Fast John

Hello Rennlist,

I have a 1990 964 C4. 189K miles or so. I have had the car a couple years and drive it when conditions allow. Although I have extensive experience with both domestic and foreign cars for 35 years and have built a few hot rods and race cars, I am learning all about my C4 the hard way (my son and I have nicknamed it "The Hubble" for reasons I'm sure most of you understand).

Here's my problem: I suddenly have a hard brake pedal, but NO warning lights while casually driving to work today. Let me outline the events that preceded this event.

A month or so ago I had the hard brake pedal problem WITH warning lights. After researching the fine Internet 964 forums, I was able to diagnose the problem as a bad pressure switch on the ABS pump unit. I replaced the switch and bled the system two days ago per instructions in Adrian's book. Although the brake fluid in the system looked fine, I ran about two quarts of new brake fluid through the system during bleeding to be sure no old fluid remained in the system.

I drove the car to work the next day (70 miles round trip) and everything was working fine. Then on the way to work the day after that (another 35 miles) the hard brake pedal was suddenly back but no warning lights this time.

I have been unable to find any reference with these same symptoms in the forums and I wonder if anyone could suggest what I should be checking that could be causing this?

John
Texas / New York

Current Stable:
1972 Datsun 240 Z - SCCA Solo II FP race car
1986 Toyota MR2
1990 Porsche C4
1996 Corvette C4 - Collectors Editon
1997 Chevy Suburban
2002 Z71 Z71 Suburban
2003 BMW SMG M3
2005 Mercedes Benz C230 Kompressor

KaiB

Good luck and welcome John.

We'll spare you the this...thread...is....useless....without...pictures.

If, and only if, you post photos of yer 240Z.

I'm chasing an ABS glitch myself, but can offer no bright ideas to you. Help should come soon.

Indycam

Did the pump run when you turned on the car ?
Do you have fluid in the reservoir ?
What happens when you use the drive line lock up **** ?

Fast John

Made some time to do some troubleshooting last night as the root cause of thing was chewing at me:

Opened up accumulator bleeder: Result, No Pressure. Unexpected as there are no warning lights coming on, but that explains the hard pedal.
Check Pump Relay operation: Relay operating (contacts closing) with key-on but pump not starting/running
Hotwire pump to check: Pump Runs - Pump good
Check Fuses: All fuses good.
Conclusion: Must be bad wiring or connector causing pump not to come on.
Additional troubleshooting: Peel cover off of pump relay and wedge contacts closed with pencil to energize wiring and start checking circuits with DVM. Grounds are good but no juice on the hot side. Plug pump and pressure switch connectors back in and re-check things again starting with fuses. I'm hoping I missed something with the fuses as I hate to have to start un-taping the wiring and poking holes in the insulation to look for a break. As I pull out fuse I bump the pencil I had wedged in the pump relay and the pump starts momentarily then stops.
Eureka! Dirty or corroded pump relay contacts are the root cause of the pump issue. BUT, why no warning lights? Could it be a problem with pressure warning switch? I ran out of time to chase that one down last night, but more to come when I do.

240Z pic attached

Indycam

"bad pressure switch on the ABS pump unit. I replaced the switch"
Can you jumper the switch to get the motor to run ?
If the switch is faulty and saying the system is up to pressure , the pump would not run and the computer would not see anything wrong .

Fast John

The switch is energizing the pump relay as it should. I just replaced the pressure switch with a new one last weekend so I'm kind of hoping it is something else causing the warning lights to not come on.

budge96

John I had a similar occurence on a previous TT where there was no light emitted but pedal was mushy
and often required a pump to gain pressure..
Your hard pedal sounds like the accumulator from all past threads I recall but my fix was the ABS pump.
Returned to regular pedal feel and no intermittent light..Bert

KaiB

John, I'm wondering if it might be worthwhile to run by an Indie with a Hammer. Pay him a few bucks to hook up and peek.

Nice Z, I've loved them since they were new (ow!!!); there is a gorgeous well driven one here at Hallett which is hard to get around.

Indycam

The switch is a complicated little bugger , has has to say turn on to the pump when the pressure drops to a point and then tell the pump to shut off at another pressure . I think a third state is not enough pressure , put out a warning of low pressure .

When you ran the pump with a jumper , did the accumulator store up pressure so that the brake didn't feel dead ?

Fast John

Well, I believe I finally tracked this down. It turns out the 40 amp pump fuse was only working intermittently. It looked and tested good and was in the holder good and tight but just stopped working at times. I replaced it and all is good. Thanks for the suggestions though. I've got a few other things on this car to keep me busy though:

1) right rear turn signal blows fuse
2) heat/ac fans in dash don't seem to work
3) heat/ac mixing doors don't seem to work, vents usually blow hot air but sometimes not
4) seat belt, air bag warning lights stay on
5) small oil leak around oil thermostat area
6) door seals shot
7) black ribbing missing from outside door sills / ground effect panels
8) missing lug wrench, jack handle, tool kit, air pump
9) motor fan belt missing, sensor not working
10) alternator belt is toast

KaiB

Good job.

Contact me regarding (8).