TT

Transistors are normally an all or nothing affair, they work or they don't. The extra heat would come from increased current or increased average dwell (duration). I would suspect the coil for any increased current, your dwell table will be stock even for the APE chips.

You can download a schematic from FR Wilks' site:

Motronic Diagrams

IIRC it is really from a 911 DME and there are a few minor errors, but it is good enough for the basics and has the parts listed of the section you say is getting hot.

Danno

Tom, the coil-impedance should be 0.4-0.6 ohms across the primary. If you have an aftermarket coil with lower impedance, you should use a ballast-resistor to bring it into range (many come with these ballasts). Someone posted on here or Pelican that they were having overheating problems with their DME and misfires after installing an aftermarket coil. Installing a ballast resistor solved that issue.

The block-grounds are used for the starter, cruise-control, cooling-fans, etc; pretty much everything in the engine-compartment. The grounds on the inside of the firewall is for the dash gauge and computers. If your block-grounds were disconnected, like you forgot to re-connect them after a clutch job, the 1st time you go to crank the engine, your DME would be toasted.

As for the Huntley injectors, he sold quite a variety over the years. Former employees can tell you more about why he chose different ones all the time. The latest batch I've see were all high-impedance Rochesters. Due to the slower-opening times and inconsistent hold-period, their maximum delivery flow-rate at 100% duty-cycle is about 20-25% lower than their rated static flow-rates.

awilson40

Should I be concerned running an MSD Blaster coil? I read that their impedance is .3 ohm. I dont want to fry a DME

Tom Carson

funny, I just looked at FR Wilks download yesterday. The ignition 'final stage' appears to be a Darlington amp/switch in a TO3 package (2 cascaded transistors in a single case). The 968 final stage is in a TO220 package and, as already mentioned, the other 16v are external from the DME altogether

TT

A Darlington pair acts like a high gain transistor to handle the high current loads. They are used for the ignition and injector drivers.

The injector driver in the 951 DME is a darlington pair in a TO220. Got a PN for the 968 final stage?

Tom Carson

in the 968 DME the it looks like a Bosch 30014...of course nobody can cross reference it. I'm thinking a NTE263 might work, but dunno. if you try one let us know.

Mike

The part you describe as getting hot is almost certainly the BOSCH 1012 ignition driver.

I replaced the one in my DME 3 years ago:

https://rennlist.com/forums/showthre...threadid=73501

https://rennlist.com/forums/showthre...threadid=76706

Tom Carson

Then from what Mike says, it looks like the TO220 package version would be BU941ZP and good for 15amps. Though an NTE263 (rated 10 amps) may work too.

The older TO3 package would be a BU941Z.

see:

http://us.st.com/stonline/books/pdf/docs/1008.pdf

TT...I wonder what year the 951 DME changed from the TO3 ('top-hat' round metal case) to a TO220 case. What year is your DME?

TT

I have DMEs from an 86 944, 88 944, and 89 951 and have had many others pass through my hands.

AFAIK The ignition drivers for the 86-on 944/951 DMEs are all TO3 packages. The fuel injector drivers are TO220. I've used a TIP141 and BD911 as replacements for the two transistors in the injector driver circuit with success. I haven't had to replace any ignition drivers... yet.

The specs look good for the BU941Z, but it only has two pins and case compared to the DME ignition driver's 3 pins and case. I don't have a cross for the 1012.

You have an 85, is your PN the same for the ignition driver. On the really early DMEs I've seen, they sometimes have the native PNs instead of Bosch relabeled parts.

Tom Carson

I didn't find a TIP141 or BD911 available in a TO3 case, which normally has 2 leads and the metal case acting as the 3rd...the collector

the BU941Z (and ZP) specs simply looks more like a 'heavy duty' version

the 968 has the TO220 case (actually prolly a TO218) dunno what's inside my '85, never opened it

Danno

What about replacing these drivers with a higher-power rated ones? Anyone tried that? I wonder if the heat-sink would still be OK...

TT

These are TO220's in the fuel INJECTOR driver circuit. The IGNITION transistor is in the TO3 package that I don't have a cross match for.

If you used a replacement that had less voltage drop or maybe even a MOSFET the heat sink would be more than sufficient. Anybody have a DME with a dead ignition circuit willing to loan it out for possible repairs in the name of science?

Tom M'Guinn

Many many thanks to all who posted on this thread. The technical info available on rennlist is unreal. With that help, I think I found the source of my hot running DME. The heat was being generated in the round "1012" device (the ignition driver transistor I now understand).

The coil resistance was within spec. HOWEVER, many moons ago, a certain tuner from So Cal (rhymes with Schmuntley Racing) talked me into installing a Nology PowerCore Coil Amplifier. It is a do-hickey that attached across the primary coil terminals with no power source of its own. It was out of sight and out of mind for years, but when removing the coil it dawned on me that this PowerCore thing may be increasing the draw on the DME -- i.e., increasing the draw on the round 1012 ignition driver transistor. So, I disconneted that thing, and low and behold, the DME runs at normal temp again. The 1012 transistor gets warm, but not sizzling hot like before.

All hail the power of rennlist!!!

washington951

ya know, tom....i've got the same coil amp from the same source.....hhhhmmmm....

Tom M'Guinn

Hmmm....interesting indeed....

Since you just invested in a replacement DME, you may want to check this out. I'd be very intereted to hear if yours is running hot too, with that amp installed. The round transistor is about the size of a stack of 3 or 4 nickels and is stamped with 1012. It sits vertically in an aluminim mount/heat-sink along one side of the dme. If you pull the top housing cover off the DME, you will see it and can check how hot it is (you do not need to separate the boards). Be careful, I burned my finger on mine. It still runs hot, but now I can put my finger on it for several seconds without getting burned. With the AMP installed, it was like touching an iron.