Years ago I had an alarm system installed in my 1988 Vanden Plas. The alarm system is designed so that the car will not crank and start when it is armed. With the alarm off I am now experiencing, on occasion, no crank when the key is first turned. The car eventually cranks, but only after several turns of the key and usually many minutes later. I have replaced the starter relay, added an additional ground strap to the chassis, and ruled out the Park/Neutral position inhibitor. Before I consider replacing the ignition switch I would like to check the alarm connection to the ignition to see if that is the culprit. Where do you suggest that I look to find the connection?
Thanks.

 

I think that the first suspect would be the alarm system. If you have a schematic of the system which would show where it is connected on your car, just try disconnecting it and the power source. I am probably oversimplifying, but if the alarm has no power maybe your car will work..

 

Hi Ernest,

I can't help with how to check your alarm installation, but cars without alarm systems have the same kinds of symptoms as yours, so perhaps it's not the alarm system.

My first suspect would be low battery voltage at the starter due to either a weak battery or corrosion on various battery power connections and ground connections. It's good practice on a Jag to periodically clean the battery cable terminals and battery posts, and the connections at the alternator and starter, and to also clean all the engine bay grounds, including the ones referenced by the engine management system (two on intake manifold studs and one on the firewall behind the cylinder head) and the engine ground strap from the lower starter bolt to the body. A battery may still be able to retain a charge of 12.6 volts (or close) but not be able to supply sufficient current to turn the starter motor, which in most vehicles is the component that requires more amperage than all others. I have a sophisticated battery analyzer and am surprised how often I will test a battery that has a static charge of 12.4 volts or more but load-tests at less than half of its rated Cold Cranking Amps (CCA) capacity.

Another suspect is your ignition switch, the electric switch in a round red plastic case that mounts on the left end of the ignition key lock barrel. One way of testing this is to try pressing the key firmly into the key barrel while turning it to the Start position to see if the car starts more reliably. If not, try wiggling the key in the barrel before turning it to the Start position. If that test is inconclusive, you can remove the lower steering column cowl (just a few screws) and test the ignition switch with an ohmmeter at its electrical connector to see if it is consistently making all the correct connections in each position. You can find the wiring diagram for the switch on pdf page 11 of the Electrical Guide at the link below.

Jaguar XJ40 Electrical Guide 1988-1989

If the ignition switch seems to make the correct connections consistently, you could confirm that the starter is consistently receiving, and responding to, the proper B+ and control voltages. The large terminal at the starter should have battery B+ voltage at all times, even when the key is removed from the ignition switch. Realistically, the voltage will measure less than the 12.6 volts or so at the battery terminals due to resistance in the wiring, but you should still see close to 12 volts at the starter B+ terminal. The smaller wire at the starter should receive battery power when the key is turned to the Start position.

If the above checks out, your starter may be failing intermittently. A traditional test of the starter is to use a screwdriver with an insulated handle to carefully jump the B+ terminal and the terminal for the smaller wire. This should cause the starter solenoid to force the starting gear to engage with the flywheel and the starter motor to turn. The challenge with attempting this test on an XJ40 is the very limited access to the starter.

One test that will rule out the ignition switch and its wiring is to remove the starter relay, bend a large paperclip into a U,and use it to briefly jump terminals 30 and 87 in the starter relay socket. The starter should run. I recommend you wear rubber gloves or otherwise insulate the paperclip, and use one hand on the paperclip and put your other hand in your pocket to prevent battery current from giving your heart a jolt.

Cheers,

Don

 

Try Don’s advice! Here’s some alarm and starter kill info in case you need it:

Aftermarket alarm starter kill systems are typically installed by cutting the heavy gauge starter wire between the ignition switch and the starter solenoid and connecting a relay, which is energized only when a signal from the alarm module reaches the relay. If the alarm signal isn’t getting to the relay, the contacts won’t close, and the signal from the switch won’t reach the solenoid.

Our cars don’t have high current ignition switches, but rather, a low current switch that grounds the starter/ignition/accessory relays. The immobilizer/starter kill relay could also have been cut into the ground wire between the ignition switch and the starter relay.

Incidentally, the factory Neutral Safety Switch in our cars do exactly the same thing. The ground signal from the switch passes through to the starter relay only when the selector is in Park or Neutral.

Under your dash somewhere is likely the aftermarket starter kill relay and the alarm control module. Relays can get sticky and many aftermarket alarms/remote starters are not wired the best, so there could definitely be an issue there like a backed out connector or twisted-and-taped rather than soldered wires.

Kill relays are usually wired so that when the system is armed, the ground signal to the relay is lost and the contacts open, which “cuts” the starter wire. When the alarm is disarmed, the ground signal is sent to the relay to throw the relay contacts and allow current (or ground signal in our case) to pass through the kill relay and reach the starter relay.

If the alarm module is faulty and not sending ground, the car won’t start. You need to either remove the kill relay and revert the wiring back to original, or provide the kill relay with a ground source, that could be a hidden switch or a wire to ground. You could also unplug the kill relay and jumper the contacts to allow the signal to flow.

The problem is that the transmission switch and the kill switch are going to show the exact same symptoms when they fail, so you have double the investigating. Don’t throw parts at it, you need to use a meter to see where the signal isn’t making it through to the relay.

Nick

 

Thanks Don, that's helpful. I am thinking that it might be the alarm system because the car wouldn't start this morning, so I armed it and then disarmed it and it started. This is the first time I've tried that, so I won't know for sure until several more times having the same result. The car has had this issue for about 10 years. The first time it happened it was in my garage and it wouldn't start for days. I finally had it towed to the repair shop and it started on the first turn of the key.

 

NTL1991, It seems that I am getting closer to the root of the problem. It may be a faulty alarm module. This morning after the car wouldn't start I armed it and disarmed it, and it started. I will try this method a few more times, and if proven successful I will attempt to disconnect the alarm. If I can't, I will take it to an alarm specialist and have them do it. Thanks.

 

Jerry, I am believing more and more that you are right. Today when it wouldn't start I armed and disarmed the system, and when I turned the key it started. I will try this method a few more times to verify. Thanks.

 

Jerry, I am believing more and more that you are right. Today when it wouldn't start I armed and disarmed the system, and when I turned the key it started. I will try this method a few more times to verify. Thanks.

 

NTL1991, It seems that I am getting closer to the root of the problem. It may be a faulty alarm module. This morning after the car wouldn't start I armed it and disarmed it, and it started. I will try this method a few more times, and if proven successful I will attempt to disconnect the alarm. If I can't, I will take it to an alarm specialist and have them do it. Thanks.

 

Thanks Don, that's helpful. I am thinking that it might be the alarm system because the car wouldn't start this morning, so I armed it and then disarmed it and it started. This is the first time I've tried that, so I won't know for sure until several more times having the same result. The car has had this issue for about 10 years. The first time it happened it was in my garage and it wouldn't start for days. I finally had it towed to the repair shop and it started on the first turn of the key.

 

I think the problem was the alarm module. I’ve removed it, and now the ignition is firing instantly when the key is turned. Prior to removal there was a slight delay to ignition after turning the key.
I will post an update if it turns out to be something else.