My Mack when I turn the heater blower or any heavy draw on the voltmeter will turn off like the ignition has been turned off and none of accessories that need the ignition in the on position will work. I have replaced the ignition switch same problem sometimes it will come back to to normal but I need to figure this out. Any and all help would be great thanks in advance John
Any circuit breaker in the circuit? Maybe a weak relay?
Is that the relay in dash that looks like a Ford stater solenoid?
The ones I'm familiar with look like this one: \ Usually mounted about in the center of the dash. I'd check to see if there was power to one of the small terminals then turn on heater and see if power goes away when power stops. Then if the power stays on to the small terminal when power fails see it there is power at both large terminals. One large terminal should be hot all the time and the other should be hot when key is turned on.
Don't forget your grounds. If it does this predictably, then is should be a matter of using a multimeter with some alligator clips to track down where the voltage drop is. Put the alligator clip on the suspect terminal, turn on your heater and see how much the voltage drops. Run your ground back to the battery to check the ground.
Does this truck have one or two terminal strips on the front side of firewall on the left side of the truck? I have a wiring diagram for the double terminal strip version not sure if that would be right for a 1992. Two parallel strip with 14 terminal studs I do recall either version having a ground wire at the end of the terminal strip(s) that did at times cause problems when they got a bit of rust in them. Looking at the wiring diagram I have for the double terminal strip on the front side of fire wall it shows the ground wire that is hooked to one of the mounting screws at the end of one of the strips being the ground wire for the accessory relay, that could be your problem. Hope this isn't a snow plow truck because looks like you have some 6-8 inches in the next couple days! Not sure if the diagram will come through but I am attaching them to this reply. Note you will have to splice them into one! Look at the bottom where it says R,U,DM to figure out how to match them up.
Doesn't a '92 have the big relay that looks like a Ford starter switch behind the dash? I'd start there. I wouldn't replace any parts until I cleaned all the connections on that circuit, positive and ground. What you are describing is usually a bad connection.
This may or may not be of any help but managed to find some notes I made when I had a similar problem back in 2003 while working on a 1996 Mack: Intermittent electrical problem, everything goes dead. Seems to come alive when wires moved near terminal strips on firewall. Cleaned connection on start relay where alternator, batt feed wire and power feed to cab connect. Found circuit breaker on firewall near brake light switch that when disconnected causes everything to be dead too. One note while checking problem at BC plant I think I had power on the output side of start relay, when circuit breaker is Unhooked there is power on output of start relay. Will order new breaker to install if problem comes back. Also there could be something shroting out causing breaker to trip. Breaker P/N 1MR315M9
not sure if anyone is still on here but I have a 1996 Mack rd690s I already replaced the alternator 4 times and changed that ford like relay and tried a few other things and I can’t get it to charge my batteries. If anyone could help that would be great.
Have you put a meter Red Lead on the Positive Terminal of your Alternator, while putting the Black test lead on the dead center of the Positive post of your Battery while running the engine at 1/2 throttle. This will test for voltage loss on the positive side of your alternator. Then put your Red meter lead on the case of the alternator while putting your Black test lead on the dead center of the NegativePost of the battery while running the engine at 1/2 speed. Would be good to put some lights on and fan on to force some current through these circuits. Please report back what the meter reads on each test and will respond back. Simon C
Might be an idea to post a picture of your alternator cable connections as you may have an excitation problem. Don't keep throwing money at it without doing more testing first. I would figure it out if I was there but not there. Simon C
R model schematic I found years ago.
Was looking at the battery and alternator set up and there is a couple of choices. Are you on 12 volt start or 24 volt start. Is your's a Delco or a Leece Neville alternator or Electrodyne. That is a pretty decent schematic heymccall posted and should help. Let us know what yours is whether you are with 12 volt batteries parallel or series circuit . I doubt if you are using 6 volt batteries, but who knows. Simon C