Can not find a short -- halp!!

[moga]

The Mounty began blowing fuses on Monday. I've had no hazards or turn signals since. To say that it's getting old is an understatement.

On the way home from work Monday, I replaced relay, then fuse Tuesday morning on the way into the office when the first part swap didn't make any impact. The fuse is burned through the moment it slides into position.

Today was the first op I had to poke around under the hood. Disconnected both terminals from battery and pulled out both front and back lens covers to take a resistance reading at each connector for the directional flashers. 2.5 ohms on right side front and rear lamps, about 18 for the both on left side. Ground lead of the multimeter for each of the previous readings is attached to chassis at the ground point for the chassis to frame groundstrap of the drivers side using alligator clamp. I inserted the red probe into the appropriate slot on each connector.

I thought "ground short somewhere between the combo switch and left signals" based on those numbers, so I took a reading at fuse #21, the one that keeps blowing, to get a baseline. This is the fuse that protects the hazard circuit, and is not shared with other components. One probe in each contact point of the fuse box. 10k ohms when flasher is in the up (open) position, 550k when it is pushed in (closed). Now I'm thinking it's the hazard switch that has a short. It's 12 years old -- its plausible.

I spend all afternoon ripping out the dash to get to the steering wheel before separating the wiring harness from the combination switch. Then I take a reading at the internal fusebox location that controls hazard flashers, expecting infinite resistance after the switch is removed from the circuit. Wrong. Resistance is still > 500k at fuse 21. But resistance is infinite at the light connectors of the four corners after removal of the combo switch.

This is my first foray into troubleshooting an automotive electrical problem. Thank goodness it is a basic issue, yet the resolution is above my paygrade here. I've got the appropriate model year Ford Repair Manual inbound to walk me through the troubleshooting next weekend but until it arrives, can anyone make any sense of any of this? I guess I expected the short to be after the switch but before the lamp connectors. Yet how do I interpret such a high resistance on the hazard circuit when the switch is removed? Does that mean that the problem is between the internal compartment fusebox and the harness that connects to the multi-combination switch? Heavens I hope not.

I can try to upload the chassis electrical schematic for a 1998 AWD tomorrow if it would at all help. Thanks for any and all suggestions.

 

[budwich]

Well if I understand you correctly, your "method of testing" resistance might be a little "suspect"... Your "intent" is good... ie. checking at various locations, starting at the "furtherest" point and working inward, etc BUT how you setup your measurement and "direction that you are looking" may not be quite so "good". Remembering that resistance measurements is a measure of anything and everything that is placed between the two leads is important. Measuring across the two contact points of a bulb will tell the resistance of the bulb ... good will be some value, very high will be bad / broken bulb, and low / short (unusual) will likely be an internal problem in the bulb stem.... but of course, you know that. Further measuring from a contact on the connector (assuming it has been removed from the sockets / bulb housing and / or the bulb is removed making an open circuit in that "direction") to a KNOWN ground, results in a measurement of "what's in the circuit" to ground is the right idea / method (ie. to known ground). In this case, not so good though BECAUSE it is likely that now you are looking back towards your battery (likely) depending on the state of the circuit (ie. key on, key off, switch on, etc). Further measuring the resistance of a battery isn't good and some meters don't appreciate this... You want to keep "looking" away from any battery sources not towards them. However, in this case, you likely "lucked out" and the switch (light) was off so you got a measurement of "something" (not sure what but maybe at least an indication of which leg in the circuit is bad... maybe) when you did this. Moving to the fuse position was good, but hopefully, now you see that placing the meter across where the fuse was (but removed) isn't going to tell you much as one end of the fuse contact goes to the battery (in some form) and the other goes towards the light system. What you want to do, is do your "good method"... of measuring resistance (from a given point) to a KNOWN ground. Of course measuring on the contact going towards the battery is a waste of time... so that leaves you with only one contact (your other contact) which is going towards your lights... great.... not quite so fast... but you are almost there. Remember there is a light switch in the circuit so, IF your light switch is "OFF", its likely you won't measurement anything (ie. high resistance -> open circuit).... makes sense.... normal.... IF not, then you know you have a problem between the fuse and the light switch. IF you switch your lights "ON", and the resistance goes to some number (x ohms) and IF you take that number and divided into 12 and the result is greater than your fuse, you have "confirmed" that the circuit an issue (likely since you have observed the physical result ... blown fuse). Further "isolation" and readings (from the same point) are then done by removing parts of the downstream circuits by unhooking splices or connectors along the circuit path. As you surmized, you need a schematic to make a wise choice on what parts of the known circuit you need to remove. Hope this is understandable... sorry don't mean to be "lecturing" but electrical measurement seems to have "mystery" about it sometimes and people don't always get around the "mystery".... hopefully now your paygrade can be raised...

 

[2000StreetRod]

Fuse 10?

I don't have the 1998 wiring diagrams but they shouldn't be much different from my 2000. You stated that your exterior lamps don't flash for either hazard or turn and that fuse 21 in the central junction box blows. The turn/hazard flasher unit receives voltage from two sources. Fuse 21 (15 amp) that is hot at all times (for the hazard function) and fuse 10 (7.5 amp) that is hot in run. Is fuse 10 also blown?

Lamps rarely fail in a shorted condition but swiches and relays do. A quick isolation test is to disconnect the turn/hazard flasher from the turn/hazard switch. They are connected by two bundled wires (light blue and white/red). Disconnect the bundle at either end (the easiest). If fuse 21 still blows then the short is in the turn/hazard flasher which is what I suspect. If it doesn't blow then the short is in the turn/hazard switch or beyond.

 

[Bill Kemp]

Take a look under your rear bumper on drivers side and see if you have a trailer wiring harness and then if so check that out or remove it & see if it helps.
Have heard of the harness being fubaed before.

 

[moga]

Sorry don't mean to be "lecturing" but electrical measurement seems to have "mystery" about it sometimes and people don't always get around the "mystery".... hopefully now your paygrade can be raised...

Not at all Sir. I freely admit that I have no experience in attempting to troubleshoot electrical problems and only a cursory understanding of electronic circuitry. I need some one to analyze and diagnose what I've done so that I can improve my methodology and hopefully find the problem. I appreciate your taking the time to point out the areas in which I've made mistakes.

Well if I understand you correctly, your "method of testing" resistance might be a little "suspect"... Your "intent" is good... ie. checking at various locations, starting at the "furtherest" point and working inward, etc BUT how you setup your measurement and "direction that you are looking" may not be quite so "good"......In this case, not so good though BECAUSE it is likely that now you are looking back towards your battery (likely) depending on the state of the circuit (ie. key on, key off, switch on, etc). Further measuring the resistance of a battery isn't good and some meters don't appreciate this...
You want to keep "looking" away from any battery sources not towards them.

I want to make sure I understand what you've said and am not trying to be obtuse here.

I believe that there is a ground short somewhere on the path of that left rear turn signal because of the fuses being blown and the high resistance reading on that side of the car. My thinking was that the known ground to the chassis is the “return path”, and the probe on the positive contact of the connector with its bulb removed is the “beginning” of the circuit. When the circuit is operating normally, this would have resulted in an open line. But because there is a short, I didn't expect the measurement to make it back to the battery, but even if it had, there wasn't a fuse in 21, so the circuit was opened at the internal fuse box. In addition, the battery leads were unfastened from the battery posts in the engine compartment. Was I really measuring the resistance of my battery? Thank goodness I still have a multimeter if that's true.

However, in this case, you likely "lucked out" and the switch (light) was off so you got a measurement of "something" (not sure what but maybe at least an indication of which leg in the circuit is bad... maybe) when you did this. Moving to the fuse position was good, but hopefully, now you see that placing the meter across where the fuse was (but removed) isn't going to tell you much as one end of the fuse contact goes to the battery (in some form) and the other goes towards the light system.

OK. I follow.

What you want to do, is do your "good method"... of measuring resistance (from a given point) to a KNOWN ground. Of course measuring on the contact going towards the battery is a waste of time... so that leaves you with only one contact (your other contact) which is going towards your lights... great.... not quite so fast... but you are almost there. Remember there is a light switch in the circuit so, IF your light switch is "OFF", its likely you won't measurement anything (ie. high resistance -> open circuit).... makes sense.... normal.... IF not, then you know you have a problem between the fuse and the light switch. IF you switch your lights "ON", and the resistance goes to some number (x ohms) and IF you take that number and divided into 12 and the result is greater than your fuse, you have "confirmed" that the circuit an issue (likely since you have observed the physical result ... blown fuse).

When I measured at the fusebox on 21 and compared the resistance when the hazard plunger was up (open) against down (closed), the resistance jumped by a factor of 50. Isn't this the opposite of what should have happened?

 

[moga]

I don't have the 1998 wiring diagrams but they shouldn't be much different from my 2000. You stated that your exterior lamps don't flash for either hazard or turn and that fuse 21 in the central junction box blows. The turn/hazard flasher unit receives voltage from two sources. Fuse 21 (15 amp) that is hot at all times (for the hazard function) and fuse 10 (7.5 amp) that is hot in run. Is fuse 10 also blown?

On the 1998, the equivalent of 10 on your 2000 is 22. It is not blown. Is that important?

Lamps rarely fail in a shorted condition but swiches and relays do. A quick isolation test is to disconnect the turn/hazard flasher from the turn/hazard switch. They are connected by two bundled wires (light blue and white/red). Disconnect the bundle at either end (the easiest). If fuse 21 still blows then the short is in the turn/hazard flasher which is what I suspect. If it doesn't blow then the short is in the turn/hazard switch or beyond.

The assembly on the steering column that houses the flasher plunger and the turn signal arm has two harnesses of wires that connect to it. I recall that one of the connectors has the light blue and while/red connected to it. So those attach the hazard to the switch? So if I pull out those terminators from the back of the connector while it is still attached to the switch assembly, and the fuse still blows, the the short is in the flasher? I will try that next time I open up the vehicle. Thanks for the suggestion.

Take a look under your rear bumper on drivers side and see if you have a trailer wiring harness and then if so check that out or remove it & see if it helps.

Have heard of the harness being fubaed before.

Thanks Bill. I know that there is one on the towing apparatus under the bumper but I don't know if it is connected. I will look into that too.

Thanks for all the suggestions gentlemen and keep them coming. I will report back on Friday with any progress or new observations when I next have a block of time to try and resolve this problem.

 

[2000StreetRod]

Less wires at the flasher

On the 1998, the equivalent of 10 on your 2000 is 22. It is not blown. Is that important?

The assembly on the steering column that houses the flasher plunger and the turn signal arm has two harnesses of wires that connect to it. I recall that one of the connectors has the light blue and while/red connected to it. So those attach the hazard to the switch? So if I pull out those terminators from the back of the connector while it is still attached to the switch assembly, and the fuse still blows, the the short is in the flasher? I will try that next time I open up the vehicle. Thanks for the suggestion.
. . .

Look at the flasher unit connections. It should be easier to disconnect the wires there than at the steering column. Also, if it is the flasher unit that's bad like I suspect, you'll have to go there anyway.

 

[moga]

Look at the flasher unit connections. It should be easier to disconnect the wires there than at the steering column. Also, if it is the flasher unit that's bad like I suspect, you'll have to go there anyway.

Copy that.

 

[moga]

OK -- finally I've acquired the two volumes of the Ford Shop Manual plus an accompanying EVTM booklet. Still, I have come to a stand still in the diagnosis of the problem. The flow chart calls for a resistance reading on a particular relay contact, but I can not find a diagram for the relay module box ANYWHERE. Not in the Shop manuals, not in the owner's manual, and not in any of the aftermarket box store manuals that I now own.

If any one owns a Ford shop manual for any year 2nd gen vehicle, please chime in if your year manual has a diagram for the relay module. I know where the enclosure is in the vehicle and all. I just don't know which relay corresponds to what function. Thanks.

 

[budwich]

can you back up a bit... now that you got schematics and had some "fun" taking things apart to get at wiring and stuff.

IF you can, can you disconnect the cable running from the flasher to the MFS ... probably at the MFS, since the connector at the flasher has all the wiring including powering in the same connector. When you do this, you should have a very basic circuit.... your battery supply, going to a fuse (actually two for the flasher), and the flasher.... nothing else is in the circuit (not quite true cause your ignition switch is on one side of the circuit also)... but anyways, you get the idea. This is the "tough" way to test but it only costs a fuse... put you fuses back for both side of the flasher, connect your battery cable. Do any fuses blow... hopefully not. Then turn your ignition to on, do any fuses blow? IF nothing is blowing, your problem is from your MFS to the tail lights/ front lights. IF the fuse goes your problem is from the open cable (that you have dangling, right) towards the flasher / hazard module.

 

[KPT]

I can not find a diagram for the relay module box ANYWHERE.

The black box under the hood with all the maxi fuses and relays? Isn't the description of everything on the inside of the cover?

 

[2000StreetRod]

Auxiliary relay box

If you're asking about the Auxiliary Relay Box I have a drawing in my 2000 wiring diagrams on the last pages of the power distribution section. There is a trailer tow park lamp relay in auxiliary relay box #2 but it is not involved with the turn/stop/hazard lamps.

 

[moga]

The black box under the hood with all the maxi fuses and relays? Isn't the description of everything on the inside of the cover?

No, not that one. This box of seven relays resides under the dash board, behind the LH side of the console that houses the temperature controls for the heating/cooling unit and the radio control head. Eventually I was able to locate a diagram for it by thumbing through each page of the EVTM manual one at a time.

can you back up a bit... now that you got schematics and had some "fun" taking things apart to get at wiring and stuff.

IF you can, can you disconnect the cable running from the flasher to the MFS ... probably at the MFS, since the connector at the flasher has all the wiring including powering in the same connector. When you do this, you should have a very basic circuit.... your battery supply, going to a fuse (actually two for the flasher), and the flasher.... nothing else is in the circuit (not quite true cause your ignition switch is on one side of the circuit also)... but anyways, you get the idea. This is the "tough" way to test but it only costs a fuse... put you fuses back for both side of the flasher, connect your battery cable. Do any fuses blow... hopefully not. Then turn your ignition to on, do any fuses blow? IF nothing is blowing, your problem is from your MFS to the tail lights/ front lights. IF the fuse goes your problem is from the open cable (that you have dangling, right) towards the flasher / hazard module.

I began to do this today budwich, but didn't get as far as removing the steering wheel to check the MFS. Thank you for the suggestion btw. I removed the hazard/turn flasher to isolate a small segment of the circuit as you suggested. The fuse 21 blew as soon as I slid it into position. At that time, the ignition was in the 0 position (off). Fuse 10, which is only hot when the key is turned, isn't and has not blown at all. Doesn't that pretty much mean that at least a part of my problem is a ground short in the physical wiring between the load side at fuse #21 and the appropriate contact at the turn/hazard flasher (circuit 383)?

BTW, here's a diagram of the circuit in question taken from the shop manual. I cropped it just to make it easier to read.

I also want to apologize for how infrequently this thread has been updated. The weather in the south has been unbearably cold. It's colder in Atlanta now than in Anchorage, and has been this way for about two weeks or so. I haven't had many chances to get out as a result, as my garage is unheated and unfinished. Yet I'm definitely not orphaning this thread and fully appreciate all the suggestions.

 

[moga]

If you're asking about the Auxiliary Relay Box I have a drawing in my 2000 wiring diagrams on the last pages of the power distribution section. There is a trailer tow park lamp relay in auxiliary relay box #2 but it is not involved with the turn/stop/hazard lamps.

2000SR, that's precisely where I eventually found it. I don't think the email notification of updates to this thread is reliable. Good heavens, I wish that I knew that the answer was here for the last several days. Good man for looking into it.

 

[budwich]

moga... I am not sure what you said about your "latest test".... but if you didn't disconnect the cable at the MFS switch but just plugged your flasher back in.... you haven't done much then in terms of isolation. yes I understand you didn't get to the MFS for weather reason... I'll try not to send any more cold stuff your way... brrr its dam cold up here too.... Anyways, until you are able to disconnect the MFS from the "circuit", you really can't isolate your problem further... Having said that, you could also cut the wires on pin 1 and 3 near the flasher area... but you need to do it so you can re-connect them easily later. Hopefully, it will warm up soon.... maybe february... Please correct me if I misunderstood how you did your latest test.

 

[Gus Gorilla]

You don't always have to cut wires to isolate. If a connector has to stay together for testing because of other circuits then use something like a pocket knife, paper clip, scribe, or safety pin to release the pin inside the connector and just back it out. They can be tricky, but it's the best way.

 

[moga]

moga... I am not sure what you said about your "latest test".... but if you didn't disconnect the cable at the MFS switch but just plugged your flasher back in.... you haven't done much then in terms of isolation. yes I understand you didn't get to the MFS for weather reason... I'll try not to send any more cold stuff your way... brrr its dam cold up here too.... Anyways, until you are able to disconnect the MFS from the "circuit", you really can't isolate your problem further... Having said that, you could also cut the wires on pin 1 and 3 near the flasher area... but you need to do it so you can re-connect them easily later. Hopefully, it will warm up soon.... maybe february... Please correct me if I misunderstood how you did your latest test.

Battery Hot > Fuse 21 > Turn Hazard Relay > MFS > Instrument Cluster > Exterior Lights > Ground

is the layout of the circuit. If I pull out the relay module, then the incoming circuit to the relay box is isolated from the outgoing circuit to the MFS, no? The relay has to be in line (installed) for the circuit to be closed. My thinking is if there is still a ground short, then it must be in the short segment between the outgoing circuit at fuse 21 and the inbound terminal of the turn/hazard relay, or wire 383 of the diagram, which would have momentary continuity with a new fuse. Am I way off base here?

 

[budwich]

Are you saying that you have removed your flasher relay... put the fuse in and it blows.... with the flasher removed.... then you right... your problem is from the fuse contact to the flasher contact and the wire in between..... BUT you can't say that if your flasher is left in place (connected in the circuit) and the cable is connected to the MFS switch. Hope we understand each other. Good hunting.

 

[moga]

Are you saying that you have removed your flasher relay... put the fuse in and it blows.... with the flasher removed.... then you right...

Yes, this is how it happened. Now, the big question. What do I do about it? Does anyone have ideas about how to tackle the task of replacing that length of wire? I know the color of the suspect wire and everything. I just don’t know how to get the fuse box and relay modules out of the dashboard to make a new run between them. If I could get them out then I’d simply snip the existing wire and solder a new one in its place. Does that amount to re-inventing the wheel?

 

[budwich]

look for problem in and around the steering column... tracing the wire as best you can. Your alternative of running a new run is OK.... but I would think you should be able to find the short... even though I know its ugly working under the dash in that area... good luck.