New 928S with Electrical Issues

[CVJR]

Today I purchased a 1986.5 928S! Yeah! 31,000 miles/ auto.

Problems I need to sort:

1. Rear wiper blows fuse as soon as switch is pushed--how to diagnose
2. Horn stays on constantly--until it blows fuse in just seconds that is.
3. Original Blaupunkt radio turns on, but no sound. Swapped in a similar Blau with same result. Also tried external antenna, no change.

Thanks for any suggestions. I'm not super with electrical issues, but have a voltmeter and basic understanding.

Thanks!

[AirtekHVAC]

nice looking car! Not sure about your problems tho....perhaps factory amp for radio is not working?

[CVJR]

Thanks Ron. Where is the Amp located?

[DougM]

Amp should be under the cover on the door side of the passenger seat.

For the rear wiper, first lift the hatch and then try the wiper. See if it moves back and forth without the wiper attached which takes some extra load off the motor. Just work your way back checking connections and possible loose grounds.

Grab the horn pad with both hands and pull it straight off. It takes a decent tug. Then have a look at the contacts. Something is probably a little bent or just sticking.

Even though the car is low miles, it would be a good idea to go through all the grounds and clean them up.

[harborman]

Clean looking car and low miles. I bought mine with 37K miles. Yep, you will need to go through a lot of things electrical and also fuel system lines, Timing Belt(a must do) if not done since new and every 5 years or 50K. Even though a nice clean car you can expect to put some good money into it.

I did away with the original radio and updated with a nice JVC which has all the new features. I also recently put in a rear tire sub woofer and amp which made a 100% improvement in the sound. Check my other posts here on the radio.

Horns: Mine did not work so I installed newer ones from Autozone. Horns are in right front fender below head light. You have to remove inner fender to get to them.

You have a nice car so take care of it!

[JK McDonald]

What a great score. It looks like it's been very well cared for. The electrical issues you mentioned will probably be minor problems to chase down. Like Harborman recommended though - get the timing belt changed before being tempted into putting any miles on her..... Check over the forum topics discussing the damage that could be done to your valves if the aged timing belt breaks.

Again Nice Car, Michael

[Danglerb]

Very nice looking 928, welcome to the forum.

First thing I would do is find a good mechanic with a LOT of experience with the 928 and have a thorough inspection done. Despite low mileage its still 25 years old and may have a long list of safety related items that need attention as well as deferred PM.

Owning a 928 means that either you or your mechanic will be fairly expert in electrical issues soon. The insulation used on wiring in that era stinks, it wears through where it rubs on metal, and everything under the hood is cooked and brittle "eventually" so that moving wires around may break off bits of insulation and cause shorts to ground.

The basic tool of electrical work is a meter and probes, but more advanced tools may be very handy to have. This is my list as hobbyist and amateur penny pincher;

Several DMM, digital multi meters, with the usual current, voltage, ohm, and some features.
Probe DMM, a long skinny DMM with the + red probe built into the end of the meter.
AC/DC current clamp DMM, allows current measurement in a wire by putting the clamp around it, but not damaging the wire. Most as AC only, with DC a LOT more expensive, but I do ok with a Sears Craftsman 82369 thats only about $60.
Power Probe 3, "like" the probe DMM, but with a flip of a switch the tip can have fused power or ground for testing.
Load Pro, a special meter probe with a button that changes a voltage reading from very low current to about half an amp to test for weak connections in the circuit.
Short detector, clips in at the fuse and puts a RF signal on the wire that allows tracing with a small hand held meter which will show a sudden drop in signal at the short to ground.
Fuse buddy, plugs in place of a fuse to measure circuit current.
Back probes, needle sized probes made to poke into the back of connectors.
Relay jumpers, wire with connectors matching the ones on the bottom of relays so the relay can be bypassed for testing.
Big battery charger, medium sized, and a handful of trickle or maintainer types. Almost all 928 have some kind of minor drain, a current that flows even when the key is off for various reasons, but the bottom line is that they need either the battery disconnected or a charger connected.
DeOxit contact cleaner
Pink erasers for scrubbing contacts.
OxGuard electricians grease.
Spare relays.

[Danglerb]

Almost forgot.

A good set of wiring diagrams printed in large format, like 18x36 for your model year.
Fuse and Relay chart for your model year.
Fuse and relay pliers or pullers.
VISA card, sometimes you just have to back away and let a pro, or a kid with a LOT more flexibility and tolerance for stress positions, to do the work.
Various tools and things to clean the ground points on the car, think metal scrubbing things.

[CVJR]

Thanks for all the comments. I totally hear 'ya on the timing belt and finding a well qualified 928 mechanic. I have adequate $$ to get that and whatever else I can't handle done.

Today I replaced the horns, the high note was fine, but the low note was dim sound and quickly overheated. I replaced both, now the horns is on constantly but no longer blows the fuse. I did swap the relay for another of the same value to no avail. I also pulled the horn pad off and the black wire connection and studs on the wheel looked good. Not sure what to try next...

More suggestions please....

[Spin-Master]

Nice score!. I would add to the list, get a set of workshop manuals and make sure the fuel lines are ok. Along with the timing belt you just as well do the waterpump also while you are in there. Welcome to the list.

[Rupesy]

I am assuming that either pulling the fuse or pulling the relay both stop the horn from working. Is that correct?

The horn is activated by completing the ground for the relay when you push on the horn. When you pulled the horn pad off and disconnected the black wire, does the horn stop working or does it still continue? If it stopped, then you have a problem with the horn button. If it continues, then you are grounded somewhere between the horn button and the relay. To confirm if grounded, pull the relay and you could start by checking for continuity to ground for that wire on the steering column that hooks to the horn button. If you find continuity to ground, then you have some chasing to do and you really need a diagram for that.

Let us know what you find and post back more details.

Try to hunt down a wiring diagram for your year.

[19psi]

Beautiful car!

Grab hold of the steering wheel and see if you have play in it. NOT rotation but up/down/left/right.
If you do, then a Delrin ring broke apart letting all the little ball bearings fall out. This will cause the steering column to drop down and put pressure on the horn ring which may cause your problem. It can also break off the tab on the horn ring which auto cancels your turn signals.

If that happened, a new bearing, collar and horn ring are dirt cheap and very easy to replace.

[Eplebnista]

Congrats on your purchase.

There are a lot of 928 owners in the Baltimore/DC area who can lend a hand etc. Some are here, some are on rennlist. I live close to BWI.

It is a good idea to join the 928 owner's club as well.

There is a set of pristine shop manuals on Ebay right now being sold by a 928 owner in Gambrills. He is not giving them away, but I know this guy and if he says they are clean, then they are.

I have had to sort a few electrical gremlins in my 86.5 as well. All the above advice is good. Remember, search is your friend here and at rennlist.

Enjoy your car. She is a beauty!

[MPDano]

You've got a short for sure somewhere. Finding it will be the challenge. I had the same type of issue, I would turn the wipers on and the fuse would blow. It didn't happen until after I got my car back from the AC Shop. I traced where he would be working and sure enough, he mashed (shorted) some wires near the Expansion Valve he replaced.

The electrics are not real special that a good auto electrician couldn't figure out for you. You'll see from this forum, we like to fix these issues ourselves. These cars can get real expensive if you don't wrench on them yourself.

[stepson]

For the rear wiper issue, disconnect the wires at the wiper motor. Replace the fuse and turn on the switch. If the fuse still blows, you will have at least narrowed it down to the wiring. If the fuse doesn't blow, you will know that it is your rear wiper motor.

For the horn, pull the horn relay. The relay should be a "53" relay. Replace the relay with new. You said you replaced the horns. Check that the wires to the horns are not bare and/or touching somewhere they shouldn't. Disconnect the horn wires at the horns and put a new fuse in. Does the fuse blow? If yes, then check the horn wires where they plug into the harness. Are they plugged into the right connections? It sounds as if the low horn wires are your problem. Disconnect that horn only and see if it honks constantly and blows the fuse.

For the radio, well, if you figure it out, please post here, cause I got the same thing going on with mine and I can't figure that one out.

Here's a diagram of your relays and fuses with the amps for the fuses and the correct numbers for the relays:

https://www.928gt.com/t-8586fuse.aspx#top

[SanDiego928]

Your car is awesome and don't ever sell.......

[CVJR]

Rupsey, thanks for the tips. Removed horn pad, horns still blow. Replaced rely, no change, horns still sound. Since replacing horns, fuse does not blow, so feeling like some progress is being made.

19PSI, wheel is tight in all directions. I do hear lots of clicking when turning the wheel lock to lock, more than I can recall in 911 or 944 I've owned.

Stepson, thanks for the wiring diagram. Wires to horn seem completely sound, no chaffing outside the fenderwell at least.

What's next, pull the steering wheel and check horn contact at tuns signal/ wiper unit?

Question, see pic, an early cell phone was installed, can I remove the leads that looks like a wire was added--there are two in the photo, a brown one and a red one.

[19psi]

The wheel should turn smoothly and quietly with no clicking.

Remove the nut and pull the wheel to see what's binding up in there. Loosen in a downward motion towards the seat. If you try to break the nut free going up you stand a good chance of taking out the windshield.

[Rupesy]

I didn't mean to tell you to replace the relay. I believe your problem is somewhere between the wheel and the relay outlet. To confirm a short between the horn pad and relay, you will need to have the horn pad off and the relay completely out. This would technically isolate the black wire and there would be no potential for grounding unless somewhere the wire in that circuit has shorted out. If you touch you multimeter to the black wire and ground and get continuity, you have confirmed a short. You will not get continuity if there is no short. You should do this first as this is a simple test and may save you a lot of time. If you have a confirmed short, then pulling the steering wheel off would be your next step. You definitely need a multimeter when working on wiring issues. You get get a good one at Autozone for a reasonable price. Worse thing you can do is start replacing things blindly. A multimeter can check a lot of electrical parts.

Does the horn stop when you pull the relay? If it does, there is no doubt in my mind you have a short on the wire in the column or somewhere on its way back to the fuse panel. That relay is normally open and only bridges the power to the horn when a ground has been made. I attached the circuit diagram for a 79 however I doubt its much different. A very simple system. Let us know what you find.