PorscheDoc

Hey guys,

I am trying to help a freind get his 86 928S running again. He drove it, parked it, and the next day it would not start. He has been working on it for a couple of months in his spare time, with no luck. Unfotunately, I don't know exactly what all he has done. I went by and he had obviously tried installing relays, etc, but was good enough to label the old relays as well so we could swap them back in and out, etc.

I am leaning towards a bad LH computer at this point, but would appreciate any input:

1. He says the car will try to start off a little starting fluid (I haven't verified).
2. Crank the car, no power to fuel pump. Jumping the fuel pump relay, fuel pump runs and there is fuel at the rail. Car still will not start. Tried new fuel pump relays with still no power to pump.
3. Verified spark at the plugs (haven't checked timing). Also pulled a few plugs, they don't look the greatest, but should still fire.
4. Tried installing new and old relays in various configurations....no start (fuel pump relay, LH relay, etc).
5. Lost tach bounce about half way through the diagnostics, replaced the crank position sensor and got that back
6. Fuse good at #42 and tried other fuses in that spot.
7. I don't know what else he has swapped, etc unfortunately. I don't know of any good computers in the area that I could borrow unfortunately.


Any ideas?

John Speake

Iassume the LH relay has been swapped for a known good one ? The LH2.2 as fitted to that model are usually very reliable.

Is the car an auto or a 5 speed ? If a 5 speed, make sure he hasn't put a relay in the kick down relay position, as this will give the symptoms you descibe.

Landseer

Back where the CPS is located, at the edges of the V of the engine, there is a bracket across there that has groups of ground wires on each side. Its all down under the fuel reg/ damper.

Anyway, these are the grounds for the critical engine electronics.

Not always the problem, but I've had them cause exactly what you report before on 85 and 86.

Also, if the MAF boot is twisted and the wires frayed beneath, similar situation will occur. Faced this on 87 (same MAF setup).

If no good, then extract the CE panel (easier than it looks), flip it, look for melted wires.
Some of the defroster circuits, prone to over heat, can partially melt into the critical relay wires for the LH and other things within the spaghetti of wires. Look for melts.

I'm a layman at this, John above and others are more versed, but I've seen this stuff mentioned take the cars down.

Also, ground wires engine to frame on passenger side under engine, and believe it or not, deep hidden corrosion of battery groundstrap beneath plastic shroud has taken down cars.

Positive little wire from battery is crucial, too.

Ed Scherer

You probably know all this stuff, but: Troubleshooting Porsche 928 Starting Problems (from JDSPorsche).

Electronik Repair has some pretty good "no start" info, too (both on their main page and the diagnostics page); but note that some of this is biased toward the LH 2.3 (87 and up) models. Most of the same tips probably apply to LH 2.2, anyway.

Too bad it's not an LH 2.3 scenario, or I'd be happy to lend an LH brain and/or various kinds of diagnostic equipment (JDSPorsche spanner, "Theo" DT999 diagnostic tester, JDSPorsche SharkTuner II).


It probably wouldn't hurt to check the various critical input signals at the LH connector and EZ connector (or, if the '86 uses 25-pin LH/EZF connectors, see Euro '84 No Start for diagrams; sorry, I'm not familiar with the '86 MY).


IIRC, the workshop manuals have a comprehensive "no start" trouble shooting sequence to go through that might be worth reviewing, if you haven't already.

fbarnhill

Has anyone checked the timing belt? I have had 2 failures over the last 20 years and the first happened exactly like this. I turned it off and it just wouldn't re-start. After the engine died on the second one, the symptoms were the same. Don't mean to scare you but at least check it out. Of course, both my failures were on the OB engines so things might be different on the 32v engines...

Best of luck,

PorscheDoc

Are the LH relays different from the fuel pump relays? At first glance (at least if he has them labeled correctly), they look similar, so we tried a 2nd new fuel pump relay. Again, everything was just kind of strewn about on the passenger floorboard, so I assume the old relays were labeled correctly. If they are different, then I would pick a new one up and give that a shot first.



Sounds good, I will go in and clean what I can. The MAF harness looked decent, but even if it were bad, is that going to kill power to the fuel pump?

PorscheDoc

Yup, first thing I checked. There wouldn't be spark at the plugs if the belt broke.

Landseer

The LH (edit) drives the fuel pump relay. I think it can.

And its all grounded together, too.

I had mouse pee do a job on those grounds, such that I had to cut one of them ( with 6 ground wires for various things ) and solder-in a new ring terminal. It included MAF, CPS shield wire, O2 sensor, LH and a couple other things.

PorscheDoc

Thanks Ed, I was just re-reading through some of those links yesterday

Ed Scherer

LH and fuel pump relay are both the very common "...253.B" a.k.a. "53" ones.

'85-'86 fuse/relay chart

PorscheDoc

That wouldn't be out of the question as the car does sit out in his detached 40x80, so I assume there are probably some mice running around. I'll try that next!

Landseer

Doc, have you got the 86 wiring diagram?

We can post it. You might have to hold your ctrl key down and spin the mouse wheel to zoom in, but it would be readable.

Ed Scherer

BTW, it wouldn't hurt to replace the EZF relay (XVI), too. IIRC, a failed EZF relay will effectively disable the LH.

Some other stuff you might want to check:

• 86 nostart no pwr to relay
• Euro 84 No Start (has nice LH / EZF related digrams and some nice diagnostic info, courtesy of Rich, the Electronik Repair guy)

Landseer

Lots of support on-board today, which is good.



Here's the important parts of the pertinent 86 diagram:

(BTW, the alarm will shut it all down too. A close look will show the bypass route. )

Mrmerlin

Should have started here, clean the battery connections all of them, Then

get a fuse relay chart here 928gt.com on page 2 under tips and links print one off this will let you see what the correct relay and position it should be in .
As Wally said the computer is probably OK ,

BUT it is probably a relay thats bad.
check all of the fuses and make sure the correct fuse is in the correct slot same goes for the relays make sure the correct relay is installed as well as the empty spots being empty.
Clean all of the fuses with an eraser, clean the ground connections above the CE panel there are 9 of them on the 2 bolts just above the top edge.
Remove all of the relays and check them for corrosion,
any relay that has corroded pins should be opened and smelled for burning and the internal contracts cleaned.

Remove the computer plug connections and inspect for corrosion( just to the right of the CE panel)

Check the hot post connections and the 14 pin connector above it for corrosion.
Check that all of the sensor plugs are installed on the engine,
Note on the aircleaner temp sensor make sure the idle valve harness isnt crossed with the temp sensor,
IIRC the temp sensor wires will have 5 volts and the ISV will have 12V ,
if they have been swapped then the temp sensor is toast,
turn on the key to run and check with a volt meter.
Also check the throttle position sensor wires have not come off,
its pretty difficult to see but its on the right side of the intake as sitting in the car.
Make sure the temp 2 sensor on the water bridge is connected ( bosch type connector).
To test it , one pin to ground,
then the other pin to ground,
Note it is a dual sensor feeding 2 different systems, so each pin to ground is the testing mode.
Make sure the MAF sensor wires and boot are in good condition.
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