My 89 928S4 Euro has the following issues:

1. The car starts and runs great temporarily from a cold start
2. The voltmeter registers approx. 13+ and holds steady even after loading up with AC, lights, stereo, etc., it barely budges!
3. After about 15 minutes as it approaches operating temperature near 85 cel. the fan kicks on and the voltmeter hits 14v.
4. As I slow down and come to a stop the voltmeter drops to 12v and when I switch on the AC it drops to almost 10v before bouncing back.
5. The engine will then almost stall while idling in traffic with the voltmeter staying around 12v.

The above sequence is very predictable and happens every time I take the car for a drive.

It seems that the problems occur upon reaching operating temperature? At first I thought the Alternator was causing the problems but it seems to run fine when the engine is cold. The voltemeter goes wonky only when the engine is hot?

I'm suspecting there's an igntion system problem in combination with the charging system?

I'm not an expert and have only had the car a few months. I bought a WSM and went through some tests and upon reading some of the postings here I've also checked and cleaned as many ground connections and sensors as I could find.

I'm taking it into the shop next week as they have the diagnostic tools but if anyone has any hunches it would be helpful!

 

Hi Brent!
I'm guessing you have an automatic?
Glenn

 

Sounds like it could be the dreaded TBF.
I did have similar symptoms on my car, ie. ran good from cold and almost died when hot. This was ever since I stripped down the intake to fix a rough and high idle. It turned out that the vacuum pipe supplying the fuel pressure regulator was completely plugged. After replacing the pipe, all is well!

Hope its something as simple for you.

Ben

 

First of all, don't trust the voltmeter in the dashboard- it is good for relative information only. Test your system with a multitester at the battery to find out where you really are.

It occurs to me that you have either a huge drain when the fan comes on [short, or the fan motor is working too hard, drawing too much current], or a failing voltage regulator. Any decent auto parts store should be able to test your battery and charging system for free. I'd start there.

N!

 

As Normy said start with the basics. Get yourself a good digital voltmeter measure the battery voltage, engine off, measure again engine on with the a/c etc off. Then start loading the system while still measuring ie. turn on lights, measure voltage, a/c on measure etc etc.

 

It sounds like the problem is only evident when the system is fully warmed up, do you checks with the engine fully warmed up. Starting with the minimum of electrical load and turning things on while checking voltage. As suggested above pay particular attention to the voltage when the fan kicks in. This should lead you to the load source that is creating the problem, if not it could be that your voltage regulator or alternator are failing to perform when hot.

 

Monitor the voltage at the coils. The voltage should stay around 12 volts. That will prove or disprove a voltage problem. When it's hot and it almost stalls will it roll over slowy if you try to stop and restart it?

 

Do all the previous listed checks for a start. If the actual voltage to the battery is above 13.5V when cold and below 13V when hot, then it is probably the voltage regulator in the alternator. Bosch tries to adjust the output voltage of the alternator with temperature and this is a typical failure.
Replace the brushes and regulator in the alternator (they are one part). You did not list how many miles are on the car. If approaching 100K, then the slip rings are probably also worn out and you would probably be better off with a complete rebuild of the alternator.

 

Glenn, Yes I do have an automatic

Ben, I'll check out my vacuum line and see if there is an issue there

Normy, I have read several threads about not trusting the voltmeter, which I don't always but once it goes real wonky something is up? A few weeks ago my fans were really loud when it was hot outside and were cycling on constantly. I looked under the hood and found only the left fan was running. The right fan's fuse had blown. Checked my fuse chart for an 89S4 and it indicated a 15A. I replaced it and it worked for a while but blew again. The fuse chart indicates the left fan requires a 25A fuse? I don't understand why the difference so I put in a 25A for the right fan and it hasn't blown since. Both fans are working properly and are very quiet! However there may be an issue there so I'll check the reading with the multimeter and start loading the system and see if anything shows up. I've already done voltage checks at various locations when the engine is not running and everything reads about 12.3V.

I orignally suspected my alternator, voltage regulator or diodes and mentioned it to my mechanic and he wasn't convinced because he said they tested it when it was last in the shop?

I'm going to check the coils and the various tests as per the WSM and report back.

Thanks for the input guys!

 

There's probably something wrong with that fan or its wiring.............putting in a larger fuse is not the solution; dare I say fire? Your fuse chart is correct with the different rated fuses.

Is it possible you have a fan motor sticking or running rough ( bad bearing?)

 

bwoyat... without further info I'd pursue the alt./regulator.

Does your '89 S4 come with a shroud & duct on the alternator
(for cooling)?
Are they there, properly connected/plumbed ?


G'luck

 

What is the proper way to test for a bad fan? I've suspected it for a while! Can I test the voltage at the jump post when the fans come on and see if one is drawing more than the other? Would I pull the fuse at one fan to get a reading then try the same with the other fan? If I'm not correct would the voltage drop on the bad fan be much greater than the good one? Wondering why they would have different fuses for the same type of fan?

I haven't checked the shroud and duct to the alternator but I'll take a look, I suspect if the alt. is getting pretty hot it won't function properly.

I'm beginning to think it's a combination of bad fan and alternator issues?

Someone asked me the mileage on the car. It says 23k km but I suspect the odometer was replaced and not rolled forward. My mechanics best guess based on the wear and tear underneath it's more like 50-60k kms. They mentioned that they tend to see more problems with low mileage cars that have been sitting and not driven.

Brent

 

It's not the voltage but the current draw of the bad unit; its loading the circuit too much and blowing the fuse.

If your not familiar doing in circuit amperage tests do a resistance test on each fan. The fan thats the problem may have a substantially higher resistance indicating a bad motor or as previously mentioned a bad bearing causing extreme drag.

 

Brent, If you need I have a complete double fan/shroud off my 90 that I don't need. $200

 

Brent, thanks for the offer for your fan/shroud. If I need it I'll get in touch in a week or so!

Thx

Brent