1996 Rd688 Electrical Issues

[therabbittree]

Ok I'm working on a 1996 Rd688. The truck has a few electrical issues. I don't have any wiring diagrams. The breakers, and relays are not marked as to what breaker and relay does what. The first problem is: when the head light switch is turned on, the headlamps come on but a breaker or relay immediately trips, and the tail lamps /clearnace lights will not light up? Seems like a short some where in the wiring. I checked the easy spots and no breaks or cut wires were found. I changed the Courtesy light to a new one also. I added new led lights in the rear and have working brake, stop, turn, and hazard signals. Just no tail or clearance lights. Also the 3 middle clearnac elights do not come on either, only the outside two? The second issue is: the dash lights don't come on or dim? ..I changed the bulbs out and still nothing. this may be related to the first issue. What is the indicator light thats is to the left of the hi beam indicator light? These are between the speedo and tach. It is not the low air, as that is on the middle dash near the trolley brake. The truck is parked outside and I need it up and going so this is one of the last stumbling blocks before we get it inspected for the road.

I am going ok to play in the cold to try to figue out the problem. Any help will be appreciated.

Thanks,

Deo

Here's a picture of the truck... slowly trying to get her on the road.

[Lmackattack]

do the headlights turn off when the braker trips? on alot of macks the 3 center roof lights turned on/off with the head lights. the clearance lamps were on a diffrent switch. if those 3 lights come on but are turning off when the braker "trips" it sounds like you have a short to those lights or to the rear tail lights. could also be the dash gauge lights. it will be a PITA to find. Start with the dash panels and see if you can wiggle the switch and make it stop, then check the rear tail lights, if you still on the chase remove the interior over head consol. the main wire loom to power the roof lights,radio, etc... runs just above the windshield. look for a louse wire touching metal or a worn spot.

Have FUN!!!!!!

[therabbittree]

I just came inside for a break from the cold and to check the forum. The head lights come on and stay on when the breaker trips. When the headlights are on, the 3 middle roof lights stay off. The outer clearance lights only come on with the clearance light switch. I just took the center console down and the overhead wires look "ok" nothing obvious. I changed the dash light bulbs and nothing. No power at the dash bulb connectors. I raised the dump body and followed the tail light harness forward and didn't see and crushed or pinched areas. I couldn't see well under the cab but the tail harness when it enters into the junction terminal strips looks factory. The 180 bend to enter the middle of the terminal strips looks factory too. I think its a dash / cab / breaker panel issue. Like you mentioned it looks to be a PITA. Does any one have a picture or diagram of what brekers and relays do what? i figured the removeable panel covering the box would have diagram or something similar, but no luck.

Thanks,

Deo

[Lmackattack]

hmmm... your glove box door (back side) does not have the diagram of relays and such.. I think that was the factory location for some of the newer R models?

. What about your tail lights do they go off when the relay trips?. just trying to isolate where to look. one other option is to go straight to the junction block on the outside fire wall. find the terminal to the roof lights and see if putting power directly to them lights them up. check the tail lights and gauge lights at the same time. if you can get them to lite up from the junction block that would suggest the short is between the relay panel and switch or from a bad wire in the dash. most of the electrical issues I have had were ground issues or a louse wire behind the dash.

[bobo]

hey does it have 2 light switches? the one on the left is a clearnce light switch(CL Lights) and the on the right is head lights(up) and markers(down) check the left switch thats what powers the outer clearnce(cab) lights. I have had the same problems with my RB's and RD's over the years. Try jumping from the left switch to the right, and check outer clearnce lights. If they power up, dont even bother trying to find the short(I NEVER HAVE!) just take the left(CL lights) switch out of circut, and wire all light to the top of the right(head lights) switch. I have ran all my trucks off the 1 switch for years with out a problem. Other spot to look at is on the fire wall abouve the motor is a black plastic relay junction box, over time the seal on them turns to shit and green death creeps in. also check on fire wall where buss bar is(left fire wall where steering shaft comes though), check for broken wires and broken or loose conection on the solenoids in the same spot, they rot from the inside out so you cant really see the corrosion in the eyeletts, use pliers and give them a shake. good luck bob0

[justincase]

I chased a headlight issue for weeks. Headlights would work then breaker would shut off cool down and lights would come back on. I was sure I had a short and finally replaced breaker and haven't had a problem since. That happened on heater and fog light relays as well on a 93 RB

[therabbittree]

Thanks for the replies so far. Every littel bit helps, Especailly when your frezzing and burnt out toubleshooting.

When the headlight switch is up nothing happens to the tail lights. But they work fine for stop, turn and hazard, and the reverse lights also. I haven't closed everything up yet as I have a license plate light and 3 little marker lights to run off the tail lamp wiring when I can figure out which one is the tail / clearance light wire.

There is no guide for the breakers on the back of the glove box door. Although, some of the relays are marked on the breaker / relay panel for tail - stop etc. But no info on the breakers. I have 1-24 on the breakers. I did find a old 1983 R model wiring diagram on here or on the net. I'm not sure how different it is.

The truck does have the separate clearance light switch and the headlight switch. I was wondering the reason for this as many trucks have then tied into the headlights switch. I will try your suggestion with eliminating the clearnace switch. I'd like to wire all the clearance lights / markers together also ie all 5 on the roof and all the others etc. I was jumping the headlight switch to test it an no I don't here the breaker or relay working argh. I think I will order some replacement breakers and relays. I will check the terminal barr connections on the firewall better. They are all connected, but they could be bad inside the crimp or eyelet I guess ..I have wondered how those connections hold up, as they are all exposed to the elements.. seems like a bad design. I wonder if the terminal bar is the source of the issues any how?

I had or have a intermittant jake issue. The wouldn't work sometimes, same with the radio & speedo some times they would work and sometimes they don't. When the jake does work its doesn't sound correct seems to not sound even, like some cylinders are dropping.

I am thinking of adding a decent junction box in the rear for the rear lights etc. do you guys have a recommendation for a source or brand? Betts? etc. something thats water proof and going to last with shock loads etc. Currently the electrical issue is the main thing holding me up. The rest is simple, mudflaps, fix the crank case vent tube (rubbed through), install, new boost and egt guages, install new radio. We're having a local shop run the overhead, check the jake solenoids, do the state inspection, fluid & filter changes, fix th ewindow regulators and door locks, door handles etc.

Thanks,

Deo

[rhasler]

I'm guessing that your truck is VMAC II. Here is a list of the fuses, applications, and ratings

1 ------25A------------Heater, Air Conditioner

2 ------15A------------Speedometer, Tachometer, Panel A, Panel C, Panel D

3 ------15A------------Back Up Lamp, PTO Switch

4 ------20A------------Windshield Wiper/Washer

5 ------15A------------Low Air, Park Brake, CAC Bypass, PDLO, Hook Up Lamp, Transfer Case

6 ------20A------------Heated Mirrors

7 ------20A------------Aux. Cab Heat/Spare, Window Lift, Ether Start, Inside/Outside Air

8 ------25A------------Sleeper Heater/Air Conditioner or Spare

9 ------10A------------Engine Brake

10------20A------------Air Dryer, Heated Air Dryer, Heated Drain Valve

11------25A------------VMAC Power, Engine Shutdown Override Switch, Speed Control Switch

12------20A------------Spare

13------20A------------Courtesy Switch, DRL Module, Headlamp Switch

14------20A------------Illumination, Tail Lamp Relay, Cab ID Lamps

15------20A------------Clearance and Trailer Side Marker Lamps

16------20A------------Flasher, Stop Lamp, Trailer Stop Lamp, Dircetional Signals

17------15A------------Ignition Switch, Hobbs Oil Pressure Sender, C.B. Post, Tachograph, Radio

18------20A------------Horn Relay, Dome Lamp, Cigar Lighter, Clock, Spare

19------20A------------Fog Lamps

20------20A------------Communication Connector, C.B Posts

21------20A------------Spare, Torque Limiter

22------20A------------C.B. Posts and Relay

23------20A------------Spare

24------20A------------Spare

[therabbittree]

Well, after being sick for almost 2 weeks i finally got to mess with the truck a little today. I replaced the Headlight switch, courtesy switch, dimming switch, all the pertainent relays, the pertainent breakers. I cut out the old spliced rear light harness. I added a junction box in the rear with the trucks "factory 4 wire rear harness" in and 7 wire out to the trailer connection.

The breaker / relay will not trip now. and the forawrd lights work correctly, so i think the problem was in the old spliced harness i cut out. But I haven't hooked the rear lights up, as I ran out of eyelets. I noticed this truck has a 4 wire rear light harness instead of 7 wire... is this normal?... The truck only has red, black, white, & brown rear light harness. The mack rd basic wiring diagram #8-50 I found online shows the rear lamp wire colors including green and yellow for the directionals, like the regular 7 wire trailer wire. Although, the wiring diagram also shows a small 4 stud terminal strip with green, red, black, & White going to the rh and lh stop,tail,dir,& backup lights with the black wire splitting to both lamps. So is the 4 wire red, black, white, & brown rear harness corrct?, if so, which will connect with the 7 wire? The diagram shows the 7 wire as: yellow - left directional, green - right directional, clearance - black, tail - brown, red - stop, ground - white. This is like the regular 7 wire sytle light hook up. If this 4 wire tail harness is not correct I will run new 7 wire cable from the front firewall terminal strip following the wire diagram. Any ideas or tips?

Thanks, & happy new year!,

Deo

[poonamt93]

I would pop the hood and inspect the battery. If you have some corrosion on your terminals then you have found your culprit. Just get an acidic soda like coca cola or root beer and pour it on the terminals as this will eat away at the corrosion and make cleaning them easier. After you get a wire brush and scrub down your terminals go to your local auto parts store and get some battery protestant its somewhere around two bucks a can and you only need one. Spray on the terminals until evenly coated then you are good to go. Also make sure all your contacts are secure. If you still have this problem then I would get the charging system inspected and if that's the original battery from 2000 then that's most likely where your problem will be at. But most likely its one of your contacts is fouled up so clean everything up and tighten everything up and you should be fine.

[therabbittree]

The batteries are not corroded. I had cleaned and used the battery protectant on them already. This truck is a 1996, not a 2000, maybe the forum posted your reply in my thread by accident? Cleaning the battery terminals wouldn't help correct the problem I had. I've yet to seea RD mack with the 4 batteries under the hood.

Thanks,

Deo

[Dieselfixer]

Well, after being sick for almost 2 weeks i finally got to mess with the truck a little today. I replaced the Headlight switch, courtesy switch, dimming switch, all the pertainent relays, the pertainent breakers. I cut out the old spliced rear light harness. I added a junction box in the rear with the trucks "factory 4 wire rear harness" in and 7 wire out to the trailer connection.

The breaker / relay will not trip now. and the forawrd lights work correctly, so i think the problem was in the old spliced harness i cut out. But I haven't hooked the rear lights up, as I ran out of eyelets. I noticed this truck has a 4 wire rear light harness instead of 7 wire... is this normal?... The truck only has red, black, white, & brown rear light harness. The mack rd basic wiring diagram #8-50 I found online shows the rear lamp wire colors including green and yellow for the directionals, like the regular 7 wire trailer wire. Although, the wiring diagram also shows a small 4 stud terminal strip with green, red, black, & White going to the rh and lh stop,tail,dir,& backup lights with the black wire splitting to both lamps. So is the 4 wire red, black, white, & brown rear harness corrct?, if so, which will connect with the 7 wire? The diagram shows the 7 wire as: yellow - left directional, green - right directional, clearance - black, tail - brown, red - stop, ground - white. This is like the regular 7 wire sytle light hook up. If this 4 wire tail harness is not correct I will run new 7 wire cable from the front firewall terminal strip following the wire diagram. Any ideas or tips?

Thanks, & happy new year!,

Deo

Hi

The lights for the rear of a dump truck only use 4 wires as the turn & stop lights work off of the same wire, it works sort of like a car, turn signal on cancells the brake light for that side. Check each wire 1 at a time with only 1 thing turned on( Left turn,right turn etc.) and it is pretty easy to figure out. As far as your question about the light on the dash next to the hi beam light it is for the parking brake, comes on when the parking brake is on.

[therabbittree]

yep I got it going today. Cutting the old mess of wires out solved my problem. The junction box worked out great. I appreciate the help. Now I want to wire all the roof lights together instead of the middle 3 and outer two being separated. Not sure why mack does it like that. I also noticed the rear clearance lights don't blink with the courtesy switch. I want to get that straightened out too.

Thanks,

Deo