Need some electrical know-how for diagnostics

I have an 88 MJ, 4.0, BA/10, yada, yada, yada.:repair: Anywho, my horn does not work and my reverse lights do not come on when I throw it in reverse. I need to get these things ironed out this week so that I can pass PA inspection. I have a good digital multimeter (not auto-ranging), but I don't know how to use it. What setting do I use? Any other tools I need? I have figured out the continuity test, but that's it. How can I test to see if I have power to my horn and my reverse light switch on the tranny? It's not the fuses, I've replaced all of those, unless it's a fuse in the engine compartment, b/c I haven't checked any of those. Somebody please help me! I've tried searching, but there is too much BS to wade thru, I don't want to read about NSS because I haven't got one. I need help with diagnosing these problems. Please point me in the right direction....

A test light would be better for you.

joshv98xj said:

I have an 88 MJ, 4.0, BA/10, yada, yada, yada.:repair: Anywho, my horn does not work and my reverse lights do not come on when I throw it in reverse. I need to get these things ironed out this week so that I can pass PA inspection. I have a good digital multimeter (not auto-ranging), but I don't know how to use it. What setting do I use? Any other tools I need? I have figured out the continuity test, but that's it. How can I test to see if I have power to my horn and my reverse light switch on the tranny? It's not the fuses, I've replaced all of those, unless it's a fuse in the engine compartment, b/c I haven't checked any of those. Somebody please help me! I've tried searching, but there is too much BS to wade thru, I don't want to read about NSS because I haven't got one. I need help with diagnosing these problems. Please point me in the right direction....

Click to expand...

Reverse lights on the BA-10/5 are driven by a switch that is, as I recall, on the driver's side of the gearcase (I could be wrong tho - check both sides.) It's a hex-shaped "plug" with an electrical connection - pull the connection straight off, and you'll see two posts. Replace the switch - it's fairly easy, and you should not lose any gear oil doing it.

As far as the horn goes, there are a few things to check. First, make sure you're actually getting power to the horn when you press the button (disconnect the horn, hook up your voltmeter - 20VDC - and have a helper press and hold the button. You should see battery voltage at the horn - but you may need to have the key ON. The engine need not be running.) If you don't get power to the horn, there is a horn relay, but I'd have to look it up. You've probably lost that. It's probably a DIN relay, less than $10 pretty much anywhere.

The next thing to check would the the switch in the steering wheel - I'd have to look that one up as well.

If you are getting power to the horn, then just the horn wants replacing - and you probably already know where that is (if not, it's behind the front bumper on one side - I don't recall which.)

Thanks 5-90, that gives me a good place to start. I did order a new switch for the reverse lights, it should be in tomorrow. I pulled the existing one and it reads that the circuit is complete when I depress the ball - is this an indication that the switch is okay? or maybe not?
This may be really elementary, but which probe am I putting where while I test the horn itself? I am guessing that the red goes to the wire and the black gets grounded to the bumper or something near?

1) Sounds like you may have a good switch - check to make sure you're getting good contact with the posts, then. Maybe take a pick and "close up" the sleeves in the electrical connector a bit?

2) Yes, the horn is self-grounding. Connect the black lead to any convenient ground, and the red lead to the wire. NOTE - with a digital multimeter, if you get a "negative" reading when you're expecting a "positive" reading - you've got the leads backwards. Most automotive readings will be "positive" voltage - you can discount the polarity of the reading, unless you really need to know for some reason (this is one case where having a DMM can be useful over an analogue...)

Alright, so I checked power to the horn. Nothing. I replaced the horn relay and re-checked it. Still nothing. I pulled my steering wheel and cleaned all of the components to the horn set up, but I did not see a wire coming in anywhere. Should there be? I assume there should be a brown wire, as the wire running to the horn inside the bumper is brown. Does anyone have a pic of what all of this is supposed to look like at the steering column?

I'll have to pillage around for my 88 FSM - but I've got to finish taxes, first...

I know it's possible to get wiring faults in the steering column - I've got to tear into the column on one of my 89's to find out why I had to disconnect the horn. I'd give you wiring colours, but I don't know them offhand. However, I may have wiring diagrams up at my RENIXPower group (groups.yahoo.com/group/RENIXPower) - you can also use a 1987 wiring diagram, but NOT a 1989 (ChryCo changed some wiring colours in RENIX when they took over...)

Good luck!

Cool. I just found the diagrams on an E-FSM that I had on my computer. That'll be a start anyway. On the reverse light switch, I checked power to the switch and it seems to read okay, or at least I do have some voltage there. When I turned the key to the "on" position, engine off, I got a reading of about 3.5-4 with my DMM set at 20VDC. Is that enough current, or should it be reading 12V?

First off - "current" and "voltage" are two entirely different things...

Anyhow, I'd have to check the wiring diagrams myself, but you should have +12VDC to one side of the switch, but not the other. When the switch is closed, it allows +12VDC from whatever source to travel through the rest of the circuit (down the wiring, through the reverse lamp bulbs, and through to earth.) I'd be interested in knowing why you've got the readings you did - how did you check them? Try measuring each terminal relative to ground (the transmission case should serve for a convenient earth, or you can use a crossmember screw. Try to get clean metal, if you can...) and see what you get - you should get something like what I told you. I don't recall any relays being in the reverse lamp circuit (unless you added one while I wasn't looking,) so that will give a better picture of what's going on in there.

While you're under there, check the switch itself - set your DMM to its "continuity" fuction, set it where you can see/hear the thing, and shift into reverse. The display should "zero out" and you should get a beep, if you have an audible continuity test. If you don't have the "continuity" test function, just set it to a low resistance range and make sure you can see the thing when your rig is shifted into reverse.

If the switch doesn't react, pull it out and press the button with your finger to re-check. That will tell you if it's the switch itself that's going out, or if whatever is supposed to trip it inside the gearcase is wearing down (unlikely, but possible.)

The main rear lighting harness runs down the driver's side of the XJ, you can find it if you pull up the doorsill trim panels (a stubby #2 Phillips should serve,) and the connectors are behind the driver's footwell kick panel and behind the driver's side rear inner trim liner (which should also come out with a #2 Phillips.) It's possible that a connection wants cleaning.

That's all I can think of for the moment - I just finished this year's taxes, and I'm spent...

Alright, I've got some updates: (Oh yeah, this is in an MJ, so the rear harness, etc, is a bit easier to get to)

Horn: I checked the horn itself, inside the bumper, with a wire direct from the battery and I nearly jumped out of my shorts even though I thought I was ready. It works. I replaced the horn relay, and I have power to the relay from the ignition, so my problem is somewhere from the steering column up to the horn switch. Can someone tell me where the horn wire is supposed to be? I don't see mine and that worries me.

Back up lights: The switch checks out okay, continuity-wise; though I had a tough time checking voltage on it, most of the stuff around it is rusty, there is power there. I checked power at the rear harness, but I've got nothing. What is in between? Anything? What else could be wrong?