dimmer

just checked ... the tach is bouncing around 1000-1500 rpm when I crank it ... I suppose that makes it unlikely that the CPS is the issue.
I won't be able to go into much more depth until I get it home .. hoping that it's something simple like a relay.

Speedtoys

Yup, CPS is good.

FredR

Jeff,

We have been led to believe that is the case when we turn on the ignition but I am not convinced that is the case so hesitate to recommend such given such is not stated in the WSM diagnostics.

I must pay more attention when cranking!

Speedtoys

Twice when I have had CPS issues, the hint was 'no tach', and led me to CPS directly both times..once was just simply not plugged in all the way.

If the tach has signal from the the ECU has CPS signal...that's where it comes from.

ZEUS+

The basic no start diagnosis is to determine if there is fuel and spark, unless it is a no crank condition or a mechanical failure. The easiest test is to remove the large vacuum hose from the intake next to the linkage mount. Squirt in carb clean for a few seconds and connect hose. If engine starts, problem is fuel related and has ignition. A simple test to determine direction of diagnosis.

Pagnobito

The loom to the fuel pump was incorrectly routed under the frame on mine rather than behind it. My rear suspension fully compressed in a severe dip at 70mph on a Belgian motorway. It cut the live wire to the fuel pump.
Check your 14 way connector in the engine bay is secure also.

dimmer

I have the car home and started diagnosis.
The 4 relays required for starting have been checked.
Fuel pump works when relay is jumpered.
Took a closer look at the CPS connector and saw a crack.
Pushing on the cps connector with a screwdriver caused more of the connector to break off.
Although the tach is bouncing when I turn over, there might be an issue with the signal due to the damaged connector.
I don't have an oscilloscope to test with, but the CPS needs to be replaced regardless.
I'll start soaking it with penetrating oil and order a new one .... hope that is the cause of the issue.

FredR

As I stated in an earlier post the male connectors seem prone to failure- this also happens on the knock sensors and the Hall sender- whether that is your issue remains to be seen- I have successfully repaired two CPS units by fitting new 3 pin Junior Powertimer connectors as per the thread I linked. Roger does an aftermarket sender that seems to work well for modest bucks- that has a plastic body that inserts into the block and that should eliminate the tendency of the stock item that commonly jams in the block when removal is attempted- I put that down to galvanic corrosion with the alloy corroding slightly and the liberated aluminium oxide that duly forms taking up more volume and thus jamming the thing and possibly forming a cement just to add insult to injury.

Kevin in Atlanta

Fingers crossed.

Mrmerlin

Fred the reason some CPS units are stuck in the block is due to overheating of the block ,
then depending on how hot things got will make the unit melt into its hole.
some do have corrosion but if its really stuck its been melted.

NOTE fitting a new connector end onto an old piece of electrical wire/ equipment is not a good idea in the 928.
as the not only has the connector degraded but the wires and insulation have as well.
Though it might be hard to measure the degraded performance..

Fitting new parts like a CPS , Hall sensors and Knock sensors and TPS short harnesses remove the doubt of the band aided repairs

FredR

Stan,

The shielded cabling of these devices stands up to the environment way better than the male terminals and replacing the disintegrated body of the connector is so easy using the original spades that invariably are in good nick. If like in this case the device is viable then extending its life with a new connector body is a no brainer for a few bucks. Needless to say one does not know whether the fix will last 5 years or 5 minutes and we as owners have the luxury of making such decisions whereas professionals like yourself do not. Simply put your time is probably worth $100/hour and your reputation is priceless- my time only has value to me and I am retired.

As for "overheating of the block" causing a failure wherein these things jam in situ I am somewhat sceptical about that claim but remain open minded considering I have never come across evidence of such to date. Given my location I would expect such to be more of a problem in my locale were this the case. My take is that the body material is likely non ferritic thus stainless steel and in direct contact with the block opens the potential for a galvanic attack if moisture gets in there in say a humid climate like mine. Given we can purchase reasonably priced alternatives from our treasured parts suppliers that is also a "no brainer" but if I had to fork out the Porsche price which is now nearly US$300 a pop I know what I would be doing.. My revamped CPS unit is currently in one of my spare parts boxes as a standby unit.

I revamped my engine mangement harness a few years ago and by and large the biggest problem was the final few inches of cabling running into the engine bay connectors. Get back into the main runs underneath the crispy fried heat shrink and by and large the individual cores and insulation were in reasonable condition. If the insulation flexes OK without showing signs of distress and the copper is not obviously corroded then I reckon it is good to go. The US$100 or so I spent on new connectors and cabling is probably the best value I have had in terms of bang for the buck in 24 years of ownership. A new complete harness with computer plugs runs somewhere in the region of US$3k thus a good incentive.

Bottom line- I fully agree with you that if these things are original something needs to be done with them.

Mrmerlin

Fred once the wire insulation has been open to the air then all bets are off as to how good the shielding is,
moisture will cause corrosion and that can degrade the effectiveness of the shielding.
I am talking about components and the connectors and the insulation failing,
and I think your talking about the car side of the harness,
so that might be where we are not meeting ideas.

Once you have worked on more than a few cars that did get an overheat episode,
and you then get the CPS out look carefully at the plastic pieces you remove,
you will see the surface has a shine to the plastic that indicted it melted in position.
Usually these require the wire pull to remove or worse total destruction.

FredR

Stan,

The male connector failures I have had on my 928 include at least two knock sensors, my current Hall sensor and the previous CPS. In all cases the insulation was in visually perfect condition, perfectly flexible and no cracks or obvious deterioration surprising as it may seem considering the poor state of the front of engine harness and then the engine management harness. Interestingly enough the Hall sensor connector had been like that for about 15 years as it was clear that the thing had failed and someone had tried to seal it with RTV or whatever and that was not done by me! With the CPS it either works or it does not and mine was functioning before the connector fell apart and after it was replaced [same for a friend's CPS that I revamped for him]. Whether or not different ambient conditions will have any impact remains to be seen- I have often wondered how my 928 would fare if exposed to the UK's cold damp miserable climate.

As for temeprature excursions impacting the CPS sensor body not sure I understand what you mean. The metal temperature at the mounting point is about 80C- thermoplastics melt in the range 200C to 250C so no chance of melting due to immediate thermal exposure. A friend's 928 caught fire in the main dealers workshop due to a damper diaphragm failure that spurted fuel out and caught fire- the CPS was well and truly goosed as was the harness and the MAF and that was over and done with in seconds. If there was an elecrrical fault I suspect the CPS would pack up immediately but...?

dimmer

The original CPS is bosch 0261210003.
I ordered a bosch 0261210002 … it is identical, except 4” longer cable (used on many BMWs of the same era). It is about half the cost … I can loop the cable.
It’s going to take a few days to come in. This is where I envy my friends in the US. Car parts are so much cheaper and readily available there.

It was a bit of a chore getting the old one out, but i managed to pull it in one piece by getting a small screwdriver under it and leveraging slowly.

dimmer

ugh ... the new CPS sensor did not fix the issue.
No spark when cranking.
Fuel pump does not turn on when cranking, but will run when relay is jumpered.
Relays for fuel pump, LH, & EZK confirmed.
CPS resistance tested at EZK connector pin 23 & 6 ... 1000ohms ... within expectations.
EZK pin 13 to LH pin 1 confirmed ok
LH Pin 20 to the Fuel Pump Relay pin 85 confirmed ok
Clutch access cover lowered and inspected for debris .. none

i am starting to think that either the EZK or LH is the issue.
The LH was rebuilt about 10 years ago with a new daughter board.
Is there a procedure to test the computer modules?