Alarm Troubleshooting Advice

Quasimoto · 06-07-2019, 06:27 AM

Was out on a Sunday drive and the alarm starting acting up. I've read quite a few of the alarm threads on here, but thought I might ask for some troubleshooting advice before pulling things apart.

The symptoms:

When the door is opened, the car goes into alarm mode. I can hear the relay on the firewall cycling.
Initially, the alarm horn sounded in synch with the relay, but stopped on its own. Just the relay now.
Car will turn over but not fire while the door is closed.
If you open the door very wide, alarm relay stops.
Car will start and run while the door is completely open. You can close the door and drive although the relay cycles for a minute or so, then goes quiet.
The spot in the door travel that the alarm activates/deactivates does not seem to correspond to the door jamb switch.

I've did not get an alarm key when I purchased the car last year, and don't really plan on using the alarm in the future.

Have seen the details of how to bypass the alarm relay and this is tempting, but thought I might take a stab at fixing the problem before I go that far.

Any recommendations or advice would be really appreciated.
Thanks!

Quasimoto · 06-07-2019, 10:02 AM

Forgot to ask...

Are the "door switches" shown on the Alarm schematic just extra contacts on the regular jamb switch, or are there separate switches?

pavegeno · pavegeno's Garage

Sticky door jamb switches? That could explain why door position doesn't seem to correlate with alarm activation. They need to be replaced every few years.

pavegeno · pavegeno's Garage

They alarm switch is the same plunger/switch that activates the interior lights.

76FJ55 · 06-07-2019, 11:44 AM

I will guess that one of the wires to the alarm switch has failed in the door harness. the alarm switch has two 4.7K ohm resisters in side it. when the alarm is deactivated the current only runs through one of the resisters. when the alarm is armed the current runs through both resisters in parallel (2.4K ohms). the system is designed so that if the module doesn't see the single resistor 360 ohm value it triggers the alarm. since your system was not armed using the key I can only assume that the door harness has an issue. either a broken wire or bad insulation that breaks connection or shorts the alarm switch wires. the module sees the out of range low (shorted wires) or high (broken wire) resistance and triggers the alarm.

for reference here is a thread I posted recently on how I made an alarm key. it has a couple pics of how the alarm switch is constructed.

Quasimoto · 06-07-2019, 11:54 AM

@pavegeno - thanks, I had a hunch they were, but good to have confirmation.

@76FJ55 - your thread was one of the ones I had read during my search, great info.

After fiddling with the door switch a bit, I think that the idea that there is a fault in the door harness makes a lot of sense.

Right now I am wrassling with the ventilation fan so I can get to the alarm module to check it...but have a feeling I will be digging into the door harness.

76FJ55 · 12:50 PM

If you look inside the trunk located between the booster and the door hinge area you will find a round multi pin plug. coming from the door. this plug is for the mirror wiring, but this door side harness also attaches to the alarm switch. the alarm switch wires IIRC are the 2 wire connector next to the round mirror wire plug. if you unplug this connector and check for continuity o the door side, you should get the 2.35K ohm or close to it of the alarm switch. then while watching the meter move the door throughout its opening range and see if you get any fluctuation in the resistance readings. if they very based on door position you know you have a door harness issue, and depending on if they go up or down you may get a clue on what type failure is occurring. in addition if you do determine that it is the door harness, you can always leave this connector unplugged and bridge the car side with a 4.7K ohm resister this will tell the alarm system that the alarm is deactivated and should make the car operable until you do a final fix on the harness.

Edit: the 2 wire plug for the alarm switch should have a green wire and a brown wire.

Quasimoto · 06-07-2019, 04:51 PM

Excellent, thanks for taking the time to put that down...very helpful!

spoke · 06-07-2019, 09:39 PM

I had the same alarm going off with door open widely last year. I've never used the alarm and never intend to. For my '86 930, the alarm module was in the front trunk under the ventilation unit. Removed the ventilation unit and measured the resistance of the 2 wires from the alarm door key. It was 4.7k ohms. Sometimes...

As I opened and closed the door, the resistance would change from 4.7k ohms to zero then to infinity. The wires were breaking sometimes shorting and other times completely open.

To fix this, I cut the 2 wires from the door and soldered in a 1/4 watt 4.7k ohm resistor. Problem solved.

Quasimoto · 06-08-2019, 06:25 AM

Times like these I'm even more annoyed that Radio Shack went out of business...erf

Thanks to all for the assistance!

spoke · 06-08-2019, 06:36 AM

You can order resistors from Digikey or Mouser. The resistor will cost you about 10 cents with $7 shipping.

Quasimoto · 06-08-2019, 08:46 AM

Yep, got one on order... This morning I have done the 87a/15 and 61/61 jumper on the alarm module in order to be able to drive the car on this glorious weekend. I'll have the car back in the garage for exhaust install next weekend and I can piece the alarm back together then.

Meanwhile...

Being new to 911's, one of the first things I did was read Dempsey's "101 Projects". This blurb stuck with me for some reason, and it's been fairly prophetic...

Quote:

There probably isn't a place on the car with more gadgets and devices that break than on the door. Not only do you have window glass and seals that leak water, but you have door handles, mirror and window switches, window regulators, door stays, and door panels: all of which are very susceptible to damage and breakage. Even if you work on your car only moderately, there is a very high chance that you will need to dive into the door to fix something that has broken.

spoke · 06-08-2019, 09:00 AM

Quote:

Originally Posted by Quasimoto

Yep, got one on order... This morning I have done the 87a/15 and 61/61 jumper on the alarm module in order to be able to drive the car on this glorious weekend. I'll have the car back in the garage for exhaust install next weekend and I can piece the alarm back together then.

I did that fix at first but found the fuel pumps ran 100% of the time the key was on. The 1/2 second pump up when key turned to on wasn't happening as the pumps ran all the time with engine off.

Do your fuel pump(s) run all the time now?

Quasimoto · 06-08-2019, 01:53 PM

Interesting question. In the threads I read about the bypass jumpers, I must have missed this issue, and hence didn't check. I'll have to take a look tomorrow.

Thanks for the heads-up!

Quasimoto · 06-25-2019, 02:45 AM

Quote:

Originally Posted by spoke

I did that fix at first but found the fuel pumps ran 100% of the time the key was on. The 1/2 second pump up when key turned to on wasn't happening as the pumps ran all the time with engine off.

Do your fuel pump(s) run all the time now?

Finally got a chance to check this out. I cannot hear either pump come on with the key in the middle position.

I don’t yet know how the DME sequence works. Are the pumps supposed to come on with the key in the middle or in the start position?