No power to half of rear box...NO START

daddysgirl · October 03, 2016, 09:12:34 AM

Hi all.
I'm just past loosing my mind, so I thought someone has thought of something I have not.
I have 1974 (Sept 73) MC8: 8V71...HT740

The bus wont start. Batteries perfect...Starter has plenty of power...but it will not start. Rear electrical panel: No power to relays or circuit board. BUT Power to the lower bars.
WTH?

Help? Anyone?

Oonrahnjay · October 03, 2016, 10:31:18 AM

Quote from: daddysgirl on October 03, 2016, 09:12:34 AM
Hi all.
I'm just past loosing my mind, so I thought someone has thought of something I have not.
I have 1974 (Sept 73) MC8: 8V71...HT740

The bus wont start. Batteries perfect...Starter has plenty of power...but it will not start. Rear electrical panel: No power to relays or circuit board. BUT Power to the lower bars.
WTH? Help? Anyone?

I don't know the exact procedure to check it but the first thing I'd check is the neutral switch in the transmission. But electrical stuff can be *anything*

Good luck.

bevans6 · October 03, 2016, 11:46:12 AM

First question - it will turn over but won't start, or it won't turn over? Thought of that only after I wrote all the bumpf below, but I actually think I covered off both scenario's. First paragraph is won't turn over, second is turns over, won't start.

There are two bus bars in the rear junction box, one high power one is fed from the alternator positive connection directly to the batteries via the master disconnect switch. The other one is a controlled bus that is powered from the master switch on the dash and comes back via stud 14 in the front electrical panel to studs 24 and 25 in the rear panel to the rear bus bar. So a lot of the relays get their power from the master switch. The start circuit is basically master switch on the dash to the start push button to the front/rear switch to the neutral relay to the start relay to the start solenoid. The starter relay is grounded through the fuel pressure switch, if you have one still. So I would check for power at stud 14 when the master switch is on, if you have no power on the rear bus bar, then on stud 24 and 25. You can bypass the neutral relay by jumping stud 15 to stud 26 in the rear panel, if you think it might be bad.

Check to see if your engine stop air cylinder is retracting when you switch master power on. Symptom would be if it turns over but there is no smoke out the exhaust indicating the engine is in no-fuel. It should push out under air pressure when the master switch is off, and retract when the master switch is on. It is controlled by a Skinner valve that gets power from the same rear bus bar as discussed above, via stud 14 etc. If it does not retract to let the engine run, and you have power on the rear bus bar, then there is a 6 amp circuit breaker that might be bad (it should be an auto reset breaker) and if you have the engine stop relay option that whole mess could be bad. If you have the engine stop relay option it controls power to the engine stop skinner valve and it is controlled by the hot engine sensors, the low oil pressure sensor, etc. You can bypass it by connecting the 6 amp circuit breaker directly to stud 53 in the rear panel.

Check some of this out and see if you find anything wrong. Obviously, first check that the master switch is on and the engine run switch in the rear control panel is on, and the front/rear switch is in the correct position. Also check if you have fuel and it's primed.

Dave5Cs · October 03, 2016, 10:14:34 PM

Didn't flip the switch under the red shield cover on the dash did you? If you did you need to re-set it in the back by the right rear near the top of the valve cover.
Dave5Cs

daddysgirl · October 04, 2016, 11:13:33 AM

This is the darndest thing. #14 has plenty of power. Master switch has power. Push button has power but nothing happens when you push it.
about half of the studs in the front and rear have very little, maybe 5-6 volts. There is no fuel pressure switch anymore.
I'll try jumping the studs, but I put all new relays in (except the starter relay), and even tired to jump the starter from the + bolt to the little nipple on the solenoid. Oh, and I know better than to hit that covered switch on the dash

eagle19952 · October 04, 2016, 11:17:16 AM

Quote from: daddysgirl on October 04, 2016, 11:13:33 AM
This is the darndest thing. #14 has plenty of power. Master switch has power. Push button has power but nothing happens when you push it.
about half of the studs in the front and rear have very little, maybe 5-6 volts. There is no fuel pressure switch anymore.
I'll try jumping the studs, but I put all new relays in (except the starter relay), and even tired to jump the starter from the + bolt to the little nipple on the solenoid. Oh, and I know better than to hit that covered switch on the dash

low voltage VOM reading indicates a bad ground.

daddysgirl · October 12, 2016, 08:42:55 AM

UPDATE... On the 74 MC8. The 8V71 and HT740 were new in 2000.

OK. Still hasn't started, BUT I did remember having to use the kill switch last year, sure enough...she isn't getting any air. BUT I have managed to locate and fix a few electrical issues. AND THAT leads me to another question...

Until my son (helper is awake) when I push the start button, should I not hear the starter click? I don't. Now, could that be a bad connection to the stop valve next to the fast idle at the top?

And does anyone have any suggestions on how in the hell I can get to the rear of the rear electrical panel? Near the Penn Air switch? I can read voltage from panel, but if I could reach it, I could re-insulate the harnesses and do some engine compartment body work to boot.

gumpy · October 12, 2016, 12:06:00 PM

If your rear switch is in the front position, when you push the start button, it should turn the starter. The stop valve has nothing to do with the starter turning or not. It's simply opens the fuel rack to the motor. If you starter is not turning, first check that you are getting power to the starter solenoid. There's supposed to be a fuel switch on the secondary filter that disconnects power to the starter when fuel pressure is reached. I think you may have said this was removed. Did you ground the wire that was attached to it?

B_K · October 12, 2016, 05:09:31 PM

Where are you located? Maybe another busnut is close by and could stop in and give a helping hand and show you a few things that should help you understand these buses better.

BK

brmax · October 12, 2016, 05:57:25 PM

I believe there was mention of jumping the starter somehow and no result, there is something to check further here maybe that I didn't pick up on. For the starter circuit anyway. Care full proper jumping it at the starter should in all cases "turn" the motor over if there is full battery voltage, now weather it starts is a separate circuit issue.
When the ground stud on the starter case end has been a culprit many times for me, it can look ok and believe me have caked white corrosion inside and "between" the stud and steel case. I'm just saying once or twice ok but these big dog starters that's a big problem area.
Floyd

daddysgirl · October 14, 2016, 11:09:09 AM

I'm near Richmond VA...and always welcome any help, but I'm thinking the starter solenoid is gone. I've run every test there is, and it won't turn, click...nothing. It has plenty of power, and a solid ground. I just made new cables for both. But it won't even turn (even after a few swipes with the hammer) when I directly jump it.
But I am in the process of rebuilding this bus (first was 16 years ago when dad and I did it) so I have a lot of systems to keep me busy. If all else fails, I'll have it towed somewhere they'll pull the engine so I can replace a few harnesses I need to make.

BUT...the power to the majority of the boxes is back. At least that's one.