6520TIF-90

Dunno. It really doesn’t make any difference, as polarity is not an issue on an injector.

I understand, but in this case, all the black/pink wires must have the same 1 position (for example).
Correct ?

Jean,
No. The solenoid inside the fuel injector does not care which position each of those Pink/Black wires are in as long as there is one Pink/Black wire to each fuel injectors and the other colored wires go to the correct fuel injectors.

Paul

Paul,
It’s clear.
But I can’t understand why my injectors “make noise” when start, and stop when crank.

I will make new the wiring loom with injectors. Maybe I have an issue there ?

I change my fuel injector main relay - with diode (near the battery) with an up to date Bosch’s one. (the old Bosch reference number was replaced by a new reference Bosch number).

the fuel system seems to be ok, come in and out untill the tank.
the electric system have some mystery for me, especially the relay.

I have 2 in trunk (main fuel and fuel relay) and four in front of the engine, on the radiator.
from left to right (I think) :main relay, fuel relay, and ?, and ?

You also need to replace the yellow fan blade on the radiator fan- they can explode.

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Possibly the shielded white wire from the ignition module- it gives a timing pulse to the ECU to turn on the fuel injectors. Module is labeled “Lucas AB14” on the left intake manifold.

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It helps to understand the sequence of events that occur for starting. One big thing to note is that the ignition system and fuel injection system are very separate with the one caveat that the EFI system is dependent on some signals including the ignition system in order to run.

When the key turns on, the EFI Switching Relay turns on (if the inertia switch is ok). (A relay not present on some other years). This in turns switches on the Main Relay (in the trunk) providing power to the EFI ECU and the Injectors.

When the ECU wakes up from key on, the first thing it does is flip the fuel pump relay on (grounds it) to pressurize the fuel rail. (You should be able to hear this with the trunk open and key first-on for about 5 seconds). Assuming there’s no leak-down issue (failed pump check valve, or problem outgoing fuel pressure regulator) the fuel rail is up to PSI and ready for injection.

The ECU will not power the fuel pump back on until it gets a +12V signal from the starter relay (indicating cranking) and then for sustained operation, the RPM signal from the Lucas AB14 Amp on the white shielded wire going back to pin 18 of the EFI ECU (one of the 3 way splits off of the Amp’s control of the negative side of the coil). This can be a common failure point.

With RPM signal, it will begin looking at the sensors and performing injection. Without the RPM signal, it will do nothing. The injection system is a peak and hold type, meaning that the injectors are OPENED by a high current pathway in the ECU, and then within milliseconds switched through the power-resistor pack on another pathway to sustain holding them open. (Sometimes the connector there needs a little cleaning). The amount of injection is LARGELY determined by the RPM and Coolant Temperature Sensor (2 prong unit) behind the B bank thermostat housing). A problem connection here will often result in non-starting, poor starting, or massive massive over-fueling to the point of non-starting. Most of the other sensors are simply trim inputs and unless the signal is plain out wrong from them, they can be unplugged and a car will usually still start. (Including the IAT, the TPS, O2 sensors, Load Switches, and even the MAP sensor vac tube (to a limited extent on that last one).

The injectors are wired in 4 sets of 3, but actual firing is done bank to bank as the internal circuits in the ECU are fused into an A bank and a B bank control system so that each side can be independently trimmed by O2 input (when they warm up).

IF the ECU turns the fuel pump on with key-on, that circuit is functioning and isn’t a problem during starting. It’s also worth noting that the control side of the Fuel Pump Relay gets its power from the Main Relay.

One way to check the ECU’s ability to fire the injectors is to turn the key on, and listen for them to all click ONE time with full depression of the gas pedal (what’s really happening is the turnstyle hits the full load switch and, assuming it is working, it will trigger a single burst of the injectors).

The ECU controlls the injector’s negative side. The injectors all get power from the Main Relay on their pink/black wires, but will show +12V power on the other side, unless at least all the 3 injectors on that control circuit, and often the entire 6 on that bank are unplugged because the power will just flow through the pathway back over to the control side of the injector (until it is grounded to operate).

The most similar problem I’ve seen where a car like this starts but doesn’t was a guy w/ a problem connection at the CTS connector, resulting in the car starting IF he pulled up the inertia switch (cutting off the fuel system) but NOT when it was depressed. The reason was the car could start once the fuel had leaned out (with injection off and fuel vaporizing in the manifolds) but NOT with excessive fueling from the ECU because it had a bad input from the CTS.

The ignition system:

The AB14 Lucas Amp box is really nothing but an enhanced GM HEI 4 pin module. The input trigger side is given by a 2 wire VR sensor in the distributer that creates a rising AC voltage of 12 voltage spikes per rotation of the distributer shaft (because it has a 12 point metal star wheel attached).

On the other side of the amp module inside the box (the other 2 pins), one side gets a +12V input with key-on (as does the positive side of the coil) along with a points-style suppression condenser / cap hanging off of it (can cause failure if it internally leaks to ground), and the negative side is split 3 ways —well 4. One is direct to the coil to cut charging and trigger the spark, and the other two are through a white silicone blob with resistors inside. One leads to the Tachometer, and the other connects to the EFI ECU on pin 18 through that white shielded wire (RPM signal source.) The 4th connection is to a zener diode bonded to the metal case for voltage spike suppression.

As noted that RPM signal wire to pin 18 of the EFI ECU can be problematic… usually in the engine bay.

If it ends up you need to go through the process of detailed continuity testing, I’ll post some diagrams to help with that, but hopefully this will sort out as something simple.

This is mostly right, but the connector is upside down here. The long row would really be on the bottom, so you kind of have to flip everything but the incoming connector mentally:

Sometimes this helps if it becomes necessary to make a wire-loop in the engine bay (like between the pins of the CTS) and check that continuity on the proper pins at the ECU. PITA work you want to avoid if you can solve it any other way.

~Paul K.

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More visuals:

Sometimes helps to clean these pins & spread the tips very gently w/ an xacto knife:

Wiring:

Harness Illus:

~Paul K.

Paul,
First of all, a huge thank you for your documented and informative response. I’ll try to answer step by step to your post, responding chapter by chapter, as you wrote.

Yes, I notice that fuel and ignition system are separate.

Correct, when key on, the fuel pump is working, and the fuel is coming in the injectors rail

My starter engine is working, so I think my starter relay is OK ? I’m right or not ?

« the RPM signal from the Lucas AB14 Amp on the white shielded wire going back to pin 18 of the EFI ECU (one of the 3 way splits off of the Amp’s control of the negative side of the coil). «

=> I must check this point… I will have a RPM signal just with key « on » ? I’m not sure….

The injection system is a peak and hold type. Yes, I understand that, with « black/pink » wires for the High current and « orange family » wires return to power resistor.

Power resistor very OK and connector as well. I made the same scheme as you for the power resistor.

I have 2 coolants temperature sensors, new. 1 near the A bank, and 1 near the B bank.

=> c onnectors on the sensors are super OK. I must check continuity of the wires from sensor to ECU

I noticed that the injectors go 3 by 3.
I made a diagram of their interactions like you did (and much better than me)

This also confirms that the position of the wires on the injector pin do not have a precise position ?

The fuel pump is working nicely (new fuel pump).

=> I have to check if the Fuel Pump Relay gets its power from the Main Relay (the main relay is side by side with th efuel pump relay, correct?)

I put a new fuel pump relay (with diode, as original). But the referrence from Bosch has changed now, it’s no more the old wellknown reference type. (finishing by 112)

« One way to check the ECU’s ability to fire the injectors … » When I put the key ON, I hear the sound of a small drill, then it stops. I checked that the gasoline arrives at the injectors. However, I never used the accelerator pedal for this test .

The ECU controlls the injector’s negative side “ Where is the negative side of the injector ? Which injector’s wire ? (I thought at the beginning it is “orange wires family”, but today I don’t know.

bad input from the CTS” . So he changed the CTS unit, and everything was fine ?

=> I have to check if the Amp module is OK . Connectors are OK and wires as well. I will check if it internally leaks to ground or not

=> I will check the signal wire from engien to ECU pin 18.

The diagrams are very clear and do not scare me.

I already made wire-loop in the engine bay…
I usually remake whole harnesses for cars, but without ECUs or injectors! (I’m not an electrician, but with years, I learned and I find that interesting.

I will let you know in the following days (if I work quickly !!)

Jean

Best of luck… I hope the operation description helps you visualize what might be going wrong.

~Paul K.

first off, welcome to jag lovers, always glad to see new people, and im an 88’ xjs owner myself, so its nice to meet you

yeah my was sitting for 10 years and i did none of this, maybe i should. kinda needed a checklist :joy:

if you have any questions regarding interior im one of the guys to ask, i love car interios and know my xjs from floor to head linning and can help with that. this includes all the interior electronics like the radio as well

Good to know for the interior !

Yeah no problem, I’m not extremely active on here, for example it took me 18 days to respond to you but yes if it comes to the interior I would give a response, maybe just not right away all the time