Apologies and another question

[mikefamig]

Did you have the P212S connected to the data bus, or operating in standalone mode?  If you had it on the data bus, it would be interesting to try it in standalone mode to see if it is the P212S data bus interface that causes problems, or whether it is truly a drop in the output voltage that then caused the zone expanders to have trouble.

 
Good point, I have always run it in M1 supervised mode. I plan to leave it alone for a while to see how it behaves.
 
Right now I am spending my spare time struggling with an electrical problem in my old 68 Chevy and I'm going to go waaay off topic because I think htat you may be able to help me. 
 
There is a problem with the starting circuit that I haven't been able to correct. I'm getting a large voltage drop between the battery and the starter solenoid that comes and goes. For years it would occur only when the engine was hot and then recently it failed altogether. I went through the circuit replacing switches and cleaning up connections and now it is back to a hot engine problem or at least it seems that way at the moment. In the past few weeks it has been failing randomly both hot and cold as if it has an intermittent connection but I haven't been able to find it. It is very hard to find because it is not consistent.
 
In addition to the possibility of an intermittent connection it has been suggested to me that the problem is that the electromagnet in the starter solenoid is getting hot causing the resistance in the wire windings to rise and causing the solenoid to fail to energize. When I put a volt meter at the solenoid when the problem occurs the voltage is about 8vdc which is a 4.5vdc drop from battery voltage and not enough to energize the solenoid. If it is true that the resistance had risen in the solenoid due to heat wouldn't I see less of a voltage drop due to less of a load created by the solenoid? When the system is operating correctly the voltage at the solenoid is about 10 - 10.5 volts.
 
Mike.

 

[LarrylLix]

Isn't the solenoid energising the starter motor? Low voltage would be fairly normal at that load. Replace the starter. The solenoid grease is probably elastic and the contacts inside burned out.
 
At some point in history there was a ballast resistor added in series with the solenoid so that when the starter pulled the voltage down the solenoid would stay in despite the low voltage. I believe it would initiate with resistor in series  for lower voltage and when it picked up the resistor would be shorted out on a smaller set of contacts. The solenoid was designed to work on about 6-8 volts.

 

[mikefamig]

Isn't the solenoid energising the starter motor? Low voltage would be fairly normal at that load. Replace the starter. The solenoid grease is probably elastic and the contacts inside burned out.

 
When the problem occurs the solenoid is not activating so no, the starter is not in the circuit at that point. I've owned the car since 1988 and am very familiar with it and the first thing that I did was to install a new solenoid. I didn't have anyone to turn the key while I took voltage reading at the time and the solenoid is cheap so I replaced it new.
 

At some point in history there was a ballast resistor added in series with the solenoid so that when the starter pulled the voltage down the solenoid would stay in despite the low voltage. I believe it would initiate with resistor in series  for lower voltage and when it picked up the resistor would be shorted out on a smaller set of contacts. The solenoid was designed to work on about 6-8 volts.

 
As above, the solenoid does not move so there is no starter motor load at this point. This is a common problem for old Chevies and a lot of guys replace the GM solenoid with a Ford type solenoid located away from the heat of the engine on the firewall or fender skirt which they say works great.
 
But back to my original question. If heat is causing the solenoid windings to build resistance, what would you expect the voltage reading to look like when it is switched on and failing?
 
Mike.

 

[LarrylLix]

But back to my original question. If heat is causing the solenoid windings to build resistance, what would you expect the voltage reading to look like when it is switched on and failing?

Higher if there is resistnce in series with it.
 
Copper has a low resistance increase coefficient. Unless it is red hot I doubt any change could be noted.

 

[RAL]

Your symptoms remind me of a problem I had once with an Audi.  Actually, there were two symptoms, caused by the same problem – dried and cracked insulation on 2 wires that allowed them to short together.
 
The first symptom I saw was that occasionally, after starting the car, the starter would fail to disengage.  The engine would be running, but you could hear the whine of the starter motor spinning at high speed.  If I put the car in gear and drove a short distance, it would usually disengage.
 
The other symptom was that I would be driving along on the highway and all of a sudden, the tachometer would drop to 0 rpm, and the engine would die.  The car was still in gear and kept moving from its own momentum.  Then, a few seconds later, the engine would begin firing again and the tach would return to normal.
 
At different times, I pulled both the ignition coil and starter motor, but after bench testing them, they both seemed to be in good operating condition.   When I went to reinstall the starter, as I attached the heavy wire that delivered power to it, a little insulation flaked off, and that led me to the real problem.
 
It turned out that the ignition circuit used a resistor wire on the primary side of the ignition coil. When starting the car, the ignition switch would bypass the resistance wire to allow the coil to deliver a higher voltage to the plugs.   This wire ran through a protective wire-loom-like tube that also contained the heavy duty cable ( #1 AWG) that powered the starter motor.  The tubing ran very near the exhaust manifold and was exposed to high heat.   Over time, the insulation on both wires became brittle and dried out and began to break away.   Because of the tubing, this was hidden from view, but the wires would occasionally short together. 
 
When starting the car, the resistance wire was able to deliver enough current to the starter motor to keep it engaged after you allowed the key to move from the start position to run, causing the first symptom.
 
While driving, if the wires happened to short, the starter motor would try and engage, but couldn’t draw enough current through the resistance wire to do so.  But that was enough to reduce the voltage being delivered to the primary side of the coil, killing the ignition circuit and causing the engine to die.  After a few moments, road vibration would cause the wires to separate again, and things would return to normal.
 
Looking at the schematic wiring diagram for the car, you’d never suspect that these two wires would be the source of the problem, since they were nowhere near each other in the diagram.
 
The lesson I learned was that sometimes it isn’t a bad component like a starter or ignition coil, but the actual wiring that’s causing the trouble.
 
So check for worn wires on your Chevy.

 

[pete_c]

@Mike,
 
Let us know what you figure out with your Chevy. 

 

[mikefamig]

@Mike,
 
Let us know what you figure out with your Chevy. 

I'm going to start a new topic in the wiring closet to continue this conversation.
 
Mike.