964 C4 Hydraulic Pump Blowing Fuses

Pilot_951S · 11-04-2004, 06:41 PM

Hey guys I've recently acquired a 1991 964 C4 Coupe. I've been having a problem with the 40 amp fuse for the hydraulic pressure pump that feeds pressure to the brake booster, center diff, etc.

The fuse keeps blowing but not all the time. The only time the fuse blew consistently was when I tried to autocross the car. Then it would blow half way through the run. I never was able to pinpoint if the problem was related to heavy braking loads, or transfer of power in the center diff. On the street the fuse seems to last for several months and then pop at random.

So far I have replaced the fuses, the relay, and the pressure accumulator. The pump and the booster are mighty expensive so I'm hoping my problem is related to something else.

In any case, has anybody seen this problem, or one like it before?

Don Haney · 11-05-2004, 07:30 AM

Hi there,
There is a factory update for a hydraulic pump relay. The 91 relays would fail, letting the pump run and over heat. Maybe your pump has not failed completely (if that's possible), but is creating enough of a load to pop the relay.
I replaced my relay early on and never had trouble with mine even after the Big Red upgrade.
Good Luck,
Don

Pilot_951S · 11-05-2004, 05:56 PM

I changed the relay after the first fuse blew. Is there more information on this issue with the pump running too much and overheating? Maybe a Porsche TSB or something?

Is there a possibility that the pump overheated at some point in the past and it is now drawing too much power and popping the fuse when under heavy load?

Don Haney · 11-08-2004, 09:22 AM

Thats what I would think...

Have the car pluged in at Porsche and they can tell you what is causing the problem.

jc964rs · 11-21-2004, 09:34 AM

I confirm the problem on C4 1991. One friend had the same problem but we changed the relay too late and the pump was damaged. No fuse problem, but random start of the pump and then assistance failures

73ess · 08-18-2019, 11:20 AM

Can someone tell me exactly which relay and fuse control the hydraulic pump on a 1990 C4? The brake and "diff" warning lights are staying on while driving and my brake pedal is REALLY hard to press. I'm hoping it's just a bad relay or fuse.

Black_Hat · 08-21-2019, 06:03 AM

In case it’s more than a fuse or relay….

My electric brake booster on my C2 Turbo started making noise and blowing fuses before it just stopped.

The motor died.

I found a really inexpensive solution.

https://rennlist.com/forums/964-forum/1134959-c4-abs-booster-pump-unobtainable-rebuild.html

Avedis · 12-02-2020, 12:43 PM

Quote:

Originally Posted by 73ess

Can someone tell me exactly which relay and fuse control the hydraulic pump on a 1990 C4? The brake and "diff" warning lights are staying on while driving and my brake pedal is REALLY hard to press. I'm hoping it's just a bad relay or fuse.

I'm having this exact problem. i've replaced the DME and Fuel pump relay, but I'm getting the diff & brake warning lights staying on. @73ess - did you ever figure yours out? Thanks!

LM964 · 12-03-2020, 12:45 AM

Quote:

Originally Posted by Avedis

I'm having this exact problem. i've replaced the DME and Fuel pump relay, but I'm getting the diff & brake warning lights staying on. @73ess - did you ever figure yours out? Thanks!

PDAS/Diff lock light and Brake Pressure warning light or PDAS/Diff light and ABS light? - two potentially different issues.

Also has the accumulator ('bomb') and diff lock solenoids ever been bled in the recent past? A 'hammer' diags plug in should throw up issue details.

LM964 · 12-03-2020, 12:48 AM

Quote:

Originally Posted by Don Haney

Hi there,
There is a factory update for a hydraulic pump relay.

Hi Don, What pump relay update is this? Simple case of buying a new relay from Porsche (assuming all new relays now available are the updated version)?

Avedis · 12-03-2020, 09:07 AM

Quote:

Originally Posted by LM964

PDAS/Diff lock light and Brake Pressure warning light or PDAS/Diff light and ABS light? - two potentially different issues.

Also has the accumulator ('bomb') and diff lock solenoids ever been bled in the recent past? A 'hammer' diags plug in should throw up issue details.

I believe it is PDAS/Diff and Brake pressure. I just ordered Adrian's book and found this excellent description he wrote on rennlist of possible causes:
https://rennlist.com/forums/964-forum/315766-problems-with-the-diff-lock-on-a-c4.html

Based on my lights/symptoms, I'm guessing it is related to brake fluid/pressure and may need to have the system flushed as a first thing to try...

LM964 · 12-05-2020, 03:19 AM

A full fluid flush is a good place to start. A local respected Porsche specialist can do that using the plugin scanner/diags. They just need to know that specifically you want the want the complete PDAS system (hydraulic brake booster, ABS, accumulator, diif lock solenoids and slave cylinders) fully flushed and bled with a good DOT4 fluid. Brakes can be done at the end (everything supplied by the same C4 fluid reservoir).

Post up how you get on.