Need DME relay education

widebody911 · widebody911's Garage

Last Friday I was having a cutting-out issue on the track. Some time between the end of one session and the start of the next is when the sympton appeared - the car worked perfectly one session, and then started cutting out on the very first lap of the next session.

I limped into the pits and started checking the 'obvious' things, ie electrical connections, etc. I pulled the DME relay, and it was hot. I swapped it for another, went back out, and the problem was the same. Came back in, swapped the fuel pump relay with the one for (I think) the power windows, and everything was OK.

I want to get another DME relay, as my 'spare' is one that I know to be flaky. There is a significant difference between the one I have, and the one that came with the DME box. The 'original' one is a 30A, but the local parts house only had a 15A. At the time, the car had a bad stalling issue, and I was certain a new DME relay would fix it, so I bought it, and it did fix it. I never re-visited the issue until now.

I looked up DME relays at you know where and all they list is DME Relay (location under driver seat), 911 (1984-89) The 90's cars used DME relays - obviously - but no mention in the catalog. And I know for a fact there are at least 2 amperages available.

So which relay do I need? Which is the correct one? The 15A relay actually looks beefier than the 30A version. When I pulled the DME relay out at the track, it had that smell like some of the magic smoke had been let out, and one of the coils has visual signs of getting pretty hot.

Here's a pic of the 2 relays. The top relay is the one that came with my DME unit; the bottom one is the one I got at the local wholesale parts dealer. The 2nd pic (tries to) shows the discoloration.

(click to see larger image)

rick-l · 05-23-2004, 10:04 PM

The top dme relay must be a solid state version for the 1990 + vehicles(3.6L). I don't have the wiring diagarm with me but I will compare the schematic on the case of the new version with the wiring diagram for my 88 which uses the bottom electro-mechanical version. I suspect there must be a difference if it let out smoke.

rick-l · 05-23-2004, 10:21 PM

I'm confused. The 88 schematic shows the same wiring for the relay as shown on the removed cover. On the 88 the DME relay directly powers the fuel pump through a fuse. It also goes unfused to the OXS heater. Maybe that little white box contains two relays.

Time to remember how you did the conversion wiring. If the dme relay powers the coil of another relay to the fuel pump I wouldn't think it would matter which you used. Did you do any testing to the fuel pump relay that cured the problem?

Wil Ferch · 05-24-2004, 09:20 AM

Thom:
I'm confused.
First of all, the DME relay is alongside the DME....not part of the DME ( at least for 3.2's)
Also...the DME relay is actually a dual relay...two realys within one case
Also...you mention replcement ( separately) of a fuel pump relay. The one-side of the DME relay *IS* the fuel pump relay..
Like I said...really confused..

--Wil

tholyoak · 05-24-2004, 10:37 AM

Thom

The 84-89 DME relay is mounted near the DME and physically identical to the ones you display with the large terminal posts. These are the ones typically used on 3.6 conversions. The higher amperage one I've never seen before and may be something Timmin's has sourced out. The lower relay looks identical to the ones I have obtained from the dealer for a 3.2 Carrera.
The 993/964 series of cars do have a DME relay it is located on the main fuse panel in the trunk similar to the 944 series of cars. While identical in operation to the 3.2 relay they have much smaller spade type terminals than the ones you have pictured.

My DME relay gets warm too, not too suprising since they flow a lot of current. As long as both sides of the relay are closing the relay is functioning correctly.

On a side note you state that you still have the fuel pump relay from the CIS system (the red one) in the circut... I wouldn't recommend this and at best is redundant, since the DME relay is a fuel pump relay you should remove the old red relay from the circut to eliminate a pottential failure mode

Todd

rick-l · 05-24-2004, 01:58 PM

That 562nd12 is a dual relay made by Fujitsu. Is that a Bosch part number on the top case?

http://www.fcal.fujitsu.com/pdf/fbr562.pdf

widebody911 · widebody911's Garage

Quote:

Originally posted by tholyoak
The 84-89 DME relay is mounted near the DME and physically identical to the ones you display with the large terminal posts. These are the ones typically used on 3.6 conversions. The higher amperage one I've never seen before and may be something Timmin's has sourced out.

On a side note you state that you still have the fuel pump relay from the CIS system (the red one) in the circut...
I wouldn't recommend this and at best is redundant, since the DME relay is a fuel pump relay you should remove the old red relay from the circut to eliminate a pottential failure mode
[/B]

So what would I do - hot wire it or something? If I simply remove it, won't it just not run?

tholyoak · 05-24-2004, 03:24 PM

Thom

Just remove the relay from the circut. I assume you have the wire on the conversion harness coming from terminal 87 on the DME relay going to the fuel pump fuse on the fuse panel in the front compartment, this then powers the red relay on the fuse panel so now you have 2 relays in the fuel pump circut. Simply take the red relay out of the circut by splicing the black wire coming out of the red relay socket (this is the wire to power the fuel pump) to the other side of the fuse terminal that the wire coming from the DME relay (terminal 87) is connected to. Hope this is clear. You might want to check out Ingo's site for a good schematic on the conversion harness wiring.

Todd

Todd